How can I work out longitude and latitude through my telescope? We purchased a Celestron Astromaster 90EQ Refractor Telescope and a Celestron Powerseeker Accessory Kit about four years ago but have not used it much for many reasons. As such my husband and I are complete beginners and was hoping for some advice please?
My Dad passed away during the lockdown and we have had a star named after him.
It's a "UK" based star and we will be sent longitude and latitude co-ordinates enabling us to search for it.
On Christmas day we are hoping to use the telescope, with our children, to point out my Dad/Grandad in the sky. However, my husband has now said he thinks you have to buy an accessory or something to do this and the telescope and accessory pack we have does not have the necessary equipment ?.
Can anyone advise how you can do this please and, if we need to purchase an accessory, recommendations?
Thank you very much for any help in advance.
Sue Davis

I used to have this exact telescope. It took a lot of practise, but I was eventually able to reliably find most targets I was after. I used this scope for about 2 years learning my way around the night sky before moving on to another scope and mount. You should practise finding bright targets before moving on to dimmer ones, just to get used to the scope. However, you can use the setting circles on the mount to help you too. The setting circles are the 2 silver disks with all the numbers on the head of the mount. The Declination one is just underneath where the telescope is fitted (upper red circle in the picture). The Right Ascension is on the other rotational point of the mount head (lower red circle in the picture)

First, you need to reliably polar align the scope, which will make nailing down other objects easier. This may take some time so for a "roundabout" method, you can just point the mount towards polaris and it'll be good enough to keep objects centered when adjusting the RA knob, but I'll still go through the "drift" method for a more reliable alignment.
Drift aligning on a manual EQ mount can take a while, but it starts with a rough polar alignment by eye. Align the mount with Polaris as best you can. Now, move the telescope and find a bright star to the south. Center the star in your eyepiece then get the highest power eyepiece you have (will probably be the 9mm one in your accessory kit) and move the star so it is sitting nearly top center of the eyepiece. Wait 2 minutes or so, then move the RA knob slowly to bring the star back into view. If the star has drifted up, then your alignment is too far west. Give it a small adjustment to the east. If it drifts down, you are too far east. Adjust it westwards. Repeat until you have minimal drift.
Next, find a bright star in the east. Use the same method of centering a star then moving it to the top of the eyepiece. If this start drifts up, your latitude setting is too high, so move it down slightly. If it drifts down, your latitude settign is too low, so lift it up slightly. Repeat till minimal drift and you have a pretty good polar alignment!
Now to actually find a target!
Find yourself a starting star, which is the brightest star in the nearby area of your eventual target. Center it in the eyepiece and look up the co-ordinates on Google or any astronomy app you may have. Adjust the setting circles so that the 'pointers' point to the corresponding values in the RA and DEC axis. Do another quick check that the star is centered in the eyepiece and the setting circles are correct. Now find the co-ordinates of your target. Loosen the declination adjustment on your mount and move the telescope s that the pointer is pointing at the declination of your target. tighten the clutch. Now loosen the RA and move the scope to the correct RA co-ordinate. Tighten the clutch once there to secure the scope. Using a low power eyepiece (your 20-25mm one that came with the scope will likely be the lowest power), have a look through the scope and your target should be somewhere in the FOV.
It should be noted that this is isn't a 100% reliable method, and the setting circles do have markings very close together so accuracy can be difficult, which is why it is best to use a low power eyepiece to find your target, but it should get you very close. It may be best to practise by starting on a bright star and using this method to find another bright star. Once you get the method, you'll start getting more accurate and speedy with it.
I hope this helps.

and welcome to Astronomy Stack Exchange.
I’m very sorry to inform you that whatever star you “had named after” your late father is not recognized by the International Astronomical Union—nor by any other legit astronomy organization. Basically all these star-buying and star-naming things are scams. On the other hand, I don’t think you need a star to remind you of the person your father was, and I offer you my most sincere condolences.
The coordinates you are supposed to receive may not be the real coordinates of the star that will be shown on the photograph. I heard of one such case—actually, there was not even a star at the location specified!
Should you obtain real coordinates (for that star or any other), the simplest way to use them would be using graduated circles that most telescopes have. One of them needs to be calibrated, though, and it’s rather difficult to explain just like that. Another option would be to point a star of known coordinates and just go from there, comparing the difference in position and moving the telescope by the same amount.
Again, sorry for your loss, and sorry too for having to break you the bad news that you were the victim of a scam.
Clear skies!


Why is space and time spoken of as one thing I've never understood why it's spacetime and not space and time. In other words why aren't space and time spoken of as two different things. Can someone explain to me why this is

There are different words for different aspects of space. For example, consider: length, width, and height. Other words include depth and breadth. We can speak of them as different things if we choose to, but we generally consider them to be part of unified concept of space. Why?

It's because we understand that these words just pick out measurements along specific directions in space set by context or the speaker's viewpoint, and that there is no intrinsic difference between those directions. Depending on which way they are facing, what's length to one person is width to another.

More formally, there is a symmetry between directions in space. We can rotate freely and the intrinsic properties of space look just the same, e.g. the Euclidean metric (infinitesimal distance formula)
$$\mathrm{d}s^2 = \mathrm{d}x^2 + \mathrm{d}y^2 + \mathrm{d}z^2$$
is invariant under the orthogonal group of rotations, $\mathrm{O}(3)$. Together with spatial translations (which are like rotations about a point at infinity), this forms the Euclidean group of isometries, $\mathrm{ISO}(3)$. Because of this symmetry that "intermixes" directions of space, we think of those different directions as being unified one into one thing: space.

In special relativity, spacetime has a Minkowski metric (in units of $c=1$)
$$\mathrm{d}s^2 = -\mathrm{d}t^2 + \mathrm{d}x^2 + \mathrm{d}y^2 + \mathrm{d}z^2\text{,}$$
and it is invariant under the Lorentz group $\mathrm{O}(1,3)$ that act like rotations in spacetime; together with the translations (in space and in time), the full isometry group is the Poincaré group. In fact, the usual Lorentz transformation (say, boosting along the $x$-axis) is exactly a rotation with by a hyperbolic angle in the $tx$ plane.

That's why people began to think of spacetime as one unified whole. The reasons are essentially the same as why people think of space as one unified whole despite it having different directions: there is a symmetry that "intermixes" those directions, albeit slightly differently in spacetime because of the different sign of the temporal and spatial directions in the metric.

In general relativity, this is generalized further, but the details aren't immediately relevant here.

Why is space and time spoken of as one thing?

Because it's how Minkowski presented it in 1908. See Space and Time where you can read this: "Henceforth, space for itself, and time for itself shall completely reduce to a mere shadow, and only some sort of union of the two shall preserve independence".

  I've never understood why it's spacetime and not space and time.

You're right to do so. Because spacetime is actually a mathematical abstraction that people mistake for reality. The Earth is surrounded by space, not spacetime. Light moves through space, not spacetime. See this physics stack exchange answer where relativist Ben Crowell says "Objects don't move through spacetime. Objects move through space". When you move fast through space your measurements of space and time change, but it's you that's changed, not space, and not the things in space. For example a star doesn't actually length-contract to a discoid shape just because you step on the gas.

  In other words why aren't space and time spoken of as two different things.

They are. The thing to appreciate is that whilst a lot of popscience articles blithely talk about spacetime, people like Einstein were very particular about saying space when they meant space. See for example Einstein's 1920 Leyden Address where he referred to the continuum of space-time and yet described a gravitational field as space that's neither homogeneous nor isotropic. The distinction between the two tends to get lost. In a gravitational field space is inhomogeneous, and the inhomogeneity diminishes with distance in a non-linear fashion. When you plot it using say light-clocks at different elevations, you see a curve on your plot. Your plot depicts curved spacetime. That's the map. But inhomogeneous space is the territory. See Inhomogeneous Vacuum: An Alternative Interpretation of Curved Spacetime.

  Can someone explain to me why this is?

By and large it's because people don't have a clear understanding of relativity. Space and time are not on an equal footing. I can hop forward a metre but you can't hop forward a second. There is no real rotation when you accelerate, it's just an abstract mathematical rotation, wherein your "local motion" is of necessity reduced by your macroscopic motion through space because the total rate of motion can't exceed c, because of the wave nature of matter. See my time dilation answer here on stack exchange. Since when you look at what clocks do  you appreciate that the time is a cumulative measure of local motion, you're actually better off thinking in terms of space and motion rather than space and time.


What makes protoplanetary disks start rotating? (Initial energy needed to rotate) Planets form from a protoplanetary disk that has been rotating around its star. The initial energy that makes them rotate really matters to me.

Why did the protoplanetary disk start rotating around the star?
Where did the initial energy to rotate come from?
Why hasn't all the disk material been swallowed by the star?

Two rocks placed in space with no relative motion are going to be attracted by gravity, and hit. 3 rocks, placed in space with no carefully rigged symmetry, will likely miss each other, as the gravitational attraction of the additional rock changes their course. Those near misses are the beginning of rotation. Multiply that effect by trillions, and you have a protoplanetary disk. Of course, a lot of those molecules, dust and rock won't even start out with zero relative velocity, so near misses are inevitable.

Summarizing/adding to previous specific answers:
Why did the protoplanetary disk start rotating around the star?
The disc forms because it was rotating. Without net rotation it wouldn’t form a disc... it will stay a spherical symmetric cloud.
Where did the initial energy to rotate come from?
A small angular momentum NOT energy is required the “angular energy” gets magnified during gravitational infall while conserving angular momentum like the figure skater spins faster pulling arms in with small initial rotation.
Why hasn't all the disk material been swallowed by the star?
As it speeds up it is basically orbiting faster and faster until centrifugal force balances gravity. Equilibrium.


Darkness in Outer Space I am trying to imagine how dark outer space is. Looking at images of cosmonauts in space proved not very useful, as the exposure makes space look completely dark.

Thinking about it, I would imagine an object would be quite well illuminated, being bombarded by provided it's not in the shadow of a planet or some other celestial body. Is that correct? How about light scattering? Is cosmic dust abundant enough for the purpose?

EDIT: @zephyr made me realize that the quantity of light is a function of position, so let's say the question is with regards to the Milky Way. Still, I would appreciate some comments on what's the situation outside the Milky Way too.

Just to put in a simple answer:

if you mean (A) for astronauts just in orbit (at the space station and so on) or on the surface of the moon.  Yes, it is incredibly bright.  Just like on Earth in the Sahara at noon!

A point of confusion is that THE >> SKY << IS BLACK, but it is incredibly bright.  On Earth in daytime, the sky happens to be blue, and it is incredibly bright.  On Moon in daytime, the sky happens to be >> black <<, and it is incredibly bright.  (Same deal in orbit as on the Moon.)

(If you are, say, explaining to a young person "is there daytime/nighttime on the moon" that's the salient point to explain: on the moon, confusingly, the sky is black during both daytime and nighttime.  But everything's just as bright on the moon in daytime as on Earth in daytime, there's the same amount of light, i.e. you can trivially read a book or see the ground, just the same as on Earth.)

If your question is about (B) in the solar system - say "on Uranus" or the like.  There is remarkably less light there than on Earth, the sun is much smaller.  Many young people don't realize this; everything's basically dark all the time from about Saturn outwards!

Outstanding page explaining this:

http://www.astronoo.com/en/children/sun-apparent-size.html

Finally if your question is about either (C) inside our galaxy but not near a star, or, (D) midway between the galaxies:

That's too difficult.

The reason the night sky is dark is our distance from the nearest stars (over 4 light years) which would be as bright as our sun (93 million miles) if they were not over 2500 times further away and only the brightness of a star, not the sun.


Can there be an infinity of stars in the Universe? I have a mind puzzle.

Can there be an infinity of stars in the Universe ?

I have 2 opposed reasonings, yet I don’t find any flaw in them. Can you help me here ?

Answer A : No, the number of stars cannot be infinite.

In the whole Universe, at an instant t, there are W molecules of water. This number may be very big, but it has a definite value. The same goes for stars. Each of all the stars at the instant t has her own size, and we can give her a name. We can list all of them, and we can sort them from nearest to farthest — if we are omniscient, of course.

Answer B : Yes, the number of stars can be infinite.

Space has no finite volume. Space has no ending wall. If I advance into space, I can always go further. Wherever I go, space is by default filled with… vacuum. But it could be otherwise. [I am not sure of this point.] Space could be by default filled with… air. So there would be +∞ molecules of air in the Universe. We can apply this reasoning to stars. Let’s say there is, in average, 1 star / 1060 m3. So, in the total infinite volume of space, there would be +∞ stars. When I advance into space, I would keep meeting new stars, without end. Like in these video games in which mountains keep appearing as far as you walk.

Theoretically yes, there can be infinitely many stars. Since this space you're talking about is better described by general relativity and Einstein's field equations. One of its solutions describes the universe as being spatially infinite.

There is an uncountable amount of stars from our perspective, but space does have a limit of size which is the distance which subatomic particle and matter as traveled since the big bang and with that limit of stars. Although stars die and form beyond our ability to count so observably we can never have an exact count of stars at anytime as well.

I would bet on answer B because The universe does have a total mass of about 25 billion galaxies. We can mathematically calculate the mass of our universe.


What phenomenon causes "lunar waves"? In multiple video clips of the moon but also one of Jupiter, some kind of waves can be observed. Is this just the effect of the object moving behind layers of different refractive indexes (similar to what we observe at sunset) or is there a wave propagating through the atmosphere?

https://m.youtube.com/watch?v=xUGxysKSGEM

https://m.youtube.com/watch?v=EMSkd4Xhp0g

There is an answer on Quora, but I would love to see a more detailed answer, why we can see exactly these two rather sharp lines moving.

Video astronomy requires much more equipment than traditional eyepiece viewing. It helps to understand what is going on between the camera lens and the video screen.

Most important is the conversion of the cameras analog signal to the computer digital format. This is usually done with a frame grabber and its associated drivers. The cameras typically have 700 ish lines of resolution which are usually translated into 640 by 480 digital signal. The camera analog signal is 60 cycle interlaced. The computer display frequency is whatever the PC is set to. I'll let the Math people tell us how the drivers fit all those numbers together and make a nice picture. I think the answer is PI or at least within a decimal place or two. The numbers get rounded and every once in a while the computer screen hiccups. National Enquirer sort of news.

The frame grabbers come in a wide range of cost and reliability. There's no rhyme or reason to their reliability. They depend on the computers operating system which is not identical on every computer. The frame grabber and driver will work well on one PC but not another similarly equipped PC.

Typically the astro video cameras two output connectors, a coax and an S-Video. The person making the video could have proven the "Waves" were and artifact of the PC by simply connecting one of the cameras outputs to an analog video screen, a television with a composite input for example and the other output to the computers frame grabber. Provided the camera and analog video are functioning properly he could have shown us a rock steady image on the analog screen and the waves on the Computer screen. With a little skill he could have shown us an alien or two surfing on the waves.

Smoke, mirrors and snake oil. Suckers are said to be born at the rate of one per minute. I have a feeling the con artist birth rate is up there as well.

It is a bit of a shame, Video or near live viewing as many of us call it, is gaining in popularity. Having someone use what is not unusual for a Video astronomer to see just for a bit of sensationalism serves no useful purpose.

Another point to edit in. The uTube is a video of a video. The image we see is taken by a second video cameras pointed at a video image being displayed on a computer screen. Crunch the computer screen frequency and resolution with the frequency and resolution of the second camera. You end up with images of airplane propellers that appear to stand still or move very slowly. I wonder how much adverting revenue that sort of video would bring into uTube.

Thus far there has been no solid explanation.

However, the person who discovered it, YouTube user Crrow777 has a video which explain his observations, and handily debunk Carl Witthoft's explanation of it being atmospheric turbulence.

https://www.youtube.com/watch?v=xUGxysKSGEM

The most obvious of which being that the distortion does not extent beyond the boundary presented by the structure of the moon in the image, even while the camera is moving around.


Why do Black Holes in the middle of galaxies not suck up the whole galaxy? As stated in several sources, it's supposed that in every galaxy there is a black hole in the middle.

My question is, why do these black holes in the middle of galaxies not suck up all the surrounding matter in the galaxy?

You shouldn't think of black holes as "sucking things in". Black holes interact with matter through gravity, just the same as any other object. Think of our Solar System. All the planets orbit around the sun because it has a lot of mass. Since the planets have some lateral motion (they're not moving directly towards or away from the sun), they circle around it. This is known as conservation of angular momentum.

When talking about gravity, all that matters is the mass of the objects involved. It doesn't really matter what kind of object it is*. If you were to replace the sun with a black hole that had the same mass as our sun, the planets would continue on the exact same orbits as before.

Now, the black holes at the centers of most spiral galaxies do accumulate mass. Some of these black holes have accretion disks around them. These are swirling disks of gas and dust that is slowly falling into the black hole. These gas and dust particles lose their angular momentum through interactions with gas and dust nearby and by radiating energy as heat. Some of these black holes have very large accretion disks, and can generate huge amounts of electromagnetic radiation. These are known as active galactic nuclei.

So, long story short, black holes don't "suck". They just interact with things gravitationally. Stars, gas, and other matter in the galaxy has angular momentum, so it stays in orbit around the center of the galaxy. It doesn't just fall straight in. This is the same reason the Earth orbits around the Sun.

*Disclaimer: When you talk about things like tidal forces, you do need to take into account the size of the objects. But for orbital mechanics, we don't need to worry about it because the distances between the objects are generally much larger than the objects themselves.

its all about ENTROPY which is proportional to the surface area of the event horizon of a black hole(see below for a heuristic quantum  argument due to Moffat/Wang as to why this is so).

Assuming a Schwarzschild  solution gives a radius of 2Gm for the event horizon with m the black hole mass and G Newton's constant. Adding mass to a black hole thus increases its entropy. Given an isolated system of finite total energy, it has a finite maximum entropy which acts as an attractor for the dynamics of the system,placing a limit on the horizon.

J von Neumann defines a quantum version of entropy as follows: Let f be a normal state of a local algebra of observables O(D) acting on the Hilbert space H. Then we can write this f as a convex sum of pure states. For a system of finite energy this sum is finite since H is then finite dimensional
Von Neumann’s non-commutative equivalent of a partition is the density operator   ie the weighted sum of the projections  onto the minimal vector spaces corresponding to these pure states
    Then we have the well-known equivalence;
For such a normal state f, the von Neumann entropy is defined as  the entropy of the weights . We interpret it as an (inverse) measure of the amount of information that the quantum system in a given state will yield through measurement. The larger the entropy of the quantum system, the less information can be extracted.
The von Neumann Entropy of a Black Hole
The measurement process cannot be performed by an external observer to elements within the interior, beyond the event horizon. We thus partition the event horizon of the black hole with elements each of area   k squared, where k is the Planck length and assume the Planck area  corresponds classically to the minimal projection   of the pure vector state  . Let N be the total finite number of partitions. By the ‘no hair’ hypothesis there is no preferred location on the event horizon, so that each partition element must have the same weighting. The von Neumann entropy of this partition is thus proportional to S the surface area of the black hole.


Metal distribution in our solar system The sun dominates in our solar system.
I wonder whether in every aspect the sun plays the most important role in our system.

For example, all kinds of metals are mainly located in the sun instead of the rest combined together in our system?

Is there any complementary relation between the sun and its children (all the planets, asteroids etc..)?
I mean: for one kind of metal, if the sun has more, her children should have less.

According to Abundance in the Sun of the elements, at least Li and Tb abundance of the sun are very low ($\approx1e-8$). Furthermore, these abundances are from the solar atmosphere which is near the surface of the sun.

The metal content of the Solar system is completely dominated by the Sun. The Sun contains $\sim1\%$ of 'metals' (in astronomical language anything but hydrogen and helium is a 'metal'), but all the other bodies of the Solar system combined have less mass than that. So even if they were only made of metals (but the outer planets are mostly made of H and He) the Sun would still dominate the metal budget.
The Sun does not dominate the angular momentum of the Solar system, which is dominated by the orbital angular momentum of Jupiter. The angular momentum due to the spin of the Sun is rather modest. This is easy to estimate: the orbital angular momentum of a planet is $\sim m\sqrt{GM_\odot a}$ which increases with semi-major axis $a$, while the spin of the Sun contributes $\beta \omega M_\odot R_\odot^2$ with $\omega$ the Solar spin frequency, $R_\odot$ the Solar radius, and $\beta\sim0.1$ a factor depending on the inner structure of the Sun. The ratio of the latter to the former is
$$
\beta \frac{M_\odot}{m} \frac{R_\odot^2}{a^2} \frac{\omega}{\Omega}
\sim0.01
$$
For Jupiter, the first factor is $\sim10^3$, the second $\sim10^{-6}$, and the third $\sim180$ is the ratio of the Solar spin frequency (once in 25 days) to the orbital frequency $\Omega=\sqrt{GM_\odot/a^3}$ of Jupiter (once in 12 years).

The metals that are forming in the sun come from nuclear fusion. Everything that now exists in the sun's body came from hydrogen, which due to high temperatures is being fused into helium and helium to lithium and so on. Once hydrogen is over, that means the start of the sun's death, where the sun will shrink (meaning temperature increase) and will start expanding again and the helium will continue to be used as a fuel and there is gonna be a point were all hydrogen and helium from the core will go, due to fusion, and the sun is gonna be cool so no further fusion will occur. Eventually what will be left is a planetary nebula with a white dwarf.

I am answering to your second question:

At the beginning of creation the planets (small dust grains) where revolving around the protostar sun. Gravitational force made dust particles to come together. The inner system which includes planets up to Mars, was very hot and molecules like water could not condense so the metals and the silicates dominated and formed the inner planets. The outer planet area (after Mara) was cool enough so the water and other elements could be in a solid state. Now, about the asteroids, these are rock leftovers that did not form into planets. Asteroids mostly found in the so called "asteroid belts", which is a region between Mars and Jupiter.


When a neutron star accretes matter, will its mass increase? When a nova gets matter, the outcome is that it may lose more matter because of eruptions. Only some kind of nova's masses can increase stably.

A white dwarf may become a supernova after it exceeds the Chandrasekhar limit, and we study that much. A neutron star can get mass too, why do not we talk about their end much?

How about a neutron star when it gets matter from accretion? It is necessary that its mass will increase?
When will it collapse?

The neutron stars have about 1.1 to 3 solar masses. If the neutron star accretes mass, its mass will increase and once it crosses the Tolman–Oppenheimer–Volkoff limit, the star will collapse to form a black hole.

On way of this happening is in the case where one of the binary stars becomes a neutron star as in X-ray pulsars. The neutron star receives gas supply from its companion, which increases its mass,for example CenX3.

Source: nasa.gov

Though this method could lead tothe formation of black hole, no observational evidence exists as of today.

A nova is generally understood as an explosion of a star.  Thus comparing a nova to a stable neutron star is kind of a nonsensical thing to do.

If you're asking if a star goes nova and the core that remains is a neutron star after the resulting explosion, will it's mass increase overall?  The answer is no, since it loses a lot of mass in the process of going nova.  However, it is now more dense than before and the concentration of matter is in a much smaller space (the neutron star).  (The explosion compresses part of the star and pushes the rest of it outward off into space).

If you're asking if a neutron star's mass will increase if you throw matter at it, the answer is yes.  Just like any other form of matter.  Is there a limit to how bit you can increase it?  No; however, past a certain density it will compress into a black hole (which do not have a size limit as far as we know).

You may want to edit your question to clarify what you mean.


How did the Ancients understand the ecliptic using the Ptolemaic system I studied the Farnese Atlas and I wonder how did the Ancients discover the ecliptic, the equator, etc. with the Ptolemaic system namely without knowing that the Earth rotates on itself, especially for the Ecleptic.

I ask the question because if in their model the Earth does not rotate, so how do they explain the two revolutions of the sun around the earth:

The first one being the daily cycle (sunrise and sunset)
The second one being the tropical year

With a model where the Earth does not rotate, I do not understand how they found out that the Sun make a revolution of a one year around the Earth (whence the ecliptic) with a second daily revolution. How could they conjugate these two Sun's cycles?

The ecliptic was understood to be the path of the sun relative to the sky.

Ptolemy believed the sky rotated daily around the Earth. All the planets, including the Moon and the Sun, shared in this motion.

However, the planets had secondary motions, relative to the sky. And in particular relative to the stars that they thought were fixed to the sky. This motion was discovered by careful observations of the sky, and was very well known, long before Ptolemy.

The motion of the stars was very simple: therefore when Ptolemy was describing his system, he only concerned himself with the motion of the planets (including the moon and sun) relative to the stars. It was clear that the planets all approximately followed a great circle relative to the stars. This great circle is the ecliptic. The Sun's motion was the simplest. It was exactly on the great circle, and it's position relative to the stars moved almost evenly. The other planets (including the moon) had orbits that were less even, but by introducing a system of Eccentrics, Deferents, Epicycles and Equants, Ptolemy was able to account for nearly all their motions.

The precession of the equinoxes was noticed because the Greeks used a calendar based on the heliacal rising (the date that stars are just visible before sunrise) of certain stars. And it was noticed that the heliacal rising was changing relative to the seasons and the equinox. We understand the reason for this to be the precession of the equinoxes.

This article describes it.
https://www.ias.ac.in/article/fulltext/reso/011/03/0051-0068
Aryabhata described it and the Greeks most likely had a similar method.
The title of this article is Aryabhata and Axial Rotation of Earth - Khagola (The Celestial Sphere)" by
Amartya Kumar Dutta. There are a lot of diagrams here.


How can we see stars if they are hundred years of light far from the Earth? So the right question is, is the light years counted from that second when you look up to the sky? If that so, how can we see the stars from the telescope (which I think it starts when you put your eyes on it, same as when you look up to the sky), that they say it's 5000 of light years from earth? If you say these lights are already started 5000 years of light ago, then there should be so many changes happened to these galaxies or stars in that timeline. Thus, for example, there should be no way to know, if an earth-like extrasolar planet is still earth-like.

You are right, there is no way to know, what is the current state of these objects "now" - if you think on the Newtonian mechanics.

For example, the 1054 supernova, what created the Crabs Nebula, happened actually not in 1054, but around in 6000 B.C. It is because it is $\approx$ 7000 ly away from us.

In the Special Relativity, "now" is relative. You can't say, that the 1054 Supernova happened in fact 7000 year ago: if you would see the whole story (1. the star explodes 2. the astronomers of a planet far, far away, some millenia later, detect it) from a spaceship going, for example, with half the light speed, you would get totally different numbers. There is no "objective now" in the special relativity.

For us, "now" is what we see in the telescopes. For us, the crab nebula explosion happened in 1054. We have no way to visit a star, for example, 50 light year away, now - we would need at least 50 years to visit it. (It is the theoreitcal limit - in practice, there is no technology to send anything even to the nearest star, 4.3 light years away. If there would be, it would likely require at least centuries to get there.)

However, the overwhelming majority of the stellar evolution processes is much longer. Most stars exists between 10million and 10billion years long. Of course we have no way to watch a star billions years long with the telescopes. However, we can see billions of stars, typically some decade (centuries) long, and even this is enough to understand a lot from them.

We have a strong reason to not expect a huge difference from the "current" state of most stars/extraplanets from what we can see today on them - simply because such events (change of planetary orbits, average lifetime of most stars) last million times longer.

Light travels at 186,000 miles per second, and in the case of the furthest galaxies we can see it has been traveling for about 13 billion years. On an astronomical scale, 5000 light years is quite nearby and probably about the distance that the nearest advanced extra-terrestrial civilisation is from us. These distances are far too large to be measured in light seconds, only bodies like the moon, 1.25 light seconds away, are close enough for that. You're right in thinking that significant changes must have taken place since the light started on its journey from stars and galaxies billions of light years away, but they are changes we can never know anything about. In the highly unlikely event that we picked up messages from a civilisation 5,000 light years away, that would make for a very tedious conversation. Our reply would take 5,000 years to reach them, and if they answered immediately we might get their reply in 10,000 years, by which time our descendants might have disappeared in a nuclear holocaust resulting from an all-out nuclear war! There is a decimal point in the moon distance quoted which doesn't show up very well.


I saw TWO moving stars going at the same speed! What could this be? This weekend me and my family took the opportunity to go camping. My dad and I went star watching because where we lived we didn’t get many stars. I was looking at the stars when I saw a bright star moving across the sky. It not as fast as a falling star or a meteor but it faster and brighter than airplane or a satellite. My dad and I kept watching the star when another star like thing moving as fast, in the same direction, but not as bright as the other star, started trailing the brighter star. We watched in disbelief as the two stars went across and soon behind a tree, out of our view. I can’t explain that. Can you?

The International Space Station (ISS) and several other satellites can be seen early in the morning or late in the evening, even from urban locations.

Go to this site.

https://www.heavens-above.com/main.aspx

Enter your location then click on ISS. The site will give you a list of the times when the ISS will be visible and the area of the sky that it will move through.

The periods when the ISS or other satellites are visible come in cycles. If nothing is listed as visible in the ten day period wait a week then try again. It might take several weeks. If you live somewhere between 50 deg North latitude and 50 deg South latitude the ISS will be visible sooner or later.

Once you have seen the ISS passing overhead you can compare it to what you saw on your camping trip.

Some people are using electronic eyepieces to view the sky from light polluted areas. They often broadcast their observing sessions on the NightSkysNetwork

https://www.nightskiesnetwork.ca/

If you live in a light polluted area and are interested in astronomy you might find the site interesting.

I agree with the above two commentators, it was something to do with satellites. Quite possibly it was pieces of satellite debris starting to make their re-entry. These pieces usually burn up before reaching the ground, but very occasionally quite large chunks come down on land, though the chances of them hitting the sea are much greater.


Are there bodies in the Universe that are not spinning? Do we know of a celestial body which doesn't spin at all on its own axis? If not, why is it so?

Many red and brown dwarfs must have planets that are in full tidal lock with their host star, that means that the same side of the planet is always facing the star so there is always day while on the other side there is always night. Our Moon and many other moons are also in such a tidal lock with their planet but not with the Sun so that the often-used term "Dark side of the Moon" is verbally incorrect since on the far side there is a day too (when on Earth it is a New Moon).

If you mean non-spinning in the sense that on the same side of the planet it is day for half a year and night the other half I'm quite sure such planets exist too. On SpaceEngine I've seen similar planets. Nothing speaks against the existence of such planets I believe. There also might exist moons whose same side faces their planet for half a year and the other half faces away from it.

Every object in the universe--from a basic star to an exotic black hole--spins, and the origin of that spin can be traced back to the very beginning of time. Within instants after the Big Bang, the primordial fireball of energy expanded at an incredible speed, then later cooled and solidified into all the matter in the universe. Had this fireball been uniform in all directions, everything we see today would be completely homogeneous.

http://www.scientificamerican.com/article/what-causes-objects-such/


Simple experimental evidence that Earth revolves around Sun What are the simplest experiments or calculations that give evidence that the earth revolves around the sun? Can you please explain them and reference the history? Many simple explanations such as this cite observations such as that relative position of two stars are observed from earth vary every night - which would not be true if the stars orbited the earth. But isn't the observation also consistent with a model where the stars orbit the earth but do so at different speeds, while the earth still orbits the sun? Simple explanations would be helpful.

The answer is ironic: Without good instruments, there is no evidence. The people who thought that the Sun went around the Earth were perfectly correct as far as the actual evidence went until the early 1700s and mid-1800s when two lines of evidence opened up that showed that the Earth moved.
Aberration of Starlight
Wikipedia has a correct but over-complicated explanation. The easiest way to think about it is to imagine yourself at a stop sign in a car in the rain, and the rain is falling straight down. When you start moving, the rain's apparent direction of fall changes so that it appears to be falling from ahead of you and slanting down towards you. That's aberration.
In the early 1700s, the stars were discovered to be shifting position, and in 1727, James Bradley correctly identified it as abberation of starlight due to the motion of the Earth around the Sun. (For any star in the ecliptic, the Earth is moving towards it at some time of the year and away from it six months later.)
Parallax
Wikipedia's article on parallax is better, and I refer you to it for details. Basically, if you hold your finger up before you and look at it with your left eye closed, and then with your right eye closed, it appears to jump with respect to the background -- the wall beyond or the trees outside or whatever. Switch back and forth between your eyes quickly to see it clearly.
As the Earth circles around the Sun, nearby stars also appear to shift their position relative to the more distant stars.  A key point here is that there were good scientific reasons to suppose that the stars were much smaller than the Sun. Seen through a telescope, stars showed disks and if they were like the Sun, their distance could be deduced from those disks. And they were close enough that if the Earth really went around the Sun, parallax should have been observed. But it wasn't and the lack of any noticeable parallax was a strong empirical argument against Heliocentric theories.
In reality of course parallax exists, but the parallax of all stars is small, because they are much further away than was estimated from their disks. (The visible disks were actually diffraction disks and not true disks at all -- but it was not until nearly a century later that diffraction began to be understood.)  Friedrich Bessel first measure the real parallax of a star in 1838.

Very simply: because of relative motion, no proof exists.  Any situation that you come up with can be explained by a tweaked geocentric module. Albert Einstein came to the same conclusion when he said   "I have come to believe that the motion of the Earth cannot be detected by any optical experiment."  and "...to the question whether or not the motion of the Earth in space can be made perceptible in terrestrial experiments. We have already remarked... that all attempts of this nature led to a negative result. Before the theory of relativity was put forward, it was difficult to become reconciled to this negative result."


How young could the asteroid belt be? I heard a theory (possibly from a crackpot) that there was an event in our solar system where two objects collided and created the asteroid belt, the orbits of some of the planets were altered, and that this event took place late enough for homo sapiens to have referenced observations made during the previous orbit in their oral traditions. (Mars parsing close enough to Earth for people to observe its moons)

Is any of this possible given readily observable evidence?
Are asteroid belts ever created in this way?

Dating the age of the asteroid belt is a problem that has to date no general solution.

Rocks are usually dated in the laboratory via isotopical analysis. This is possible for meteorites, but what to do about those rocks that remain in space?

Several methods to date the asteroid belt are known to me, apart from ad-hoc assumptions that it has the same age as the planets:

Creating links between dated meteorites and asteroid families: The meteorite family of L chondrites have possible origin families in the asteroid belt. Those 'families' are small blobs in the eccentricity-inclination diagrams, that probably originated in one common parent-body breakup event. Dating meteorites originating from those families, gives minimum ages for the parent bodies of i.e. 500 Myrs for the Ordovician meteor event.
Dynamical age modeling: After a breakup of a parent body, the cloud of fragments experience different strengths of YORP and tidal forces, which causes their semimajor axis distribution to spread. Taking a given distribution, one can calculate back to an age of when the cloud was one particle, i.e. the moment of breakup. This again is a minimum age and found for the Flora family to be more than $10^8$, but less than $10^9$ years. This does exclude a very young belt, but does not say much about the upper age limit.
Crater ages: For the largest bodies, i.e. Ceres, Vesta, etc. we can assign relative ages to the lunar cataclysm. Those usually give ages in excess than 3-4 Gyrs. For bodies like Vesta, where the surface is saturated with craters, this age is a lower limit.
Dating of crustal ejecta: There is another family of meteorites that can be linked specifically to Vesta via spectroscopy, called the HED meteorites. Those meteorites look like they're coming from a differentiated parent body (melted, low siderophile element content), and they containt CAIs. Thus, we can date Vesta at least back to the origin of the solar system, as CAIs give us the effective $t0$ for the formation of the solar system.

Some of this, as you say, is crackpot stuff, but there have, long ago, been some major collisions in the asteroid belt. The asteroid belt was formed about the same time as the Earth, and these collisions took place quite early on, billions of years before there were any humans. Mars never passed close enough for humans to observe its moons until telescopes were invented, but it does come within 35 million miles on closest approach. Only Venus comes closer at a distance of 26 million miles. The only asteroid belt we can study is our own, but common sense tells us that there must be other solar systems with asteroid belts, created in much the same way as ours. Some of the fragments from these asteroid belt collisions have been propelled Earthwards and left many huge craters, but only the biggest from the last couple of billion years have left identifiable traces. Those from earlier times, especially those that fell in deep ocean, have been eroded and left no visible trace.


Does astro means space or stars In the popular TV show The Big Bang Theory Season 3, Episode 4, Sheldon Cooper said that astro means star and Raj said astro means space, I got a little confused here, various websites and others are going 50-50 on the subject. What does astro actually means?

Raj: No, no, no, no! That rate is much too low for what we’d expect from this collision. Do you understand that we’re talking about dark matter colliding in outer space?
Sheldon: Of course I understand. And who are you to tell me about outer space?
Raj: I’m the astrophysicist. Astro means space.
Sheldon: Astro means star.

Astro comes from the greek άστρο which not only means star but has a more general meaning that includes any small dot in the night sky (see https://en.wiktionary.org/wiki/%CE%AC%CF%83%CF%84%CF%81%CE%BF#Greek for details)

In today's language it still has a quite broad meaning:

Oxford English dictionary (https://www.oed.com/view/Entry/12240?isAdvanced=false&result=2&rskey=LlaQB9&):

  Forming terms relating to stars, other celestial objects, or outer space.

Merriam Webster (https://www.merriam-webster.com/dictionary/astr-)

  : star : heavens : outer space : astronomical

Conclusion: Sheldon and Raj are right

Astro definitely means star, hence astronomer, astrophysics,asteroid etc. Asteroid means star-like. Astronaut is a bit grandiose, as it means someone who journeys to the stars, which is something astronauts will never do. They have their work cut out to visit and explore some of the planets, asteroids and moons in our own solar system, and some of those will never be visited. No astronaut will ever land on Venus for example, though it is the nearest planet to us. To continue the star theme further, we also have asterisk, a star-like symbol, and aster, a kind of daisy, I'm sure there are more of them. The word comes from the Latin for star.


visualisation of galaxy density If I had a long stick that was millions of light years long, and with a width of about the Sun, and I then swiped it through the center of the milky way galaxy, what would the probability be of me hitting a star?

EDIT:

Ignore the speed of light. Assume it is possible to just swipe it through instantaneiously.
Assume that the stars in the galaxy are equally distributed. Obviously if you were to swipe right through the middle you would hit the galactic center.
The purpose of this thought experiment is just to help visualise the density vs size of the galaxy.

For your experiment, you can take it 2D, so let's ingnore the thickness of Milky Way.

So, we have 400 billions stars(the real number is between 100B-400B) in a circle with 100k ly(the real number is between 100k-160k).

The area of the circle is pi*50k*50k ~= 7800M sqly = 7.8 billions sqly.

We have then ~51 randomly stars per square ly.

But you do not sting the Milky Way with your stick, but cut through it.

If you swipe through the middle, you have 51*100k stars in a width of 1 ly.

So, you have a chance to hit a star per 1/5.100.000 ly which means 1.855.000km.

But the diameter of sun(of your stick) is 1.395.000km. Taking diameter of sun as medium diameter of a star(this is not true), you have a chance of 1.4/1.8 = 77% to hit a star. If you go with the "low density" numbers, 100B stars and 160k ly diameter, you'll get ~31%.

My conclusion is you often hit a star.

Well Urmmmmm... its a weird question but to put it one way, you wouldn't hit any as our lovely black hole would consume this stick before you could hit any. Apart from that the chance is actually high, there are so many stars in our galaxy(and even more near the centre) that you could hit on. But lets say if you to count the black hole, but it didn't get consumed, it would take so long (because of the black holes gravity) that you would die of old age before you managed to swing down through.

Edit anyone who can improve on my theory!


Warm and Hot dark matter density profiles For cold dark matter, density profiles are well known and easy to find information about - eg. NFW, Burkert, Einasto, and others.

But for some reason I couldn't find explicit expressions for the density profiles for Hot and Warm dark matter.

I need to know what are $\rho_{_{HDM}}(r)$,$\rho_{_{WDM}}(r)$.

I'm interested in the differences between cold/hot/warm dark matter and why do we use Cold dark matter ($\Lambda$-CMD  model) and not Hot or Cold dark matter. I need to back my claims with formal quantitative analysis and not just a qualitative explanation.

If you can recommend about other aspects which differentiate the models, or a good review paper on the subject, I would welcome it.

There are indeed models for halos made of warm dark matter. The profiles will depend on exactly what the dark matter is and what its temperature is. I am no expert, but a starting point could be a paper by Vinas et al. (2012), who claim that the profiles are much flatter in the centre than those of CDM halos.

http://arxiv.org/abs/1202.2860

Hot dark matter is defined as such during the early universe and will be nearly uniformly distributed at the epochs of early structure formation. However at later epochs, hot dark matter with rest mass can cool down. For example, the cosmic neutrino background would have been highly uniform when it decoupled and when  most galaxies formed. But because of the non-zero neutrino rest mass and the expansion of the universe, these neutrinos are now non-relativistic and will be affected by cosmic structures and become "clumpy". The same will be true for any dark matter particle with a rest mass greater than about 0.1 eV.

I think you're going to have a difficult time finding this. The reason being, nobody has really bothered studying non-CDM density profiles because it was pretty clear from the onset that CDM was the (most promising) answer. Why spend time further analyzing a model that looks wrong? I will say though, that there are issues with CDM models (e.g., where are all the "satellite" halos that CDM predicts), and because of that, some people have turned to WDM or a Cold-Warm dark matter model.

But how do we know these non-CDM models are wrong, you may ask. The answer for that comes from N-body simulations. There have been a number of continually impressive simulations over the decades that model the formation of large structure in our universe (see this paper for example). These simulations have included various forms of dark matter in an attempt to discover how different types of dark matter affect the formation of structure and content in our universe. The resounding answer seems to always be that hot dark matter just produces an incompatible universe from what we actually observe in ours. There's no point in thoroughly modeling a hot dark matter density profile when it is clear that the HDM universe is drastically different from our own.

I would suggest that if you want to mathematically compare the various dark matter models, that you look at the power spectra from them. A good place to start might be this paper.


Is there a way to know how the magnetic field was at the very beginning of our universe? I recently read that

  Gamma-ray bursts are catastrophic events, related to the explosion of massive stars 50 times the size of our sun. If you ranked all the explosions in the universe based on their power, gamma-ray bursts would be right behind the Big Bang,

I'm asking this question in the tentative to compare the Big Bang to a gamma-ray burst. I keep in mind that a key difference between both is that Big Bang was Not an explosion, but want to know similarities to point out.

Near the formation of a black-hole they tell in this same article that matter and magnetic field play a key role the gamma-ray bursts.

I know magnetic fields are induced by charge displacement. Is it possible there was a kind magnetic field before protons and electrons formed, or during the first expansion phase?

Here are pictures I modified from Wikipedia's chronology of the universe to illustrate:

Do we have a way to know how magnetic fields were at that time?

Close-up (image borrowed from donsnotes ):

Any way we could have information on magnetism at this point of universe creation?

In the Electroweak Era?
During the Great Unification Theory (GUT) Era?
And then in the Plank Era?

Is there a way to know how the magnetic field was at the very beginning of our universe?

I don't know of one. But then again I think there was no magnetic field at all at the very beginning of the universe. I can't point to any material that proves this beyond a doubt. But I can say that a quiescent black hole "has no hair", so it has no magnetic field, and the evolution of the universe is said to be something like pulling away from a black hole. See Hawking saying the universe is like a time reversed black hole here.

  I recently read that "Gamma-ray bursts are catastrophic events, related to the explosion of massive stars 50 times the size of our sun. If you ranked all the explosions in the universe based on their power, gamma-ray bursts would be right behind the Big Bang". I'm asking this question in the tentative to compare the Big Bang to a gamma-ray burst. I keep in mind that a key difference between both is that Big Bang was Not an explosion, but want to know similarities to point out.

There aren't many similarities. A gamma-ray burst is where some of the infalling matter is directed by magnetic fields generated by infalling matter and converted into gamma rays. The Big Bang is when the space of the universe started expanding.

  Near the formation of a black-hole they tell in this same article that matter and magnetic field play a key role the gamma-ray bursts.

True enough. But as far as I know matter and magnetic fields played no role in the Big Bang.

  I know magnetic fields are induced by charge displacement. Is it possible there was a kind magnetic field before protons and electrons formed, or during the first expansion phase?

It's possible I suppose. A magnetic field is associated with curl aka "rot" which is short for rotor. The early universe could have had an overall magnetic field, or gravitomagnetic field, if it was somehow rotating. See Mark Hadley's Galaxy sized twist in time pulls violating particles back into line. Also see the universe may have been born spinning. But I think it's rather speculative stuff myself.

  Do we have a way to know how magnetic fields were at that time?

I don't think so. The early universe was said to be maelstrom. Hence I think information about the initial state got scrambled.

Magnetic fields play a role in gamma-ray bursts (GRB) because you have a star that is collapsing into a black hole the magnetic fields serve to funnel a small portion of material into two jets coming out of the poles. If the jet happens to be pointing in the direction of the Earth, we see a very bright light source and that's what we call a GRB.

Clearly none of this has any analog in the Big Bang.


What is this rapidly twinkling red, blue, and white star I saw? Last night, I was on my balcony at 1AM (PST) and I looked up and saw two stars near the horizon (I'd guess ~30 degrees above the horizon), and they were "twinkling" about twice as fast as other stars higher in the sky, and I could clearly see them changing from red to white to blue repeatedly. Other stars in the sky only appeared white to me, and didn't seem to "twinkle" as rapidly as these two stars did. The red and blue make me think of red-shift and blue-shift, but I don't know how I would see both from the same object.

What was I seeing?

I don't know if it helps, but I am in the Los Angeles area, and I was looking in a roughly north direction. almost exactly to the east, according to google maps.

Edit: I tried taking a picture, but light pollution from the nearby street lights wouldn't permit me taking a decent picture. However, I noticed a group of three stars close together in nearly a perfect almost vertical line, and managed to find that in Stellarium. I think I found the two stars I am seeing: Procyon and Sirius

Is there anything about either of these stars that would make them show as red/blue?

It's most probably Sirius.  At this time of year (at 1 am local time) it's low in the sky in the East, so there is a lot of atmosphere in the way, and as Sirius is a bright bluish star, it will show all the colours described as it twinkles.

They are not stars I’m from Prince Edward Island Canada those red blue and white looking disco balls in the sky are something the police or military are using. Look on U tube thier are people all over the world seeing the same thing . If u watch those for long enough they will drop every 3 hours approximately. And in most cases especially mine thier is heavy drone activity when these are present. I have really good footage of a lot of this stuff as I bought good camera equipment to gather evidence. If u have these near your house it’s not a good thing as your privacy is being affected from the law or military.


How did matter first come into existence? How did matter come to be?
Why is there matter if matter and antimatter cancel each other?

Actually, I also believe that before anything should have come into existence nothing should have been in existence. So everything should have got created out of nothing as we get +ve numbers and -ve numbers from zero as follows.
0 => + 4 - 4 or 0 => +5 - 3 - 2
Wherein, the +ve numbers represent the matter and the -ve numbers represent antimatter. Of course though splitting of zero can be explained but how we explain it in physics is an unresolved mystery.


How low can the sun be seen in the sky? I'm interested in knowing how low the sun could ever get in the sky and still be seen.

More technically: what's the minimum possible elevation of the sun at which it can still be seen above the visible horizon from a terrestrial vantage point?  And where on Earth would you go to make such an observation?

Some clarification:

elevation is angular distance measured from the astronomical horizon (the great circle that is 90° from the local vertical).

For example, I imagine from a mountain top near the ocean, you might be able to view the sun at an elevation angle that is negative, below the astronomical horizon but above the visible horizon, and possibly by a decent amount.  So then I thought Mt. Everest might be the spot, but locally I'm not sure how low the visible horizon is, because the surrounding area is at a high elevation too.  So maybe I'm just looking for the highest angle possible between mountain peek and sea level, in other words, where is the lowest apparent visible horizon, but the sun may not necessarily ever rise or set in that direction.

I also looked around for a map of local visible horizon angles (measured relative to the astronomical horizon), but couldn't find one.  I suppose such a thing could be constructed from a topographical map of the Earth, right?  Maybe this would be the best way to "solve" this if nobody already knows.

I'm hoping the answer can ultimately be turned into a factoid of the following format (looking to fill in the unknowns in this):

  "From the top of mount something-or-other, looking west, the visible horizon is x° below the astronomical horizon.  From this vantage point, just before sunset, the edge of the sun could be observed to be f(x) sun diameters below eye-level."

The lowest exposed point on land is the dead sea shores, at -413metres below sea level. so theres a good chance you could find a spot that lines up correctly to give you a view of a piece of the horizon well under sea level. I dont believe that elevating yourself is the key factor here though. At Around 35000 ft you can start noticing the curvature of the Earth, the horizon being 200 some odd miles away (Pythagorean theory -theorem?- can be used for this, several sources on google explain the steps to find your equation, and far better than I can. ) but the distance to the horizon grows rapidly. So you would certainly  want to find the sweet spot of height /distance, too high and you wont know when the land has ascended back to sea level, as the terrain will become more vast and harder to judge detail or distance. Too close to the shore and your only seeing a few miles but the angle of sight wouldnt be the best possible for finding the lowest horizon. You would want to work out the math and find a spot that puts the dead sea shores as your visible horizon, bc if your standing there, watching the sun rise, the sun is rising over a horizon that isnt the lowest accessible location...


Can dark energy or dark matter affect black holes I was wondering if black holes are the greatest force of gravity how does other highly spoken about concepts such as dark energy or dark matter compare. Can a black hole absorb dark energy or dark matter? I though dark matter held galaxies together. If a black hole could absorb the dark matter would that be a problem: i.e. the universe getting smaller?

Dark matter can, and probably does, fall into black holes and affects them just as any other form of matter or energy falling into them does. It doesn't fall in a huge amount because, like anything else approaching a black hole it usually isn't aimed straight in, but rather misses a bit and swings by and escapes again. Unlike normal matter, dark matter experiences almost no friction or collisions from other stuff near the black hole (it goes straight through everything, including itself, we think) so it doesn't get caught in the accretion disk or anything. Dark matter falling into the central SMBH doesn't do much to galaxy, because its mass is still there pulling on the stars and so on just as it did before it fell in.

Dark energy is another story. In most of the various competing theories in which it exists, it is an energy inherent in space. Every cubic meter of space contains a certain (small) amount of dark energy, which can't move around or anything. It shows up as something like a pressure, causing space to tend to expand whenever there isn't a larger force (like the gravity of nearby masses) causing it to contract.

Dark matter can certainly fall into a black hole. Nobody is sure what dark matter is, but everyone agrees that its gravitational effects on galaxies prove its existence. One popular but very speculative idea is that it is made of Weakly Interacting Massive Particles (WIMPS) which hardly ever interact with normal matter, but so far particle physicists have never detected or isolated a WIMP. One thing that everybody accepts about them is that they interact gravitationally. That being the case, they would have no option but to be drawn into a black hole if they were very close to it. As, like neutrinos, they seldom interact with matter and there would be no friction or compression resulting from them, they would not brighten the accretion disc, but when they entered the black hole they would increase its mass.

As for dark energy, not everyone is convinced of its existence. It is said to make the space between the galaxies expand faster than was the case very long ago, but no one knows how it does this or what kind of particle could carry this energy. Further data may yet indicate that dark energy is an illusion, but as things stand it is thought that this speeding up of the expansion will prevent gravity eventually bringing the expansion to a halt and starting a collapse, so he universe will go on expanding for ever..


What happens to the information on the event horizons of two merging black holes? What happens to the hairiness/information on the horizons of two black holes if they collide?

After Hawkin the information of the matter which has fallen into the blackhole is encoded on the surface of the event horizon. What happens to the information of both blackholes when the event horizons merge?

How black holes encode or destroy information is an open question, which RichS touched on.  However, while he provided an answer consistent with the No Hair Theorem, I will provide an answer derived from the holographic principle.  I'd like to stress that both are equally valid, since we (as of yet) do not know enough about black holes.

In terms of the holographic principle, the information about the matter that formed the black hole is encoded in some manner (micro gravitational fluctuations maybe?) on the two-dimensional 'surface' of the event horizon.  It has been theorized that one could reconstruct this information by measuring the outgoing Hawking radiation, since this process decreases the radius of the black hole, hence the surface area of the event horizon, and subsequently the amount of information.  I would liken this to burning a book and then trying to reconstruct the book by measuring the properties of the ashes and radiated light.

When two black holes merge, they form a black hole of smaller mass than the combined masses.  Once more the total surface area of the event horizons has decreased to that of the event horizon of the new black hole, so information must have been 'radiated'.  Black hole-black hole mergers are not believed to have any optical counterpart (burst of light for sake of simplicity) and the merging process is derived from relativity, so Hawking radiation is not a component of consideration.  Where could the information have gone?  Well, luckily enough these mergers do radiate gravitational waves, now proven by LIGO's recent discovery.  Thus the information, if it does in fact encode itself on the event horizon surfaces, could be radiated/lost by the gravitational waves created during the merger.

EDIT: The above description sounds very 'hand-wavey', so I will expand on proposed theoretical method behind it.

Gravitational radiation is generated by the changing quadrupole moment, caused by the two in-spiraling black holes.  However, just as for the case of electromagnetic radiation emitted by oscillating charges, the quadrupole moment contribution is only one part of the greater multipole expansion of the oscillating masses.  Oscillations of the the event horizon would then cause deviations of the system from the simple quadrupole approximation, and result in gravitational radiation from the high-order multipole terms.  This radiation falls off with distance faster as one moves to higher-orders, making measurement of these contributions much more difficult.

Of course this is just one proposed solution to the black hole information paradox.

I'm not a scientist, just someone who likes to read and think about this stuff. I think mass would be transferred.

My reasoning is as follows:

Black holes aren't holes, they are masses. Gravity comes from mass. Each black hole has immense gravitational pull as they likely have incredibly dense masses in them.
However, they do not have equal gravity. So what happens when two masses with enormous gravity pull on each other? The stronger pull wins.
They aren't static either. Black holes do grow as they pull in more mass, so that implies that they can grow and so can probably shrink.
I don't think black holes are inescapable. Light cannot escape but light has a constant speed while the force of gravity is theoretically infinite. Think about it, they grow instead of becoming an infinitely dense singular point. That means that mass resists the pull to further contract into a denser ball. So, if a black hole's gravity can be resisted, why not from a bigger gravitational pull pulled matter away from a smaller gravitational pull?
There's no such thing as empty space. We are slowly realizing that empty space is likely filled with dark matter and other hard to detect substances. So the space that appears black isn't empty. So your scenario is still mass acting upon mass.
I believe it has been shown that black holes have merged, so that process likely started when two event horizons collided.

Again, I'm no scientist, but it seems quite likely that mass is exchanged.


Einstein General relativity and bending of space As Einstein's General relativity suggests that the space
time curves in the presence of matter, which is experimentally
proven too, this phenomenon also explains orbits. My question
is if the presence of mass curves space-time, in which direction
does it bend, if we assume that it bends in one of all possible
directions then what makes the body to choose that particular
direction. Does all the bodies bend the space-time in the same
direction ?

We cannot say that it bends in a particular direction because space-time in relativity is 4-dimensional. The space-time diagram which we see frequently is just a simple way of explaining it in two dimensions because 4-dimensional space is hard to visualize.


How can a black hole ever form? We know that an object falling to the black hole, as observed from an observer far away, will freeze as it gets closer and closer to the event horizon. So my question is how can a black hole form in the first place? I mean we usually say thing a star collapses into a blackhole. But considering the effect of "time-freezing", how could that be possible in the first place?

After a supernovae explosion from stars whose mass is more than the Chandrasekhar limit, the remaining neutron star is so dense that it collapses into a one single singularity( A point where the density in infinite and is compressed into a point).the only way we can detect a black hole is by the entropy given by the black hole. so at the point of formation the black hole is not taking any matter into it.After the black hole is formed we will see any object going into it as frozen in spacetime


How well would the Moon protect the Earth from an Asteroid? Would the Earth fare better if the Moon blocked the meteor, comet, rogue planet, or otherwise rather than a direct impact? At what point would the Moon's debris would be an extinction event?

The limit of the impactor is the size, angle of attack, or composition in which to moon would no longer be of any protection.

It would be much better for Earth if the impactor hit the moon...

In this Worldbuilding answer, I used a paper on ejecta kinematics to do calculations for ejecta velocity upon impact. Without going into too much detail here, much of the ejecta from a large impactor would not exceed the moon's escape velocity of 2.38 km/s. You can examine Figure 7 from the linked paper which shows the logarithmic relationship between ejecta velocity and the edge of the crater. Only material within a few tens of meters of the edge of the formed crater can attain the velocity needed to escape the Moon's gravity.

The mass of the asteroid increases with the cube of its radius; while the mass of material within $n$ meters of the edge of the impact crater increases with radius; so it is evident that the the larger the meteor itself gets, the lower the danger of the any potential ejecta relative to the original impactor.

Furthermore, material ejected from the Moon has the potential to land back on the Moon, enter a stable orbit of Earth, or be ejected from the Earth-Moon system; so only a (probably small) fraction of Moon ejecta would threaten Earth.

Overall, fewer, smaller rocks are much better than large rocks when it comes to getting hit by things (except maybe for your car's windshield).

...unless the impactor knocked the Moon into Earth.

Of course, there is always the possibility that a very, very large impactor would knock the moon into a different orbit, potentially one that eventually impacts Earth. That would, obviously, be the worst case scenario. Significant changes to the orbit of the moon, even if these changes do not cause a collision with Earth, have the potential to do significant damage in terms of tides; not just ocean tides but also the poorly understood effect of tidal forces on Earth's mantle.

In any case; it is probably much better for a large impactor to hit the Moon instead of the Earth, but possibly much, much worse.

A meteor is a small, sand grain or pea-sized fragment which burns up in the atmosphere before hitting the ground. Those large enough to reach the ground ae called meteorites. It seems to me the ones you are talking about are large enough to be called asteroids. The moon affords hardly any protection against asteroids, and those that strike the moon do very occasionally cause tektites (small glassy beads) and small pieces of debris to hit the Earth. The next asteroid on a collision course for Earth is most unlikely to be blocked by the moon, but there's no need to worry. Chelyabinsk or Tunguska events are fairly common and occur several times a century, usually striking the sea, but the really big ones like Manicouagan or Chicxulub occur on average about once every 150 - 200 million years. We will probably have already become extinct by the time the next one hits.


How thick was Earth's primordial atmosphere? Earth's first atmosphere was made of hydrogen and helium accumulated before fusion started in the Sun. As soon as this happened it was stripped away by the Sun. How many atm of hydrogen and helium had Earth accumulated at the peak pressure of the first atmosphere?

We have no way of knowing how much hydrogen and helium was in the Earth's primordial atmosphere. What is more important is what the atmosphere consisted of when the primeval soup from which life arose was formed, just over 4 billion years ago. We have a fairly good idea what this atmosphere consisted of: mainly carbon dioxide and nitrogen, rather like Mars at the present time, but with substantial amounts of ammonia and methane. Also very small amounts of hydrogen and helium, most of which had by then been blown away by solar radiation. There was hardly any oxygen. As for pressure, all we can say for sure is that on Earth it was a good deal higher than it is today. Conditions at that time were similar, though not identical, on Venus, Earth and Mars, perhaps on Titan as well, so it is possible that all three terrestrial planets developed a version of the Earth's primeval soup, which was essential for the creation of life. Whether life emerged on any planet other than Earth, we don't yet know. Because all three planets had oceans, water vapour would also have been an important atmospheric constituent.


What was the composition of the primordial asteroid belt? According to one theory, the region between Mars and Jupiter was once populated by several primordial planets with a combined mass of roughly the earth.  These planets provided the material that made up the original asteroid belt.  Since then, 99.9% of this material was lost.  The current composition of the asteroid belt is about 10% metallic and 90% silicate/carbonaceous.  Could we assume that the original asteroid belt had the same composition?  Or, at least a very similar composition?

We can be pretty sure that the asteroid belt once held one or more sizable planetesimals,as there is evidence that it or they were large enough for gravity & internal heating to drag the heavy metals (mainly iron & nickel) to the centre,forming a metallic core. Bodes Law decrees that there should be a planet where the asteroid belt is now,but it seems that as this planet was forming it was shattered by an impact & the remnants scattered. Some reached Earth,which is how we know that their materials had differentiated into metallic core & rocky mantle. It is likely that some of the huge asteroids which have struck the Earth in the distant past were fragments thrown out of the asteroid belt & that some hit Mars & Venus as well. There may be more material in the asteroid belt than we think,because some fragments are so small & widely dispersed that they don't register in our telescopes. I think we can assume that the primordial planetesimals had a similar though not identical composition to the asteroid belt of today.


Attach a visible light telescope to the outside of the ISS Visible light imaging of the heavens is limited on Earth primarily due to the atmosphere. I know the ISS is moving at 17000mph, but given the delays to the James Webb telescope, is there any merit in attaching a space-rugged visible light telescope to the outside of the ISS? Could it ever approach the visible light performance of Hubble?

A self-sufficient orbiting telescope is basically Hubble mkII and would never get off the ground, literally and metaphorically

Hubble was expensive because it was state-of-the-art, requiring development of many new systems. The systems it needed to function as a standalone satellite (compared to being attached to the ISS) were cheap by comparison (reaction control, power, communications) because they mostly didn't require new development.

So a Hubble successor attached to the ISS would not be much cheaper than a free-flying one.

Any telescope attached to the ISS has to deal with:

high vibration levels which degrade performance
outgassing which degrades performance
severe pointing constraints because it's surrounded by a large structure
bandwidth constraints because it has to share a transmitter with ISS operations
end of mission in 5 years when the station reaches the end of its life.

The advantages of attaching to the ISS:

easier servicing,
you get the option to store lots of data on the ISS and bring it down on a capsule. Downside is a latency of several months.

IMO you'd be better off building a standalone space telescope.

I think there is nothing wrong with your idea in principle, there is already a modified Boeing jumbo jet specifically for the purpose of carrying a large telescope above the densest layers of the atmosphere in order to minimise atmospheric interference and absorption by atmospheric water vapor.

However, the telescope and the personnel who man it use up a considerable amount of space, though  on the aircraft there are no competing activities going on beyond those necessary to fly the aircraft. On the ISS the main problems would be in finding the space to accommodate the telescope and its crew and making the necessary modifications to the fabric of the space station.

I don't think such a telescope could compete with Hubble, but that doesn't mean it couldn't do anything useful.

One advantage would be that if anything should go wrong,it could be somewhat easier to repair than it is with Hubble. Now that the Space Shuttle is no longer available, a crewed mission to Hubble would be extremely challenging and a robotic mission would require technology not yet available.


Why (the heck) is the basic angle of GAIA 106.5°? This answer to Why does the Gaia space telescope have two main mirrors says:

According to the GAIA FAQs which does an excellent job: http://www.cosmos.esa.int/web/gaia/faqs:

Why is there an angle of 106.5 degrees between Gaia's 2 telescopes?
The choice of the so-called basic angle of GAIA was a non-trivial one. On the one hand, it should be of order 90 degrees to allow simultaneous measurements of stars separated by large angles on the sky. On the other hand, it should not be a harmonic ratio of a 360-degree circle (e.g., 60 deg, 90 deg, or 120 deg). Taking these considerations into account, acceptable ranges for the basic angle are 96.8 +/- 0.1 deg, 99.4 +/- 0.1 deg, 100.5 +/- 0.1 deg, 105.3 +/- 0.1 deg, 106.5 +/- 0.1 deg, 109.3 +/- 0.1 deg, 109.9 +/- 0.1 deg, etc. Accommodation aspects identified during industrial studies subsequently favoured 106.5 deg as the value finally adopted for Gaia.

I don't understand any of that.
Questions:

"On the one hand, it should be of order 90 degrees to allow simultaneous measurements of stars separated by large angles on the sky." Well any large angle is a large angle, why should it be 90?
"should not be a harmonic ratio of a 360-degree circle" I can imagine for instrumental reasons, but why are "acceptable ranges for the basic angle are 96.8 +/- 0.1 deg, 99.4 +/- 0.1 deg, 100.5 +/- 0.1 deg, 105.3 +/- 0.1 deg, 106.5 +/- 0.1 deg, 109.3 +/- 0.1 deg, 109.9 +/- 0.1 deg, etc." and what does "etc." mean here? Is there a pattern I'm not seeing?

Any angle with an uncertainty of +/- 0.1 deg can be reached with some rational number with a denominator of 1800 or less, that's as far as I've gotten.

I did not find anything conclusive on the web, and I am yet waiting to receive feedback from ESA's contact page. In a German speaking IT forum there is something a bit less vague than the FAQ cited in the question:

The reason for the 106.5 degrees is not easy to see. It has to do with the fact that we are (also) doing an astrometric solution on a great circle and for this all angles 360x(n/m) [n, m = small integers] have to be avoided because otherwise one would have a congruence state that would hinder the solution. Values other than 106.5 degrees would also be possible. 90 degrees would be ideal for parallax determination, but unfortunately 90=360x(1/4).

This said, why not simply choosing 91° as $\gcd(360,91)=1$ (with $\gcd$ being the greatest common divisor)?
Maybe it is really only a numerology of the engineers, just like the Mars parachute Morse message. An argument for that would be that Gaia also has also exactly 106 CCD sensors. Or it is simply a logistical issue on how to mount things on the spacecraft. I am really curious what the official ESA statement will be.

1/ 90 degrees gives the most precise angle for triangulations on a 2D Cartesian plane.  You can try it out by programming 3D mesh and by glancing sideways travelling on a bike or on a satellite: glancing at 90'C, you have the highest measurement of the movement of X relative to Y compared to looking at 45'. If you have binoculars on the bike helmet (i.e. a narrow field of vision) you will find that 90 degrees views gives the best relative difference.
2/ The second statement is wrong: On the other hand, it should not be a harmonic ratio of a 360-degree circle (e.g., 60 deg, 90 deg, or 120 deg). Taking these considerations into account, acceptable ranges for the basic angle are 96.8 +/- 0.1 deg, 99.4 +/- 0.1 deg, 100.5 +/- 0.1 deg, 105.3 +/- 0.1 deg, 106.5 +/- 0.1 deg, 109.3 +/- 0.1 deg, 109.9 +/- 0.1 deg, etc. . The suggested values don't impart notable harmonic ratios in the same way that phi rotations give the best coverage of light for a plant:

The CCD time is set to 4.4 seconds and can be changed to 4.47, rendering angular resonance NEARLY TRIVIAL.
The rotation time is 1 degrees every minute, one rotation every 6 hours (4009 photographs at 4.4 seconds apiece), which can be changed to i.e. 6:20.
GAIA has already rotated sideways by ~1 degrees every time the cameras have scanned round once, and the aperture of the CCD is 1.7 x 0.6 degrees, The resonance shifts mostly depend on those figures.
The sweep has to also keep the solar panels towards the sun and the sun away from the CCDs and to provide a very constant thermal environment. some very decisive constraints compared to rotation resonance.

Here's a video of the complexity of the supposed "360 degrees".

https://www.gaia.ac.uk/science/parallax/scan


Do we know the exact spot where big bang took place? If you rewind the universe back 14,7 billion years, all matter were in one spot, and then started expanding. Do we know where this is in reference to our own solar system? And is there anything there? Or are all matter in the universe just blown inn all different directions and none is left at the "original" spot where it originated

You're envisioning the Big Bang as a cosmological "explosion" in space where the universe is the resulting material expanding in outward all directions.

The problem is, the universe doesn't work like that.

The universe isn't a region of space that is expanding outward into another thing where you can (even hypothetically) fly out to the border and say, "Yup, that's about all there is. Let's use these borders to find the center of this expanding sphere."

The Universe is literally everything there is: space, time, and everything it contains and ever will contain. And the word contains is even a bit of a misnomer in that it suggests a container with boundaries. But the universe is simultaneously both unbounded and finite at the same time. "Finite" refers to the fixed amount of stuff this universe contains (matter, energy, etc)… and unbounded because this thing we know as "the universe" has no borders in a sense that we point to or even experience. So how would you even define a "center"? There isn't one.

There's a famous way to help visualize an expanding universe that is both finite and unbounded without a center or borders; it's called "The Balloon Analogy." Imagine our friends in Flatland living in two-dimensional space… where everything they know (their entire universe) exists on the surface of a balloon. If you start to inflate that balloon (the expanding universe),  all the little astronomers on the surface will observe that all the surrounding galaxies are moving away from them — and the farther away those galaxies are, the faster they seem to be moving away.

That's pretty much what most cosmologists believe is happening to us.

Going back to that balloon analogy, picture yourself existing on the surface of that balloon-universe and ask these questions again — In my expanding universe (the surface of the balloon), where is it's center? Everything is expanding outward in all directions, so if we rewind back in time, certainly everything should converge somewhere, right? Can we just find the borders of this outward expansion and calculate the exact center? So basdically… "Do we know the exact spot where big bang took place?"

Be careful taking the balloon analogy too far, because it starts to break down in many ways. But the answer to where the expansion of the universe emanates from really is that it emanates from everywhere; everything we know is contained within the origin of our own existence, and there is no center and there are no borders by any definition of existence that we can experience.

I have pondered this issue for nearly 35 years. If the universe came into being by way of a Big Bang process we will probably never find the exact center at which it commenced:
REASONS BB CENTER MAY NEVER BE FOUND
1. We should always keep this in mind as our starting point: We did not see the beginning of our universe take place. There are no eyewitnesses who can tell us what actually happened. Therefore, it is a 13.7+ billions of years old COLD case. In other words everything we put forward regarding the beginning of the universe will always be only speculation.
We have no way to prove anything that we hypothesize regarding how our universe commenced (no matter how many theories are proposed regarding the beginning of the universe nor how good they and their corresponding math equations appear to be, there is no way to fully test them to prove out anything they make claims for).
In other words, even if the numbers do not add up perfectly you can always come up with another constant or sub-theory that makes it look more correct, none of which is provable in our real world predicament. This means it may be impossible to determine if the BB took place or if a center ever existed at all.
2. That said, even if the Big Bang is responsible for how the universe expanded to its current state, problems or barriers may prevent us from locating the exact location of the center from which it all started.
3. The starting point, regarding the BB, was supposedly a state of singularity. What singularity actually would be is not really known. However, its state would be a point about the size of a marble in which presumably all the laws of physics as we know them are broken down, a kind of state of total annihilation of all atomic structure and its various particles. If the supposed early "super" inflation phase occurred it would be opaque because photons would not be present nor released during this phase. Therefore, our view of events up to this point would be obscured.
In other words, we can never see through this phase nor would there be any way to see this phase of inflation because we would need to see photons. Only where photons are released are we able to see even part of what was there (the so-called visible universe limit).
4. Because we can not see beyond the visible universe (which would include the parts of the electromagnetic spectrum that can not be seen by the naked eye, that can only be seen through equipment that can detect invisible wavelengths) we would have almost no chance of ever finding the exact center of the BB.
5. According to one theory space exists and inflates separate from matter and energy. That is we are located within and are a part of this inflation. According to this proposal space (fabric of spacetime) is inflating in all directions all around us, that we are within the spot. The result being that we can not detect a direction in which the BB started from. Supposedly, because we are located within the BB inflation, this precludes us from having a frame of reference that would allow us to have the ability to trace back and locate the exact center or even the general location of the BB starting point.
6. The real world frame of reference for our universe is three dimensional plus time. The visible universe matter is not uniform in all directions. That is all the galaxies, etc. are not uniformly spaced along its x, y & z axis (unlike the two dimensional demonstration models with evenly spaced matter). Since, supposedly the Big Bang started with a very tiny singularity blob (at least for this theory), smaller than a marble, then there ought to be various vector trails (even if they are fragmented or somewhat askew, blurred or dissipated) of some sort going back to the marble size singularity state or at least into the outward edge area of the 'super' iflation phase (especially since matter is not uniformly spaced and would have to change direction and consistancy in order for large areas or gaps at irregular intervals to be present now). However, so far we have not been able to locate any evidence that would suggest that the BB came from a particular direction which has also precluded us from locating a BB center.
7. Unfortunately, two dimensional demonstation models lack ability to show true three dimensional effect regarding starting at a tiny point through the present state of inflation or expansion of 13.7 billion light years. In other words, if we look across through our universe sphere (matter does not exist evenly spaced on two dimensional planes here) somewhere at least near part of the edge of our visible universe we should see at least some large area where galaxies are much closer together compared to say an eight or ten billion light years area around where our galaxy is located now. So far we have not found any evidence. Perhaps this means the BB did not occur.
Certainly if the universe had a tiny starting point there should be a visible change somewhere in an inflation or expansion that covers such a huge area of more than 13.7 billion light years (there would be some indication in which direction we should concentrate our effort to find something regarding at least the general direction of the BB's original location). So far such indication has not been found.
8. Also, there may be a force(s) outside our universe that is the driving force for the inflation or expansion of our universe, in which case it would most likely make it impossible to ever find evidence of a center if one ever existed.


Why does gravity make time go slower and warp light? I know that objects with lots of gravity are capable of warping light or slowing time (an example being a black hole). I also know this is because of the theory of relativity, but I need someone to expand on that and explain why/how that works.

Also, please write the answer in somewhat simple terms, as I am not (yet) an expert on this stuff :)

Sometimes things we think of as different are actually just different forms of the same thing. Electric fields and magnetic fields are forms of each other. That is why we now call it the electromagnetic force.

Likewise, energy and mass are also just different forms of the same thing. We can even convert it too.
E = mc^2 ...

And from what I understand, space and time are connected together as well. So it gets called spacetime. If you want to think about it that way, gravity works by warping space, like a heavy marble sitting on a fluffy blanket. The fluffy blanket is spacetime. The heavy marble is a black hole or other super massive object. So now light is still going "straight" along space (the fluffy blanket) but it is bent because the blanket is bent. (Caveat: this is where i am conceptualizing light as a wave, not photon packets. It is actually both.) And if gravity bends space, of course it bends time too, because space-time is connected somehow as the same substance.


How far can you see in solar plasma at just under 1 solar radius? I was trying to compose an answer to this question here, about touching the surface of a star, and I was going to mention that the popular depiction of the sun's surface as 'basically lava' (as depicted in Sunshine, for example) is wrong.

The usual definition of a star's radius is where the optical depth is 2/3. At this radius, the density of the sun is about 1/10th of air at sea level and can hardly be described as solid or liquid.

I wanted to get a sense of what this diffuse plasma would look like if you were in it and somehow impervious to the heat and radiation. Would it appear as a thin, glowing fog, through which you could still discern distant objects? Or would it appear as opaque as, say, a bonfire?

Rephrasing in a little more technical way, What is the extinction coefficient of solar plasma at ~1 Solar radius in the visible spectrum?
What is the path length at which all light is effectively attenuated?

(Since it varies by wavelength, I mean specifically the region of the visible spectrum where the plasma is at its most transparent).

If you have a look at the top-left panel of Fig.11 in these lecture notes by Rob Rutten, you will see that the continuum opacity at optical wavelengths at the photosphere is about $10^{-6.7}$ cm$^{-1}$.

The inverse of this is the optical depth. You can see stuff through about 2-3 optical depths, so your "horizon", looking horizontally in the solar atmosphere is about 100 km.

However, the same plot shows that, because of the temperature and density variations with height, the photosphere becomes an order of magnitude more or less opaque at only 50km below or above the "photosphere" respectively.

What this means is that you can see about 100 km horizontally, but less than this (guesstimate 30km) looking downwards and more than this (guesstimate infinity - that is how the photosphere is defined!) looking upwards.

These numbers would all be revised downwards at the wavelengths of prominent absorption lines.

If you could look all around you, you would see a very asymmetric brightness/colour distribution. Overhead would be dimmer and cooler (redder); underneath, much brighter and hotter (more or less the same colour we see the photosphere).

I suspect part of the problem here is that cosmological optical depth is defined vertically (so far as I can discern from the wikipedia article), while you seem to be asking for the extinction coefficient for a horizontal path at the specified star-radius.  So you'll need to find out what the $\alpha(z)$ is at that radius, play some games with integrated radiance due to the plasma in the horizontal line of site to estimate the noise factor, and compare that with the putative radiance from the distant object you wish to see.