View from inside the black hole If an observer is falling toward a black hole with its face away from singularity then what will he observe after crossing the event horizon? The reason that why I am asking this question because as far as I know for an outside observer, the falling observer appear to freeze at event horizon i.e. time appear to stop for falling observer. So if the falling observer is able to look outward after crossing event horizon then he will be able to see an infinite amount of time which is impossible. So what will the observer see after crossing the event horizon?

Your premise is incorrect:

The external observer's view does not imply anything about an observer within the event horizon.

The event horizon itself is just an indicator of the edge of the region where light can no longer escape. Light can head inwards as you would expect.

So an observer inside the event horizon will be able to see light reaching them as normal (well, not exactly as normal, as there will be lensing effects) - there will not be an infinite amount of time visible.

If an observer is falling toward a black hole with his face away from singularity then what will he observe after crossing the event horizon?

Nothing.

  The reason that why I am asking this question because as far as I know for an outside observer, the falling observer appears to freeze at the event horizon i.e. time appears to stop for the falling observer.

Correct. Gravitational time dilation goes infinite, and the distant observer says the "coordinate" speed of light at the event horizon is zero.

  So if the falling observer is able to look outward after crossing the event horizon then he will be able to see an infinite amount of time which is impossible. So what will the observer see after crossing the event horizon?

Nothing. The coordinate speed of light is zero at that location, which means that by our clocks, it takes forever to see anything. So the falling observer hasn't seen anything yet, and he never ever will.

IMHO it's worth reading the mathspages Formation and Growth of Black Holes and paying attention to the frozen star interpretation:

"Incidentally, we should perhaps qualify our dismissal of the 'frozen star'  interpretation, because it does (arguably) give a servicable account of phenomena outside the event horizon, at least for an eternal static configuration. Historically the two most common conceptual models for general relativity have been the "geometric interpretation" (as originally conceived by Einstein) and the "field interpretation" (patterned after the quantum field theories of the other fundamental interactions). These two views are operationally equivalent outside event horizons, but they tend to lead to different conceptions of the limit of gravitational collapse. According to the field interpretation, a clock runs increasingly slowly as it approaches the event horizon (due to the strength of the field), and the natural "limit" of this process is that the clock asymptotically approaches "full stop" (i.e., running at a rate of zero). It continues to exist for the rest of time, but it's "frozen" due to the strength of the gravitational field. Within this conceptual framework there's nothing more to be said about the clock's existence..."

The author doesn't favour it, but it squares with what Einstein said about the speed of light varying with gravitational potential. The other interpretation doesn't. And note that Einstein didn't refer to the "coordinate" speed of light. He simply referred to the speed of light. So we can reasonably say that at the event horizon, the speed of light is zero, and that this is why a vertical light beam can't get out. The distant observer sees the infalling observer freeze at the event horizon. But the infalling observer doesn't see himself as frozen, because the speed of light at that location is zero. He sees nothing.

NB: SR time dilation is symmetrical, but GR time dilation isn't. If you and I passed each other in gravity-free space at some relativistic speed, we would each claim that the other's clock was slower. But when we're at different elevations, we both agree that the lower clock is going slower.


How likely are planets to form after neutron star collisions? It is well known that planetary collisions can create moons orbiting the result of the merger if they happen in the correct way, and this is how the Earth's moon is believed to have been formed. See the animations on this Durham University page to get an idea of how the mechanism works http://icc.dur.ac.uk/giant_impacts/.
It seems to me that it should be at least theoretically possible for the same process to happen when neutron stars collide, which would produce bizarre extremely-high-metallicity (or rather high-average-atomic-mass) planets. However, I also know that the physics is very different in some ways: the colliding objects are much denser; the collision is much higher-energy; radioactive decay creates a burst of extra energy from any matter thrown off the objects; the gravity and velocity are high enough that relativity matters a lot; they probably are in very circular orbits spiraling toward each other rather than hitting each other from the angles that protoplanets do; etc.
It's also possible that most of the mass of the neutron star might be thrown away and leave a low-mass remnant that might expand into an high-atomic-weight planet or white-dwarf, or that some bit of ejected matter might be thrown out at similar enough velocities (speed AND direction) to eventually coalesce into a rogue planet.
I'm just wondering whether anyone has looked into this before, or if anyone has any input as to whether this would be more or less likely than moons forming from planetary collisions, or if anyone knows how to test this with simulations.
EDIT: I've just realized the reason why it is probably impossible for a planet to form in the same way the Moon formed around the Earth: The outward force is way stronger than gravity except for close objects, which would be inside the Roche limit of the resultant black-hole or neutron-star and thus form an accretion disc or ring rather than a planet (due to the fact that any potential planet would be ripped apart by tidal forces). I haven't done any math on this, and this is just my impression, though. In addition, this doesn't mean a planet couldn't form from the ejecta in other ways; for instance, the disc of matter close enough to be held in orbit after the initial explosion might be pushed out to include a planet-forming region outside the Roche limit during a later phase of the event.
EDIT 2: I've had an idea for how this might happen, but I think this might really be a different question. The idea is that, if another star was in the same system as a kilonova (collision between stellar remnants that ejects matter and radiation), the kilonova might leave enough of the star to stay in the system, or perhaps leave enough matter for the other star to capture it somehow. One thing about this scenario, though, is that the idea of another star being in the same system as a compact binary merger rather implies that this third star has already been hit by at least one supernova, possibly multiple and maybe several novae, depending on whether a parasitic binary was formed. (This wouldn't apply if the third star was captured into the system after both of the other stars had already died, though.) Supernovae are stronger than kilonovae in terms of energy that gets thrown out, so the previous supernovae would already have had a stronger affect on the object. I believe that kilonovae are thought to produce heavier elements than any type of supernova, so stars hit by kilonovae would be different in composition than ones hit by supernovae, but it's still basically the same question: What kind of remnants can survive from stars hit by supernovae/kilonovae/novae at close range. I think it's pretty obvious that this could form some kind of remnant, possibly depending on the distance to the third star, so that already answers my question, though I don't know what compositions are possible or what masses are likely, but I think this is really a different question that should probably be asked separately if I or anyone else want it answered on Stack Exchange.

There do appear to have been some studies on the properties of potential fallback discs formed after neutron star mergers, for example:

Rosswog (2007) "Fallback accretion in the aftermath of a compact binary merger
"
Cannizzo et al. (2011) "Fall-back Disks in Long and Short Gamma-Ray Bursts
"
Lyutikov (2013) "The Electromagnetic Model of Short GRBs, the Nature of Prompt Tails, Supernova-less Long GRBs, and Highly Efficient Episodic Accretion
"

These studies focus on explaining X-ray flaring in the aftermath of gamma-ray bursts rather than the potential to form exotic planets in these environments. It does seem fairly likely that some kind of disc does form around the remnant of a neutron star merger, but it's going to be extremely hot and likely so close to the remnant that it will be unable to form planets.
As noted in Menou et al. (2001) "Stability and Evolution of Supernova Fallback Disks", planet formation in fallback discs depends on the timescales for the disc cooling and how long it takes to spread beyond the Roche limit: if the disc becomes neutral before it spreads beyond the Roche limit, spreading becomes reliant on interactions within the remaining disc of rocks. While they consider the case of merging white dwarfs (noting that this scenario leads to a more favourable environment for planets than post-supernova fallback discs around black holes or neutron stars), they do not study the case of merging neutron stars.

which would produce bizarre extremely-high-metallicity planets

My, fifteen cents: "neutron star" called so, because it consist with barely neutrons, not metals, like ferrum/iron which is core of The Earth.
10km in diameter, heavier than The Sun million times. It is very heavy bunch of neutrons on square like Moscow City.
The borders between atoms wiped out, and the whole star - like one big atom, with "tridizillion" of neutrons, each star may take 10^google*x - place in periodic table of Mendeleev.
Probably, even if you can by collide two of such bodies - extract part of its material in to the common orbit, it would not be metals, definitely not an Iron - which is product of burning of primary stars... It would be pure neutrons.. "Neutron Star".
By the way, black holes - are the results of such collides...


Can stars be seen in the background when looking directly at our solar system? Imagine viewing the solar system from the outside:

Considering the sun is quite bright, would you still be able to see stars in the background, or does the sunlight disable us to do so?

Consider this photo:

Does the light from the sun disable us from seeing stars in the background or have they simply been removed in an editor after the fact of being taken (the photo)?

Obviously there needs be some way of seeing stars from space, since looking at photos from space stations (or even from Earth) those are full of stars.

But, viewing the solar system from above (from the outside), can you see stars in the background or does the sunlight simply block them out?

Stars are much less bright than the earth in full sunlight as per your picture. So with the exposure required to give that picture of earth, no, you won't see stars.

But move the camera a bit to the side, so earth is not in shot and you can see stars.

The same goes for the sun - make sure sunlight is not entering the lens, and you can easily see stars.

Sunlight doesn't block any light out (unless you want to see a star that is actually behind the sun) it's just so bright that you need to make sure it isn't in shot.

Considering the sun is quite bright, would you still be able to see stars in the background, or does the sunlight disable us to do so?

the sun is way too bright and the reflection of the sun with black out the stars.

  Does the light from the sun disable us from seeing stars in the background or have they simply been removed in an editor after the fact of being taken (the photo)?

you would have to edit the photo with more bright features to bring out the stars.

  But, viewing the solar system from above (from the outside), can you see stars in the background or does the sunlight simply block them out?

yes they will block out.


How to lose the moon? In this worldbuilding question, the moon is gone:

  an extra-solar gas giant passed close by Earth in which the Moon was
  knocked from orbit to become a planet in an independent orbit, as well
  as sending Mercury crashing into its parent star.

So by introducing a rogue planetoid, the orbit of both the Moon and Mercury needs to be affected, with at least the former surviving the encounter.

Is this possible? If so, what are the requirements (mass, velocity, proximity)?  Would it be possible for it to happen and affect the Earth minimally, so that it remains habital?

The Earth and moon orbit around a barycentre that is "about 3/4 the radius of Earth" away from the Earth's centre: https://en.wikipedia.org/wiki/Orbit_of_the_Moon. Removing the moon would almost certainly affect the Earth's orbit around the sun due to the change in the centre of mass, and probably a resultant change in momentum of the Earth system (such as it was). That's without even considering the gravitational tug of the planetoid on the Earth, and that of the moon as it gets snatched away. I would think it spectacularly unlikely for the interaction to leave Earth travelling along the orbit of the centre of mass of the old Earth-moon system. But, I haven't actually modelled it, so I might be wrong there.


Can a star fall in a super massive black hole without getting destroyed? This is the first of many similar questions I have to build up to a specific question or scenario that I want to explore and find an answer to.

Yes, it is easily possible, but it depends critically on how massive the black hole is. A simple calculation will suffice.

If we take a Newtonian approximation for the tidal acceleration across a star of radius $R_*$ as it reaches the event horizon
$$ a_{\rm tidal} \sim 2\frac{GM_{\rm BH}R_*}{r_s^3}\ ,$$
where $r_s = 2GM_{\rm BH}/c^2$ is the Schwarzschild radius.

The gravitational acceleration at the surface of the star, due to its own mass is just
$$a_* = \frac{GM_*}{R_*^2}\ .$$

If we take the ratio of the two, we can assume the star will survive being ripped up before it crosses the event horizon if this ratio is less than $\sim 1$. i.e. when
$$\frac{a_{\rm tidal}}{a_*}= \frac{c^6 R_*^3}{4G^3 M_{\rm BH}^2 M_*} <1$$

The tidal acceleration reduces markedly for high mass black holes. We can rearrange this inequality to give us the minimum black hole mass for survival:
$$ M_{\rm BH} > \left(\frac{c^6 R_*^3}{4G^3 M_*}\right)^{0.5} = 1.5\times 10^{8} \left(\frac{M_*}{M_{\odot}}\right)^{-1/2} \left(\frac{R_*}{R_{\odot}}\right)^{3/2}\ M_{\odot}$$

To first order, this is almost independent of the type of star swallowed. It depends on the inverse square root of the average stellar density, which doesn't vary a lot.

This means that a star could fall into a black hole like that in M87 ($M_{\rm BH}\simeq 6\times 10^9 M_{\odot}$), but  would not survive to the event horizon of Sgr A* ($M_{\rm BH}\simeq 4\times 10^6 M_{\odot}$).

I think in practice this calculation can only be accurate to factors of a few. I have not taken into account shearing forces that would be present for a rotating black hole, or the compression due to tangential tidal forces, or any heating effects from hot gas near the black hole, or interaction with an accretion disk. A full hydrodynamic simulation is needed (and perhaps has been done?).

Also note, this calculation is just for getting past the event horizon. The tidal forces grow as $r^{-3}$, so any normal star will be ripped up shortly after this, even for the most massive black holes we know of.

Please note: This answer is written from the point of view of the falling star. A distant observer will not see the star (or anything else) cross the event horizon.

This is a great question! Simply put, we do not know. However, I would say that the star in question would likely be pulled apart therefore leaving the star structure destroyed but the energy of the star still intact. Imagine a mirror being shattered! The pieces (energy) of that mirror (STAR) are still existent, however the mirror is shattered (destroyed). I hope this makes sense? The energy of the star would likely continue to exist, but the formation of the star would likely be altered.


What mechanism causes oscillations of the solar system's orbit about the galactic plane? In a recent paper (news release here) Lisa Randall and Matthew Reece propose that a dark matter disk coinciding with the galactic plane together with the solar system's oscillations through the galactic plane could explain the 35 million year periodicity in mass extinctions.  They propose that the solar system passes through this dark matter disk in the galactic plane every 35 million years, disrupting bodies in the Oort cloud and causing some of them to collide with Earth.  An image of the proposed cycles from the news release is below.

My question is: What mechanism is responsible for this 35 million year oscillation about the galactic plane?  Is there a companion body to the sun? Are we orbiting around one of the arms of our galaxy?  Is this a well known phenomenon or are they proposing the 35 million year oscillation as well as the dark matter disk?

The cause for the oscillations perpendicular to the galactic plane is the gravity of the non-spherical mass distribution (needed for a plane Kepler ellipse) in the Milky Way. Simplified, there is a dense galactic plane. The density is not exactly known; therefore there is some uncertainty (a few million years) about the precise oscillation period. Details see this article, subsection 3.3.

The idea of a correlation of mass extinctions with this oscillation isn't new, it originates back probably to about 1970, or earlier.

"The outer solar system probably does not contain a large gas giant planet, or a small, companion star", see this press release.

We are probably not orbiting around a spiral arm.

The dark matter disk is a hypotheses, an idea to investigate. Usually only a small fraction of this kind of hypotheses can be confirmed definitively later, most of them can be ruled out after some time, some remain unsolved, some can be refined to match observations.

Ok, it makes a lot of sense that the gravity of the disc of the Milky Way is pulling stars up and down as they go about their circum-galactic orbit.  But this wouldn't explain why recent 3-D observations of the nearest stars using the FLAMES-GIRAFFE spectrograph on ESO’s Very Large Telescope and the IMACS spectrograph at the Las Campanas Observatory showed a definite wave-structure in the motions of stars orbiting the Galactic plane.  In other words, most of the stars in the disc are following each other in a chain or train, as if they are bobbing up and down on a CURRENT.  Which means they most certainly are oscillating on a current.  And this was predicted in 1978!

Our Sun produces a magnetic field stretching out along the equatorial plane in the heliosphere.  This field stretches throughout the Solar System where it is called the “Interplanetary Magnetic Field”.  In 1965 John M., Wilcox and Norman F. Ness published their finding of the “Heliospheric Current Sheet” which showed that the Sun’s rotating magnetic field is constantly producing waves in the plasma of the interplanetary medium.

These waves form a “Parker Spiral” and are described in terms of an electromagnetic current but they are also mechanical waves which cause the planets themselves to oscillate up and down as they orbit the Sun.  In 1978 Hannes Alfven and Per Carlqvist suggested that there is a similar “Galactic Current Sheet” carrying an electric current of 10^17 to 10^19 amperes through the plane of symmetry of the Galaxy.

Ok?  That pretty much solves the mystery of the oscillation of the Milky Way's stars.  But the problem here is that (ahem, cough cough) OUR SOLAR SYSTEM IS NOT PART OF THE MILKY WAY.  In 1994 it was discovered that we are actually part of the Sagittarius Dwarf Elliptical Galaxy, or Sag-DEG for short, which is in a 500 million year POLAR ORBIT around the Milky Way.

Have you ever wondered why they say our Solar Apex is near Vega, but Vega itself is moving towards us almost twice as fast as we are moving towards it???  Well, in the late 80's it was discovered that nearly all of the stars orbiting the Milky Way seem to be "raining" down on our position.  Which could only mean that our Solar System is moving upwards, out of the Milky Way.  Sorry to inform you guys, but even though the stars of the Milky Way oscillate up and down during their 250 million year orbit, we aren't part of that dance.  Our own path will take us high above the Galaxy, with a spectacular view at apogalacticon, and then back down again.


Why do (most of) the planets rotate counterclockwise, i.e. the same way the Sun does? Referring to the mechanisms explaining the solar system formation and to the initial rotation of the gaseous cloud that collapsed, I understand easily why the planets orbit the Sun the same way this one rotate (say counterclockwise) but I can't figure out why this apply to planets rotation too. Thinking about that from Kepler's laws and angular momentum conservation point of view, I might conclude that the planets should rotate clockwise because the velocity of the particles that aggregated during the planets formation was higher closer to the Sun...

Apart from a short explanation, I would like to have a good reference from the literature if possible.

Edit, to make my reasoning more explicit: following Kepler's laws, the particles that aggregate on the "day side" of the proto-planets in the east-west direction relative to the ground are faster than the ones hitting on the "night side" in the west-east direction. If we add all of these contributions, the planets should rotate in the opposite direction relative to the initial cloud (i.e. relative to the actual Sun rotation). I guess something is wrong or missing there (to counterbalance the phenomenon I just described) but I can't see what it is...

New edit: References I found some published articles dealing with this kind of question but I don't have the time right now to read them carefully. If someone is motivated to do so, do not hesitate ;-) If I find the answer to my question amongst these papers, I will post it there later. Of course, you may need to use the network of an institution with a subscription to these editors to access them:

R.T. Giuli (1968a) in Icarus: http://www.sciencedirect.com/science/article/pii/0019103568900821

R.T. Giuli (1968b) in Icarus: http://www.sciencedirect.com/science/article/pii/0019103568900122

A.W. Harris (1977) in Icarus: http://www.sciencedirect.com/science/article/pii/0019103577900793

J.J. Lissauer, D.M. Kary (1991) in Icarus: http://www.sciencedirect.com/science/article/pii/001910359190145J

Even though I'm a professional astronomer (but not an expert on this field), I don't have a good answer. AFAIK, this question is still open. Actually, the fact that most planets in the Solar system rotate in the same sense as they orbit the Sun is an important constraint on possible planet formation scenarios (of which still several are presently discussed by scientists).

For example, the idea that planets form out of dust collected at the centre of vortices in the proto-planetary disc can be almost ruled out, because retrograde vortices are much more stable and long-lived than prograde ones (Jupiter's red spot is a retrograde vortex), and hence planets should be retrograde, but aren't.

The outer planets of our Solar system are not only moving the same way but have almost equal angular momenta. This includes the Sun spinning on its axis. Why it is so is not clear.


Are there upper or lower mass limits for black holes? Natarajan & Treister (2008) describes a practical upper limit for black hole masses at $\sim 10^{10} M_\odot$. This is all due to the black hole's interactions with nearby matter.

However, is there a theoretical upper mass limit for black holes in general relativity? More specifically, do any solutions make note of this? Would this depend on whether the black hole described is eternal or time-variant, static or spinning, charged or uncharged, etc.?

Similarly, do any metrics make note of lower mass limits? Would it be possible for a black hole with the mass of an electron to exist (at any point in time, putting aside Hawking radiation)?

In classical General Relativity, black holes can exist at any size (mass) without any problem. The upper limit is given by the available mass of the universe and there is no theoretical lower limit.

As already noted in the question, quantum effects like Hawking radiation set up lower limits on stable black holes; the ones with too low mass will decay quickly into radiation.

The discovery of TON 618 have created a new black hole species (already fingerprinted by M87 or even IC1101 cores): the ultramassive black holes with masses greater than $10^{10}M_\odot$. As said in the previous answer, in classical settings, there is no upper limit of the mass of black holes (I am not so sure if you get a theory beyond General Relativity even in classical settings).

Maybe, one day we will learn that quantum gravity says something about that. Interestingly, any supermassive, stellar, intermediate and ultramassive black hole has a mass much much greater than Planck mass, about a microgram. The issue is that we think quantum gravity applies only to VERY MASSIVE TINY (very dense) objects, not to very massive only. Indeed, any person has a mass much greater than Planck mass, but it is not "concentrated". When you have concentrated mass in very tiny regions, we have no idea of how to handle quantum fluctuations and amplitudes excepting with superstring theory. Another related question, is if you can have black holes of any DENSITY. Again, as said, you need to consider quantum processes like Hawking radiation, ... However, there is a subtle point, called the transplanckian problem. In principle, as the black holes evaporates it gets smaller and smaller, such as at certain size the wavelength would be lesser than the Planck length. We have to expect for a definitive theory of quantum gravity before to answer the ultimate fate of black holes and thus, the destiny of both: black holes and the whole universe (even the spacetime could be metastable and provisional/transitional state).

How large can a black hole formed from the collapse of a massive star grow
 in 1 Gyr? Suppose the black hole can grow as fast as it can. Suppose, by the moment, it satisfies the Eddington limit. Then, an exponential law follows up:
$$\dot{M}=kM=M/\tau$$
where $k=4\cdot 10^{-16}s^{-1}$ for a ten solar masses inicial mass function accordingtly to the Eddington limit. Then, as

$$M=M_0\exp(kt)$$

Plugin into this formula $M_0=10M_\odot$ and the value of k, you get that the maximum mass it yields is in the range of ultramassive BH, i.e., $M_f\sim 10^{10}M_\odot$ for a timescale about 1 Gyr (be aware, the numbers are tricky). Of course, transEddington limit is tricky, but there are some reasons to believe black holes bigger than $10^{10}M_\odot$ are unstable and eject material. Of course, in the absence of any other argument, the above argument does NOT provide an upper limit in principle. Only other considerations relativo to quasars and jets seem to apply. But the issue is a hot topic of debate in astrophysics.
By the other hand, the minimal (or tiniest) black hole mass is also a mystery. In macroscale, we have NOT found black holes tinier than 3-5 solar masses (stellar black holes). However, primordial black holes or microblack holes could made some bits of the dark matter hidden in clusters and other parts of the galaxies. Again, the only hint are inflationary ideas, astronomical measures and experimental bounds (recently, it has been analyzed the probability of dark matter being totally black holes, but some evidence seems to say that that is not the case: black holes can not be all the dark matter).


What causes horizontal and vertical lines coming out of pictures of stars? What causes the horizontal and vertical lines coming out of pictures of stars, instead of them simply appearing as circles? For example, this picture from Wikipedia: .

They are called diffraction spikes, and they're artifacts from a supporting structure inside a reflector-type telescope.

(I'd make this a comment, but no reputation yet.)

Minute Physics has a good video on this! Filtering light through any lens causes spikes that identify what kind of aperture was used. That looks like a Hubble picture because of the diamond spikes.


Moon's orbit around the Sun The Earth revolves around the Sun and the Moon revolves around the Earth.
Out of curiosity I started thinking about the orbit of the Moon around the Sun and expected (assumed) it to be as follows:

But on Wikipedia and some other sites I found out that the orbit is actually like this:

I have 3 questions:

What is the reason for this difference between assumed and actual path variation?
Has this path been like this since the formation of the Moon?
Do natural satellites of other planets also follow the same orbit around the Sun?

After further searching I found out a better, easier explanation regarding the orbit path on YouTube for those interested, be sure to check it out.

Not an answer, but I thought this was a good slice of a picture of the Moon's orbit around the sun.

Source:  http://www.wired.com/2012/12/does-the-moon-orbit-the-sun-or-the-earth/

Earth, Moon and Sun relationships and the Moon’s movement around the Sun.

An orbit is a modified helical path through the solar system. The depiction of an orbit as a complete circle is incorrect when viewing the solar system. When an orbit is around a parent body (Moon/Earth, or Earth/Sun) it is closed and ovoid in shape (2 Dimension). When it is a part of the solar system it becomes open and a never-ending spiral (3 Dimension), it’s orbital path still being ovoid.
The Moon cannot spin on its own axis, as many astronomical publications suggest because it doesn’t spin, maybe it used to but it doesn’t now. It’s just a blob fixed radially to the Earth’s axis, slowly moving around the Earth once every solar month dropping back its position daily by approx. 1/30th of the orbital circumference. One Moon orbit plus 29 Earth spins gives a total of 30 passes every solar month. The Moon’s orbit will always touch the extended radius of the Sun.
The Earth spins inside the Moon’s orbit approx. 29 times every solar month and advances in its own orbit around the Sun approx. 30 degrees every solar month therefore the orbit of the Moon advances with the Earth.
The Moon is a part of two systems.

a. In its own orbit around the Earth being its Earth system and,

b. The orbit of the Moon in its movement around the Sun being its Sun system.
The Moon’s movement around the Sun is a slightly modified epicycloidal curve. By constructing this curve, it is possible to track the Moon around the Sun.
If you construct the epicycloid and where the two circles touch (the rolling circle as the Moon’s orbit and the base circle as the Sun’s extended radius) and call this point ‘NM’ then moving the rolling circle 1/29th of its circumference along the base circle then ‘NM’ will have moved away from the base circle but still within the Moon’s orbit. This is the first point on the epicycloid. Do this another 28 times and ‘NM’ will be back on the base circle about 30° later. These are the positions of successive New Moons. This will happen approximately 12 times a year. The ratio of the two circles should be about 12 to 1.
Epicycloid constructions may be found on the internet.           Thursday, 21 May 2020


Does the expansion of the universe increase with distance? I read something in a book the other day which has gotten me really confused. They were talking about the expansion of the universe, and if I understood it all, they said that the expansion of the universe increases with distance. I’m wondering if I understood this correctly, because I was reading about Hubble’s law and it said that the redshift if galaxies far away (caused by the expansion of the universe) is proportional to the distance (up to a few hundred megaparsecs away). Doesn’t this mean that the expansion is the same but the redshift increases? (I don’t see why the redshift would be proportional and a linear graph if the universe expanded from us proportionally to the distance, or at least more further away). I hope you understand what I mean by all this.

I'm not sure why you say that "expansion is the same" when you also say that "expansion increases with distance", but I wouldn't say the "expansion increases", but rather that "the recession velocity increases".

The recession velocity $v$ of galaxies increases linearly with distance $d$. The constant of proportionality is called the Hubble constant $H_0$. Thus, Hubble's law states that
$$
v = H_0 d,
$$
and currently, $H_0 \simeq 70\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$, but in the past it was higher. That means that, for every Mpc (mega-parsec, $\simeq 3.26\times10^6$ light-years) a galaxy is from us, its speed away from us is roughly $70\,\mathrm{km}\,\mathrm{s}^{-1}$. This is the speed it has right now, but not the speed we see it have, since we see it in the past.

In fact, we don't really see a speed at all. We measure a galaxy's redshift $z$, and $z$ also increases with distance, but not linearly. Thats's because it depends on how much the Universe has expanded since the light was emitted, and the expansion rate changes with time. But for small enough distances, i.e. for galaxies that are not seen that far into the past, the relation is close to linear, so that $v = cz$, where $c$ is the speed of light.

This is indeed approximately true out to a few hundred Mpc. For larger distances, the linear relation becomes inaccurate, and one must instead apply a cosmological model of the expansion history of the Universe, which depends non-trivially on the densities of its various constituents.

Acceleration of the Universe

The term "accelerated expansion rate" refers not to the value of $H$, but to the rate at which the scale factor $a$ — the "size" of the Universe — increases (the Universe may or may not be infinitely large; $a$ is normalized such that, today, $a=1$).

For a given value of $H$, we know the recession velocity of any two points separated by some distance at a given time. At a later time, these two points will have increased their distance, and hence — by Hubble's law — their recession velocity will have increased.

What was higher in the past is the Hubble parameter $H(t)$. That number will, in the future, asymptotically decrease toward a slightly smaller value of
$$
H(t\!\!\rightarrow\!\!\infty) = H_0\sqrt{\Omega_\Lambda} \simeq 56\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}.
$$
The Hubble parameter is defined as $H\equiv\dot{a}/a$, where $\dot{a}\equiv da/dt$ is the Universe's change in size per unit time. If $H$ is constant, and $a$ increases, then $\dot{a}$ also increases. That is, the increase in velocity of any two points accelerates. In fact, we can solve the differential equation and get that, in the future,
$$
a(t\!\!\rightarrow\!\!\infty) \propto e^{Ht},
$$
i.e. the scale factor increases exponentially.

The figure below shows the evolution of $a(t)$ until $t\sim130\,\mathrm{Gyr}$. On the left is a zoom-in of the first $20\,\mathrm{Gyr}$ where you can see how the Universe decelerated the first $\sim10\,\mathrm{Gyr}$, until dark energy started taking over the dynamics.

At the moment the acceleration is actually quite modest, but it will increase without bounds in the future; for instance, in 50 Gyr from now, $a\sim10$, i.e. galaxies will be $10\times$ farther from each other, but in 100 Gyr we have $a\sim150$.

To put the matter at its simplest, imagine an inflated balloon with a number of dots scattered on its surface. The balloon represents the universe and the dots are the galaxies. Inflate the balloon further and you will see that an ant placed on one of the dots would see himself as being at rest with all the other dots moving away, and with the furthest dots moving away fastest of all. The ant, of course represents you, and the expansion of the ballon represents the expansion of the universe. An ant placed on any other dot would also see himself as being at rest, with your dot moving away from him with a speed proportional to its distance. The universe deviates from this analogy inasmuch as the galaxies which are very close to us, Andromeda for example, have a proper motion which masks cosmic expansion and will sometimes produce a blueshift instead of a redshift.


Do black holes really exist? Black holes from my knowledge are created from the collapse of a star’s own gravity crushing itself, Now my question is well two questions, is it really infinitely dense to where space-time can be distorted to the point where it just stops? if so, this would mean objects caught in the event horizon would freeze and time would be stopped how is the black hole itself not effected by it’s own mass wouldn’t it just cease to exist in the first place?

The literal answer to your question is "yes". We can see clear astronomical evidence of black holes of a variety of sizes in this and other galaxies. It is possible of course that all that evidence is actually the result of some unknown physical process(es) and not black holes, but it would be a remarkable coincidence that it looks so much like we would expect black holes to look.

So I'll take your question as asking for a way to understand what black holes are. The best answer to that question is basically just equations -- solutions to the governing equations of general relativity that have event horizons, but I'm going to assume that is not what you want here.

So we are left looking for a better intuitive explanation or understanding, bearing in mind that this is at best inexact, and at worst misleading. If you have a lot of matter in a small region of space, and you look at it from the outside you see that the light coming from the surface of the region is red-shifted and (which is basically the same thing) that clocks on the surface seem to run more slowly.  As you pack the matter in more tightly this effect becomes more pronounced. But because the clocks are running more slowly, whatever process is packing the matter in also runs more slowly. Eventually, as seen from the outside the clocks on the surface approach a complete standstill, and you will receive a final few extremely red-shifted photons from the surface and then nothing. What you have is now a black hole. From your point of view it is "frozen" and all you can see of it from the outside is the gravitational field surrounding it (and maybe an electric field as well). No information can ever leave it (I'm ignoring Hawking radiation and quantum mechanics in this answer, which is reasonable for big black holes).

Relativity being relativity, things can look quite different from another point of view. If you are falling freely into the black hole (and it is big enough that tides don't rip you apart) everything seems fine, as in this simulation Light from the rest of the universe is blue-shifted and increasingly distorted by gravity, but locally everything is OK. However, no matter what you do, within a finite time (a few days for the largest black holes we know about) you will reach a singularity -- a place where the equations of general relativity break down, and some other physics must presumably apply. We don't know what happens there, but we do know that no information about it can come back out of the black hole, so we can't send in any kind of probe to tell us about it.

The honest answer is: Nobody knows for certain.

They exist only in theories. Physicists regularly construct elaborate mathematical equations, and the Black Hole is something they essentially created to fill the gaps in certain theories, or to artificially 'complete' them.

In other words, for many of their theories to be correct, Black Holes MUST exist.

In reality, though, we have no idea. Something that's infinitely dense and small destroys all logic, physics and observable reality.


How long for a complete solar system cycle Considering the solar system as a closed system, how long until all the planets pass twice by the same position relative to each other? I would say that you'd have to get the length of the year in each planet and find the lowest number that can be divided by them, is that correct or are there more things to take into account?

Furthermore, can one do this also considering the moons of the planets?

Thank you!

It depends how precisely you mean the same position.

Take 2 since my previous attempt was ... optimistic.

In the late 1970's, the outer planets were moving into a configuration that made a Planetary Grand Tour much cheaper and quicker. They will be in a similar configuration in about 175 years.

But when will the planets be in the exact configuration? Never, well, not before the Sun turns into a red giant and swallows the inner planets.

But from time to time the planets will all be on the same side of the Sun. If the planets were all in a line, then after 495 years they would all be on the same side of the Sun, then again at 500 years, 522 years, 523 years (these last two are, of course 88 days apart, the period of Mercury), 995 years, 997 years, 1006, 1183 years, etc.

By assuming the orbital periods of planets around sun and treating the solar system as closed and considering only planets Mercury,Venus Mars, Jupiter, Saturn , the least common multiple of the periods may give an idea of repeatability of particular position of planets around sun.The phases of moon is not considered here.The LCM shows the repeatability of  a configuration is about
1,87,500 solar years .The planetary periods are corrected to near integer for easy estimate of the order


The most feasible hypothesis about Theia The formation of the Moon had many hypothesis over the years, most of them involving a Mars-sized body called Theia, which presumably collided with the Earth. But even within the Theia hypothesis, is not very clear the conditions of that collision. So, what is the most plausible hypothesis about Theia today?

Theia fully disintegrates and the remnants debris form the Moon in a head-on collision(the Big Impact hypothesis)
Theia-Earth suffer a glazing collision and Theia manages to escape to space, but some of the remnants of the collision form the Moon (Reufer, 2012)
Other hypothesis, such a double Theia system

All the hypothesis have problems, but to me the most believable is 2, because it solves the majority of the problems of the other ones, although there's not much consensus on that one, the only who defends it is Reufer.

For example, some research shows that only 2% of Theia actually contributed to the formation of the Moon (I don't remember the reference), so 1 doesn't seem too much feasible because according to that one Theia fully disintegrates.

According to new simulations, a better model seems to be one where the Earth had a magma ocean at the time of impact, see here:

We perform density-independent smoothed particle hydrodynamic collision simulations with an equation of state appropriate for molten silicates. These calculations demonstrate that, because of the large difference in shock heating between silicate melts and solids (rocks), a substantial fraction of the ejected, Moon-forming material is derived from the magma ocean, even in a highly oblique collision. We show that this model reconciles the compositional similarities and differences between the Moon and Earth while satisfying the angular momentum constraint.

As I recall, when Theia collided with Earth it created oceans, and the modern atmosphere, which led to life. As a side-effect, a ring was created around the Earth, with colliding rocks forming two moons. That's right; Earth had two moons 4 billion years ago. Eventually, they collided to form the modern moon.


Can planets have orbits other than elliptical or circular orbits? Can planets have orbits other than elliptical or circular orbits? All of the planets in our solar system have either a circular orbit or an elliptical orbit (perhaps like Pluto).

Orbits are conic sections therefore can be either circular, elliptical, parabolic or hyperbolic.
Of these 4, only first two form a closed curve under 2 body hypothesis, while the later two extend to infinity.
If you talk about planet,  by definition it has to orbit a star which would require it to have a closed orbit hence circular or elliptical, with respect to the star. For any other kind of orbit the body will just fly away to infinity never to return back.  Infact comets are considered to be parabolic, but in theory they have highly elliptical orbits with their aphelion lying near the edge of the solar system.

But it is possible for a planet to have other kind of orbits if we  consider their motion from a different reference frame such as with respect to another planet.
So for an inertial frame of reference a planet will have a circular or elliptical orbit, even Pluto.

Certainly.

But you'd have to think in Einsteinian, rather than Newtonian, terms. Inertial frames of reference and all.

If you view our planet as motionless, then the orbit of Venus, for instance, would resemble a telephone coil cord.


If the Moon were impacted by a suitably sized meteor, how long would it take to impact the Earth? An answer to the question of How well would the Moon protect the Earth from a Meteor? mentions as a possibility that the Moon could get knocked into the Earth.

What is the smallest change to the orbit of the Moon from being impacted by a large meteor that would cause it to eventually impact the Earth (i.e. "circling the drain")? What timeline would that look like (minutes, hours, days, years, etc)?

As several people have said, this is incredibly unlikely. Part of the reason why is that the "circling the drain" effect you describe doesn't really happen for solid objects much less dense than black holes. Orbits are not "precarious" in that way.

So, suppose something large enough and fast enough to change its velocity noticeably, but not large enough or fast enough to shatter it, did hit the Moon. The effect would be to shift the Moon from its present almost circular orbit around the Earth, into an elliptical one. Depending on the direction of the impact, it would either get a bit nearer to the Earth than it is now, once per orbit, or a bit further away (it also might swing North and South a bit). What is important though, is that this elliptical track is stable at least for a while. Suppose it gets knocked into an orbit that is 220000 miles from the Earth at its closest and 240000 miles at its furthest, that is where it will stay. It will not "spiral in".

Over a long enough period the gravity of the Sun also comes into play and things may shift a bit, but that is a relatively small effect.

Now, suppose that the impact was really big, or perhaps there were a long series of impacts (starting to look like enemy action..) so that the innermost point of the ellipse was eventually driven down to within a few thousand miles of the Earth, somehow miraculously not smashing the Moon to fragments in the process. At this distance it starts to matter that the near side of the Moon is closer to Earth than the far side, so that Earth's gravity pulls on it more strongly. If it orbited closer than about 3000km to the surface of the Earth for long (the Roche limit) these forces would eventually pull it to pieces, and Earth would probably have a pretty set of rings for a short time before internal collisions between the bits caused them to rain down on Earth and kill everyone.

Finally suppose the impact(s) was(were) so big that they actually put the Moon into an elliptical orbit whose innermost point was so close to Earth that the Earth and Moon touched. This is manifestly impossible without shattering the Moon, but in that case, the Moon would indeed hit the Earth. The time for the impact would be about 1/4 of the Moons current orbital period, which is to say about a week.

I would argue that the lower limit on impact delay is somewhere around 1.3 seconds.

Any impact that would leave the Moon at rest relative to Earth (see previous answers) would also, ahem, structurally disrupt the Moon. As in, turn it into an expanding cloud of vapor and debris, some of which would strike Earth sooner, some of which would form a ring, some of which would escape into (or out of) the rest of the Solar System. (I'm not sure how well even Elon's Mars colony would fare in the wake of a big collision.)

So, if we're allowing disruption, just hit the Moon with a converging swarm of ultrarelativistic impactors. They essentially turn the Moon into a giant shaped charge. Dial in the energy you want to select the resulting jet's velocity, up to lightspeed-minus-a-small-margin. For a visualization of the resulting impact, look up one of those stop-motion photos of a bullet hitting an apple...


Why do I see rainbows around the moon? Regularly, I see rainbows around the moon. Here is approximately what I see:

Right at the edge of the moon, I see a bright blue ring of light. Further out, I see an inner rainbow. This is not as bright as the ring, but is still clearly visible. Even further out, I see an outer rainbow that is very faint.

I see the blue ring during every full moon. I see the rainbows more frequently in late fall and winter than in spring or summer. But every year, I see these moon rainbows during a full moon.

Since I see these moon rainbows more frequently in late fall and in winter, are these moon rainbows caused by moonlight reflecting off of ice crystals in cirrus clouds high up enough that I can't see the clouds? Also, why do I see a bright blue ring right at the edge of the moon during every full moon?

There are a couple of atmospheric phenomena that can create rings around the moon.

Corona are caused by water droplets diffracting the moon's light, they are fairly small, and close to the moon. They are coloured but not as brightly as a rainbow. Our eyes are also not so good at seeing colours in dim light.

The 22-degree halo is a larger ring, as big as two hands stretched out at arms length. Solar halos are clearly coloured, lunar halos are dimmer and I haven't seen lunar halos that are bright enough to appear coloured to me.

An eye condition, astagmatism, can also create rings, such as the "thin blue ring" (though I am not a doctor) You may be seeing a combination of all three at various times.

I think you very nearly answered your own question. The rainbows are indeed due to light diffracting off high altitude ice crystals, but more in a kind of haze than in the form of clouds. I have seen something similar in Hereford UK, but only on very rare occasions. There must be something unusual about the climate where you live for you to see them so often. The other unusual thing is for you to see the colours so clearly. The human eye loses colour vision at night, though with a very bright full moon there is still a vestige of colour vision left. The fact that your concentric rings are off centre and slightly elliptical I put down to imperfect draughtsmanship rather than faulty vision.


Do objects look larger the further away they are, beyond z=1? I stumbled upon this text which illustrates and says:

  We find that as the object is moved to higher redshifts its angular
  size first decreases (as naively expected) but soon begins to increase
  after passing through a minimum value.

Is this correct for the mainstream standard model? What would an English language explanation be?

Yes this is correct for the standard model. I believe the English language explanation would be that the angular size of a galaxy does not depend on its current distance to us, but its distance when the light we currently see from it was emitted (here using "distance" synonymous with "proper distance"). Though I also believe there are other complicating factors such as the spatial geometry of the Universe.

Look at the first diagram in Figure 1 in this paper, notice how the size of the past light cone in terms of proper distance firstly increases as we go back in time, before reaching a maximum and decreasing to zero at t = 0.

This is an interesting thought,but red shift was a theory that could be answered by assuming near light velocities.The belief in,this unproven idea,invokes argument of"If it is coming toward us then we will already have seen it"


What is the orbit of Earth around the Sun? According to wikipedia and other sources, a planet and a star always move in a circular orbit around the common center of mass of the both bodies, in case  of Earth and sun, this center of mass lies inside the sun. Therefore, Earth should orbit the sun in a circular orbit. So, my question is, why does the Earth orbits the sun in an Elliptical orbit?

And, if it does, and one focus is the sun, then what is the other focus?

According to wikipedia and other sources, a planet and a star always move in a circular orbit around the common center of mass of the both bodies ...

This is not true. In the absence of other gravitational sources, a planet and a star move in elliptical orbits about the common center of mass. Ancient scientists assumed circular orbits, but only because circles were somehow most pure. There were problems with this: It didn't match what those ancient scientists saw. So they modified these circular orbits by tacking on other circles. It was a mess.

Kepler cleaned this mess up by noting that the orbits of the planets are very close to elliptical rather than circular. This empirical result was later strengthened by Newton, who derived that elliptical orbits (not circular orbits) are a natural consequence of a force that follows an inverse square law. Gravitation is such a force.

The orbits of the planets about the solar system are not quite elliptical because planets are attracted to one another as well as to the Sun. Nonetheless, the orbits are still very close to elliptical. The Sun is more than 1000 times more massive than Jupiter. Jupiter, Saturn, and the other planets represent tiny perturbations on the nearly elliptical orbits of the planets about the Sun.

What about other perturbing effects, such as nearby stars and the Milky Way as a whole? These are immeasurably small. A passing star will perturb planetary orbits over the course of the tens of millions of years or more. The Milky Way as a whole has essentially no effect whatsoever. Theoretically, the effect most certainly does exist, but it is so small that it can be ignored, even over the lifespan of the solar system.

The trajectory of Earth's orbit is shaped not only by Sun's gravity. Earth's orbit is being changed by different sources of gravity as well, for example center of our galaxy. This is why it's not perfectly round.


Was the Universe expanding before the beginning of inflation? My point of view is that first there was the Big Bang singularity, and then the period of inflation which resulted in the observable Universe becoming many times bigger. But was the Universe expanding between the Big Bang singularity and the beginning of inflation?

This is likely unanswerable.  Inflation was in part a resolution to a fine tuning problem: without it, it seemed we needed the early universe in a very specific and precisely balanced state to get to where it is today, and there was no solid scientific way to explain why things were so perfectly arranged (other than to simply assert that they were).

With inflation, the state of the universe before the inflationary epoch is fairly irrelevant.  Mostly all it needs is that the region that inflates into what we will know as the observable universe had enough time to achieve thermal equilibrium (which we need to explain why the universe looks pretty much the same in all directions, exactly as if things well outside of light speed communication had nonetheless achieved thermal equilibrium at some point).  Curvature and such get "smeared out" by the inflation to give us the mostly flat and homogeneous universe we see today.

On a pedantic note, some researchers consider inflation and the big bang to be the same thing. At least in the sense that talking about "before inflation" is scientifically meaningless, so if the Big Bang is the (scientific) beginning then we might as well take it to be inflation.

Inflation has started with the bigbang. In fact for anything to take place you need an expansion right away. If you had a static universe just after the bigbang, it couldn't have formed subatomic particles or atoms, without any increase in size.
Inflation was always happening and will always happen. The process is behaving (in terms of duration) like radioactive material, that means that inflation slowy decays wth a halflife of about 10^-32 seconds. I hope this answer suffices.


Is stacking welder's glasses a safe way to watch at the eclipse? You can find in many place on the Internet that welder's glass #14 is good for looking at an eclipse. Tomorrow (March, 20th 2015 at 10:45 CET) there's a solar eclipse and yesterday I could only find glasses #11 and #9. I tried briefly this morning, and combining a #11 and a #9 was giving me a good view of the sun. Using 2 #11 gave a too dark image. Now, I read different opinions on the internet.

Against (Perkins Observatory):
http://perkins.owu.edu/solar_viewing_safety.htm

  Be careful that you use the right kind of glass! Welder's glass is
  numbered from 1 to 14 with 14 being the darkest. It is only number 14
  glass that is dark enough for solar viewing! And NO STACKING! A pair
  of number 7's or a 10 and a 4 together DO NOT have the same protection
  as a single piece of number 14 (see unsafe methods for more details).

Favorable (Royal Astronomical Society of Canada):
https://www.rasc.ca/tov/safety

  If SN14 filter is not available, it is possible to combine lower shade
  numbers to get roughly the same level of eye protection from solar
  radiation, e.g. combining SN 6 and SN 8 filters. However the image
  quality may be considerably poorer than that seen through the single
  SN14 filter

I could not find a table or something explaining which kind of protection gives each number; according to the Canadian website, the only concern is about how much infrared light goes through, ultraviolet does not seem to be a problem in almost any case (I was surprised to read that).

Note: It's not my intention to open here the discussion on what could be other safe methods to watch the eclipse, this is well explained everywhere around. I read too late about the eclipse to order specific glasses.

Edit:
One of the answers here report the following formula: (more insight at this link)

  13 or darker is safe enough. Also, you CAN add up welding glass, using
  the formula S(sum) = S1 + S2 -1. S(sum) should be greater than or
  equal to 13

YES

You have summarized pretty much everything available on the subject. There are loads of people saying you cannot, but fortunately for us, the Royal Astronomical Society of Canada, that you quote, includes evidence to back up their claims. They are a respected institution and the author of this article appears immensely qualified. The article itself does not include evidence that welding goggles can be stacked, but the inclusion of such for all the major points adds credibility to the entire article.

It does appear like the glasses are not exactly additive, this Reddit post goes through some calculations, but they are pretty close such that aiming for 1 tint higher will more than offset it and if anything is overkill.

I know it's a bit late for the UK eclipse, but for future reference:

Welders goggles do not cut out the same frequency of light that Mylar/Black Polymer eclipse glases do.

Arc welding glass #14 is much darker than brazing welders glass but there is no guarantee that they will block the Sun's ultra-violet and infrared rays.

The problem is there are no pain sensors in your retina so you don't feel your eyes cooking and the dark lenses (as do sunglasses) cause your pupils to dialate letting in more ultra-violet and infrared rays so they could make things worse.

I have used arc Welders glass on a camera and it worked fine but gave a picture of a green Sun. Project the image on paper to take it.

Do NOT look through a camera viewfinder even with the correct eclipse glasses on if you do not have the correct solar filer on the LENS end.


Is the Sun prograde or retrograde with respect to the rotation of the Milky Way? I recently learned about the terms prograde and retrograde. I've seen these terms used to describe the rotations and orbits of moons with respect to their planet, and planets with respect to their star. But I haven't been able to find what the relationship is between the rotation and orbit of the Sun with respect to the rotation of the Milky Way.

So, basically what I'm trying to do here is ask two questions:

Is the orbit of the Sun around the center of the Milky Way prograde or retrograde with respect to the rotation of the Milky Way?
Is the axial rotation of the Sun prograde or retrograde with respect to the rotation of the Milky Way?

Regarding question #2, I know it might be considered a slightly weird question to ask, simply because the plane of the Solar System is significantly tilted relative to the plane of the Milky Way (by about 60°), and furthermore the Sun itself has its equatorial plane tilted by a few more degrees relative to the plane of the Solar System (about 7°). But if we mentally "flatten out" the Solar System such that the Sun's equatorial plane aligns with the plane of the Milky Way, then we can talk about whether its axial rotation is in the same direction as the rotation of the Milky Way, so I think this is a valid application of the concept.

Prograde. This is usually the case for stars in spiral galaxies outside the most central regions.

A galaxy can be either rotation-dominated or dispersion-dominated, depending on whether its kinematics is dominated by ordered or random motion. Spiral galaxies such as the Milky Way belong to the first class, while ellipticals belong to the second. Irregular galaxies can be somewhere in between, with random motions but with an overall rotation.

In spirals, stars and gas generally rotate in the same direction (although they can have more random motion in their bulges, i.e. in the central parts). The Sun follows the overall rotation of the Milky Way, except for roughly 20 km/s in the direction wrt. the overall rotation; this direction is known as the Solar apex.

However, following a major merger event, the product can be two kinematically decoupled components. This is described well in Corsini (2014). Such a merger is required to perturb the system sufficiently.

On smaller scales, however, the turbulence of the gas clouds giving birth to stars are more random. Hence the rotation of a star may very well be in another direction than the overall disk rotation. As you say, the plane of the Solar System is tilted by 63º wrt. the plane of the Milky Way, in the direction that the Sun is traveling. This can be seen e.g. in this infrared IRAS image, where the angle between the bright Milky Way disk and the blue stripes — which is emission from dust in the plane of the Solar System — crosses at roughly 60º:

credit: Caltech.

The rotation of the Sun is also prograde wrt. the Milky Way (or else we would say that the tilt was 180º – 63º = 117º).

I think the answer is retrograde.

For Earth orbiting satellites, prograde means that when viewed from above the North Pole the satellite moves counterclockwise. However, when viewed from above the north Galactic Pole the solar system orbits clockwise.

Are these two norths consistent? Yes, even though the plane of the solar system is tilted by about 69 degrees from the plane of the Milky Way, it still "points north", as you suggest in your question.


Is Earth's climate significantly different when the one solstice occurs at aphelion rather than the other? I mean different in some kind of significantlly non-symmetrical way, such as it being much colder during one period rather than the other? Let's assume other factors, such as tilt due to nutation and variation in the Sun, are the same.

Seasons on the Earth are driven primarily by axial tilt. I'm writing this answer in late August, when the Earth is a bit past aphelion. That event currently occurs in early July, a couple of weeks after the June solstice. Yet it's summer in the northern hemisphere. Precession have a secondary effect on the seasons. The current situation (perihelion close to the December solstice, aphelion close to the July solstice) was reversed ~12000 years ago when perihelion coincided with the June solstice. This made Northern Hemisphere summers hot but short, winters cold and long.

The brutally hot Northern hemisphere summers back then resulted in the end of the last glaciation (colloquially, ice age). Glaciations start when summertime temperatures at far northern latitudes (65° north is the standard proxy) are so mild that snow from the previous winter doesn't completely melt. Glaciations end when summertime temperatures in the Northern Hemisphere become hot enough to melt the ice that has accumulated over forty thousand to a hundred thousand years.

The reason we aren't we seeing a glaciation start now (conditions are right from the perspective of precession) is that the Earth's eccentricity is rather low. Precession, eccentricity, and other factors have to line up to trigger a glaciation.

At present the Earth's orbit is very nearly circular, with only a 3 percent difference between distance at aphelion compared with  perihelion, too small to make a significant difference to the winter solstice. However, the Earth's orbit is perturbed occasionally by the gravitational field of Jupiter, so at intervals of about 100,000 years the distance from the sun at aphelion increases to about 12 percent, which is enough to cause ice ages.


If the Sun disappeared, could some planets form a new orbital system? If the sun were to suddenly disappear, the planets would continue to travel tangentially to their former orbits. (This I know from this answer to a somewhat related question here.)
In such a scenario, is it possible (however unlikely) that a new orbital system would form from two or more of the now-rogue planets?  For example, is it physically and mathematically possible that Earth might end up getting captured by Jupiter and thereby become one of its moons?  Or do the size and speed of the planets completely rule out such a scenario, even if the sun disappeared at such a time that the planets' new trajectories would cross?

The issue here is whether pairs of planets can become gravitationally bound to each other.
In the two-body problem the trajectories or orbits are ellipses (bound orbits), parabolas and hyperbolas (unbound). For all practical purposes, an encounter looks like they start out at infinity with some finite speed, approach each other, and then maybe fly away or remain bound. This situation (finite speed when far away) corresponds to a hyperbola, so the generic case will not form a new bound system: some of that energy needs to be shed somehow.
One way this can happen is an inelastic collision, i.e. a merger. A less extreme case is a close encounter where tidal forces dissipate some of the kinetic energy. But the kinetic energy of planets is large, so a single encounter is unlikely to dissipate enough to make them bound.
Another important possibility is a three-body encounter. This is how comets get into short-periodic orbits when falling into the solar system: an encounter with Jupiter (or some other planet) leads to a gravitational "assist" that makes it bound with the sun.
In your scenario where the rogue planets fly off along their tangents it is very unlikely that any start out bound to each other since planetary orbital velocities around the sun are much bigger than the orbital velocities of a bound planet-planet pair at a large distance. Hence they will just fly apart. In theory they might have an encounter before leaving the solar system vicinity (e.g. Mercury by sheer chance sweeping past Jupiter) and that leaves a tiny chance of a three-body interaction with moons or a merger, but the chances are truly tiny. The generic case is just scattering.
Addendum: Some estimates
For a body to be bound to another body of mass $M$ it need to move slower relative to it than escape velocity $v_{esc}=\sqrt{2GM/r}$. If we assume planets move in circles we can then take their orbital velocities for all possible positions along the orbit, calculate the relative velocity, and compare it to the escape velocity.
The result is this diagram, where I have plotted the separation on the x-axis and how many times escape velocity the relative velocity is on the y-axis. The numbers denote which pair of planets I am comparing. The blue horizontal line is $\Delta v=v_{esc}$.

Looking just at the lowest relative velocities shows that the closest to being potentially bound is Saturn to Jupiter were the sun vanish just as they are moving in parallel, but even in this case Saturn has several times Jupiter escape velocity.
One could complicate it with inclinations and elliptic orbits, but it will not change the qualitative picture: the planets are not even close to being bound to each other in the absence of the sun.

Without the sun... at current orbital velocities... The planets would fly apart... But exactly how far would they fly? Would they continue indefinitely into interstellar space? Or would they fly out into the Kuiper belt or Ooort cloud and subsequently fall back toward some barycenter.
Removing the sun removes 99% of the mass of the solar system... so this could dramatically alter the barycenter of the system. Assume all/most planets are in one quadrant at time of sun's disappearance. Then presumably those planets with roughly similar trajectory would not have escape velocity relative to their shared barycenter.
Take for example the recent alignment of Jupiter and Saturn. There was definitely a window of time this year where if the sun had disappeared, Jupiter and Saturn would have formed an orbital system between the two, and had Earth or some other planet been behind them in orbit, overtaking them like a rock in a sling that, released at the precise moment, find its mark, that planet might get caught up in their system. (especially because our inner planets have higher velocities than more outer planets)
But if you could, with your hand of god, place the planets just so that they were least optimally arranged for the formation of an orbital system in the sun's absence... how far would they fly? Would they escape into interstellar space? I would think in 99% of planetary arrangements, this would happen since these planets are so tiny relative to the space between them and relative to their velocities and relative to the mass of the sun which is 99x the total mass of everything else in the solar system.


Theoretical sudden appearance of a black hole in BBC drama show *spoiler warning* I asked this on the ‘movies’ stackexchange site but they suggested I ask here, it’s a purely hypothetical query based on a BBC drama show called ‘Hard Sun’, feel free to kill the question if it’s inappropriate here.

In the TV show’s series finale an expected ‘extinction level event’ happens and appears to be a black hole close to our sun, it already has developed a spiral of hot gas around it drawn from the sun.

Clearly this is fiction but I’m keen to understand just how badly-considered this is. Firstly the show is set in London and everyone seems amazed when the image below rises from the dusk yet clearly it's been in existence for the time taken to create the spiral (hours at least?) so nations east of the UK would have seen it for that time and reported on it globally surely?

My real question is more around is there any potential scenario where a black hole of that size could develop suddenly and get to that size quickly enough for that image to be a surprise? My understanding, very much as a scientific-layman, is that a black hole of that size would have to have the mass of at least one of our suns to get to that size (probably more) - is that a valid assumption? If so where has the mass come from? Also wouldn't the sudden existence of something else with at least the same mass as our sun suddenly appearing at the centre of our solar system have enormous gravitational impact on everything in the system, including Earth? We'd have massive volcanic/tectonic impact wouldn't we?

So again sorry if this is inappropriate, I was just incredulous at how badly-considered the scientific aspects of this fiction were and wanted to check that my understandings were correct.

Screengrab:

Firstly the show is set in London and everyone seems amazed when the image below rises from the dusk yet clearly it's been in existence for the time taken to create the spiral (hours at least?) so nations east of the UK would have seen it for that time and reported on it globally surely?

And we've kind of a large number of space based satellites that do nothing but monitor the Sun and all the people monitoring them would certainly spot this minor event.

  My real question is more around is there any potential scenario where a black hole of that size could develop suddenly and get to that size quickly enough for that image to be a surprise ?

About the only way I can think of is someone creating a Kugelblitz, although it's pretty far fetched.

Maybe a very freak collision of two high mass objects head-on, but that would create far worse problems - essentially an absolutely huge explosion and we would not be looking at the result quietly afterward.

  My understanding, very much as a scientific-layman, is that a black hole of that size would have to have the mass of at least one of our suns to get to that size (probably more) - is that a valid assumption ?

There's a minimum size for the formation of a black hole by gravitational collapse of a star and it's larger than our Sun's mass.

There is, however, no theoretical minimum for the size of a black hole if you can make one some other way.

  If so where has the mass come from ?

Ask the writers. :-)

  Also wouldn't the sudden existence of something else with at least the same mass as our sun suddenly appearing at the centre of our solar system have enormous gravitational impact on everything in the system, including Earth?

Yes that's a major issue.

Any black hole large enough to start siphoning off the Sun's outer layers would need a significant mass and it would have a significant impact on our orbit - and do so more or less immediately.

Our orbital speed is enough to maintain our almost circular orbit around the Sun.

With something as massive as a small star added close to the Sun, our orbital speed would no longer be enough for a circular orbit and we'd be in an elliptical orbit about the Sun-BH pair.  It's more complicated than than that, but that's the broad strokes.

But that ellipse would be with our current distance as an aphelion, whereas we'd have a smaller perihelion distance.  So we'd get warmer.  And it would not take much to raise temperatures here uncomfortably.  Very roughly if we were to have a closest approach of 0.72 AU (Venus's current orbit) we'd find our temperatures rise by perhaps 24 degrees Celsius, which would be enough to make the planet unlivable for us - ( maybe 60 C at the equator and enough to melt significant parts of the polar ice and permafrost ).  And as this change would happen every six months as we orbited moving from close in to far out, we'd be permanently shifting climate.  And that ignores greenhouse effects - we could end up with a runaway greenhouse effect that raised temperatures to lethal levels everywhere.

  We'd have massive volcanic/tectonic impact wouldn't we?

That's a hard one to call.  This would depend on how far apart they Sun and the BH were and how massive the BH was.

But we'd have lots to worry about.

You guys are all hinting about the effects of a black hole on the solar system, but you are forgetting about the effects on time that gravity creates.  The gravity of a black hole would slow time.  As such, the gravitational time dilation effect would let us see a slow death spiral of the solar system, even though to an external observer, the solar system would already be gone.  The sun would indeed become egg-shaped, but the spiral we see at the end of the Hard Sun miniseries would be the first sign of it.  The five year timetable would be how long it would take for the earth to actually experience the full effects of the black hole, because as we got closer to the black hole, time would continue to slow down and we would feel time dragging out even farther than before.

As for it suddenly appearing and nobody noticing, the fact is that someone did notice.  That is the reason the flash drive existed.  Remember, NASA still only monitors NEOs over 140m across.  A black hole is actually much smaller than that - and it is virtually undetectable except in the presence of matter.


Could the Universe really be 12.5-13 billion years old? My initial reaction is that "this must be wrong" and apparently that's a lot of people's initial reaction according to the article.

https://www.nbcnews.com/mach/science/universe-may-be-billion-years-younger-we-thought-scientists-are-ncna1005541?cid=sm_npd_nn_tw_ma

My question is, is this study actually making scientists scratch their heads, and, 2nd part, if accurate, how could the 13.78 (or whatever it is) estimate have been wrong, as it seemed to be the result of more and more accurate estimates over time.

One of the two estimates has to be wrong.

Because some measurements are difficult to make with absolute precision, it often happens that there is some uncertainty about them. The half life of the neutron was once thought to be 12 minutes, but in the 1980s this was amended to 10.5 minutes. In the early days, C14 dating was often substantially out by comparison with dendrochronology and other methods. It depends on the amount of C14 in the atmosphere at any particular time, and when they looked into the matter, scientists found that this varies from one century to another. When they took that factor into account, C14 dates became much more accurate and reliable. Back in the 1970s the age of the universe was thought to be about 15 billion years, but that has been revised to 13.8, and as new data comes in it may need to be revised again. This is why gravitational collapse of the universe has not been entirely ruled out, despite the advent of dark energy, which is thought to be accelerating the expansion when it should be slowing down. It is possible that like some other measurements in science, the dark energy measurement may also need to be revised at a later date.


If light has no mass, why is it affected by gravity? Light doesn’t accelerate in a gravitational field, which things with mass would do, because light has a universally constant velocity. Why is that exception?

Another way to answer this question is to apply the Equivalence Principle, which Einstein called his "happiest thought" (so you know it has to be good).  The equivalence principle says that if you are in an enclosed box in the presence of what Newton would call a gravitational field, then everything that happens in that box must be the same as if the box was not in a gravitational field, but accelerating upward instead.  So when you release a ball, you can imagine the ball is accelerated downward by gravity, or you can imagine everything but the ball is accelerated upward, and the ball is simply being left behind (which, ironically, checks better with the stresses you can easily detect on every object around you that are not present on the ball, including the feeling you are receiving from your bottom right now).

Given that rule, it is easy to see how light would be affected by gravity-- simply imagine shining a laser horizontally.  In the "left behind" reference frame, we see what would happen-- the beam would start from a sequentially higher and higher point, and that raising effect is accelerating.  So given the finite speed of light, the shape of the beam would appear to curve downward, and the beam would not strike the point on the wall of the box directly opposite the laser.  Therefore, this must also be what is perceived from inside the box-- the beam does not strike the point directly across from the laser (as that point is getting higher then the point across from it where the light was emitted), and its path appears to curve downward.  Ergo, light "falls."

Indeed, this is the crucial simplification of the Equivalence Principle-- you never need to know what the substance is, all substances "fall the same" because it's nothing happening to the substance, it is just the consequences of being "left behind" by whatever actually does have forces on it and is actually accelerating.

Incidentally, it is interesting to note that even in Newtonian gravity, massless objects would "fall the same" as those with mass, but to see it requires taking a limit.  Simply drop a ball in a vacuum, then a lower mass ball, then a lower still mass.  All objects fall the same under Newtonian gravity.   So simply proceed to the limit of zero mass, you will not see any difference along the path of that limit.  Nevertheless, Newtonian gravity doesn't get the answer quite right for the trajectory of light in gravity, because Newtonian physics doesn't treat the speed of light correctly.

If light has no mass, why is it affected by gravity?

Because light has a wave nature, and a gravitational field is a place where the speed of light varies. So light curves downwards. It's rather like the way sonar waves tend to curve downwards in the sea:

Image from FAS and the US Navy, see course ES310 chapter 20

  Light doesn’t speed up its acceleration, which things with mass would do, because light has a universally constant velocity. Why is that exception?

That's not right I'm afraid. See what Einstein said:

1912: “On the other hand I am of the view that the principle of the constancy of the velocity of light can be maintained only insofar as one restricts oneself to spatio-temporal regions of constant gravitational potential”.

1913: “I arrived at the result that the velocity of light is not to be regarded as independent of the gravitational potential. Thus the principle of the constancy of the velocity of light is incompatible with the equivalence hypothesis”.

1914: “In the case where we drop the postulate of the constancy of the velocity of light, there exists, a priori, no privileged coordinate systems.”

1915: “the writer of these lines is of the opinion that the theory of relativity is still in need of generalization, in the sense that the principle of the constancy of the velocity of light is to be abandoned”.

1916: “In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity”.

1920: “Second, this consequence shows that the law of the constancy of the speed of light no longer holds, according to the general theory of relativity, in spaces that have gravitational fields. As a simple geometric consideration shows, the curvature of light rays occurs only in spaces where the speed of light is spatially variable”.

Einstein also spoke of “the refraction of light rays by the gravitational field”. So did Newton, see Opticks query 20: “Doth not this aethereal medium in passing out of water, glass, crystal, and other compact and dense bodies in empty spaces, grow denser and denser by degrees, and by that means refract the rays of light not in a point, but by bending them gradually in curve lines?” It really is a refraction, and gravitational lensing is an appropriate phrase. Also see the GR section of Is The Speed of Light Everywhere the Same? It’s a PhysicsFAQ article by editor Don Koks. He talks about Einstein and refraction, and says this: "light speeds up as it ascends from floor to ceiling, and it slows down as it descends from ceiling to floor; it's not like a ball that slows on the way up and goes faster on the way down". Isn't that interesting?

A lot of people will tell you that light curves "because it follows the curvature of spacetime", but that isn't right either. Spacetime curvature is associated with the tidal force, not the force of gravity. See my "physics detective" articles on the speed of light and how gravity works for details and references.


How to create 3D animation of particles position vs time in python? I am trying to make a 3-D animation of stars' position vs time. I have the information of the stars' x, y and z co-ordinates over time. I was looking at the example of Matplotlib and did understood what I need to do but I don't want the connecting lines between different times. All I need is to show the position of the stars at different time and make a movie of it. This is the example I was looking at

https://matplotlib.org/gallery/animation/simple_3danim.html

After that I want to make it a bit complicated. I have the stars size and mass as well. I was wondering if I can change the size of the circles in the plot according to the radius and use different Colors for different mass range?

After that I want to make it a bit complicated. I have the stars size and mass as well. I was wondering if I can change the size of the circles in the plot according to the radius and use different Colors for different mass range?

That part has been addressed in previous questions and answers here, I remember reading maybe a year ago (+/- 0.9 years).

Here is something you may find helpful reprinted from my answer in Stack Overflow. It's based on astronomer and data scientist Jake VanderPlas' blogpost from 2013.

For your needs keep the dots but not the lines. What I recommend is that you start from this and get as far as you can, then ask a question in Stack Overflow showing what you've tried and explaining what you still need.

  Here is the Lorenz attractor both in 3D and animated. The script is in the following link (along with many goodies) in Jake VanderPlas' Pythonic Perambulations. You can learn a lot by going line-by-line through the script - it's an elegant use of matplotlib objects.

  https://jakevdp.github.io/blog/2013/02/16/animating-the-lorentz-system-in-3d/

  I added these two lines just before return in the animate function, and then used ImageJ to import the "image stack" and save the "animated GIF":

fname = "Astro_Jake_" + str(i+10000)[1:]
fig.savefig(fname)

  Note: For OSX it seems to be necessary to set blit = False in animation.FuncAnimation.

  Here is a minimal, simplified example of plotting lines in 3D based on the above:

def lorentz_deriv((x, y, z), t0, sigma=10., beta=8./3, rho=28.0):
    """Compute the time-derivative of a Lorentz system."""
    return [sigma * (y - x), x * (rho - z) - y, x * y - beta * z]

import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
from scipy.integrate import odeint as ODEint

x = np.linspace(0, 20, 1000)
y, z = 10.*np.cos(x), 10.*np.sin(x) # something simple

fig = plt.figure()
ax = fig.add_subplot(1,2,1,projection='3d')
ax.plot(x, y, z)

# now Lorentz
times = np.linspace(0, 4, 1000)

start_pts = 30. - 15.*np.random.random((20,3))  # 20 random xyz starting values

trajectories = []
for start_pt in start_pts:
    trajectory = ODEint(lorentz_deriv, start_pt, times)
    trajectories.append(trajectory)

ax = fig.add_subplot(1,2,2,projection='3d')
for trajectory in trajectories:
    x, y, z = trajectory.T  # transpose and unpack
    # x, y, z = zip(*trajectory)  # this also works!
    ax.plot(x, y, z)

plt.show()

I wrote this before I knew of PIL. Apparently you can use PIL to generate your GIFs instead of using an external program like I did.

See

Creating GIFs in Python Using Pillow (PIL Fork)
https://pythonprogramming.altervista.org/create-an-animated-gif-with-pil/
https://pythonprogramming.altervista.org/png-to-gif/

and have fun!

An example of a GIF with stars in it from the first blogpost (Nyan Cat):

Hi I did manage to write the code. Here is what I did

import numpy as np
import matplotlib.pyplot as plt
import mpl_toolkits.mplot3d.axes3d as p3
import matplotlib.animation as animation
import pandas as pd

df1=pd.read_pickle("properties.csv")
x=np.array(df1['x'])*3.24078e-17
y=np.array(df1['y'])*3.24078e-17
z=np.array(df1['z'])*3.24078e-17

data=[]
for i in range(len(x)):
    blank=[]
    for j in range(len(x[i])):
        dummy=[]
        dummy.append(x[i][j])
        dummy.append(y[i][j])
        dummy.append(z[i][j])
        blank.append((dummy))

    data.append(np.array(blank))

def animate_scatters(iteration, data, scatters):
    for i in range(data[0].shape[0]):
        scatters[i]._offsets3d = (data[iteration][i,0:1], data[iteration][i,1:2], data[iteration][i,2:])
    return scatters

def main(data, save=False):
    fig = plt.figure()
    ax = p3.Axes3D(fig)

    # Initialize scatters
    scatters = [ ax.scatter(data[0][i,0:1], data[0][i,1:2], data[0][i,2:]) for i in range(data[0].shape[0]) ]

    # Number of iterations
    iterations = len(data)

    # Setting the axes properties
    ax.set_xlim3d([-10, 10])
    ax.set_xlabel('X')

    ax.set_ylim3d([-10, 10])
    ax.set_ylabel('Y')

    ax.set_zlim3d([-10, 10])
    ax.set_zlabel('Z')

    ax.set_title('Nuclear Star Cluster')

    # Provide starting angle for the view.
    ax.view_init(25, 10)

    ani = animation.FuncAnimation(fig, animate_scatters, iterations, fargs=(data, scatters),
                                       interval=50, blit=False, repeat=True)

    if save:
        Writer = animation.writers['ffmpeg']
        writer = Writer(fps=30, metadata=dict(artist='Me'), bitrate=1800, extra_args=['-vcodec', 'libx264'])
        ani.save('3d-scattered-animated.mp4', writer=writer)

    plt.show()

#print data[1]
main(data, save=True)

Now my problem is my data shape is changing with time, the number of particles is getting reduced. How do I incorporate that in the code?


What would a gas giant consisting of only earths atmosphere look like? Putting aside how such a planet could be formed and just lets assume it exists.

What would an gas giant "look" like if it just consisted of the gas compound earths atmosphere consists of.

Could we just watch through it?
Would we see a burning core and therefore see something like a dim star?
Or would there be still causes that would block vision, even if there were no other components included?

And would such a planet be able to hold it self together?

Jupiter's atmosphere, if you could see through it, the inside of the planet would begin to glow visibly red from heat perhaps a couple/few thousand miles below it's upper clouds.

Source

The problem is, some two thousand miles of atmosphere above that visible glow is almost certainly well past the point of transparency.   Add to that, mixing and cloud formation would make seeing through all that impossible - no matter what the atmosphere was made of.

Hot gas rises and as it rises, some gas condenses from gas into liquid droplets (clouds), but these clouds form in updrafts so even though they start falling back towards the planet, the thick atmosphere and the updraft movement maintains the clouds for some time.   There's different layers of clouds too.   Jupiter's upper most clouds are Ammonia, but below that there's water clouds (and perhaps others).   go deep enough you might get sulfur clouds (I'm not sure) or more dense hydrocarbon (gasoline like) clouds.

An atmosphere that thick maintaining transparency is impossible.   Hydrogen and Helium are (to my knowledge) just as transparent as Oxygen and Nitrogen, maybe more, but the mixing and condensation gives Jupiter a constant cloud cover, different colors mostly represent different temperatures and depths from which that gas came from.

If you gave Jupiter an Oxygen-Nitrogen atmosphere but maintained it's inner core, the inner hydrogen would rise and mix with the Oxygen-Nitrogen.   Chemistry would inevitably happen and the Oxygen would bind with the hydrogen and make water.   Some of the Nitrogen would probably form Ammonia and in the end, Jupiter might not look that different.   Initially it might have a bluish tint, but only temporarily.  It would likely always have clouds and mixing some heavier/darker gases from below.   You'd never see it's core, no matter what.

Now if you had a planet with only one gas in it's atmosphere and entire makeup - avoiding any chemistry, say a planet made entirely of hydrogen or entirely of Argon or Oxygen - then you might get specific colors relating to the specific element.   But such planets are pure fantasy.

My best guess is that Jupiter's appearance wouldn't change much if you gave it an Earth-like atmosphere, but Jupiter is basically made of gas, so if you did that, it would just mix and it's inner hydrogen would rise to the top and Oxygen-nitrogen would sink.   It's the trace gases that give Jupiter it's color anyway, not it's hydrogen.

Layman's answer.  :-)

Jupiter's atmosphere is mostly Hydrogen, lying over liquid Hydrogen, on top of metallic Hydrogen, and finally at the center a large rocky core.  As you remove the hydrogen atmosphere the liquid would turn to gas, metallic to liquid, and on and on until all the hydrogen were gone and you have a massive core left approximately 12 to 45 times the mass of Earth.  (Not much is known about the core.)  So if you had only the rocky core, cooled down from it's current temperature of 36,000K, and added a nitrogen/oxygen Earth like atmosphere, you could see the bottom in an Earth like way.

It would not be a dim star, though it would glow very hot until it cooled, and also the planet would hold together.


What causes the Earth to have gravity? I've always wondered how our planet Earth or any other large spheroid-shaped thing in our universe has gravity?

Does it have a small black hole on its core causing it to have gravitational force?

Take our planet as an example: what causes it to have gravity?

Any massive body (i.e., a body having mass) like the stars, planets, moons, and any atom in the universe have, what is called, a gravitational potential which results in its gravity.  This is described by Newton's Law of Gravity. Why masses cause gravity, we do not know.

So, in short, the earth's mass causes it to have gravity.

The earth is made from MANY particles, these particles get something called mass, from a thing called the higgs feild, this mass is how much gravity something has, the earth has its mass, and mass warps space time, like when you push your finger on your bed, it creates a curve that goes down, it happens for space, and things get attracted to earth.
The exciting thing is that everything has mass! Even us, but our mass, like some tens of KGs, are too small to create a gravitational pull, to pull other things, even just a centimeter away.