Is there a plausible causal mechanism to explain why the Sun would be heating up faster/ sooner than we thought? Question for a novel I am writing a novel about what it will be like on earth 500 or so years before the death of our Sun. The Sun is heating up gradually and one day that will negatively affect life on Earth. I know that we are a long ways away from the heat being to much to permit life though. That being said, the novel is more about the social/political/religious ramifications of living in such a time. I still am very interested in making it as relatable and understandable as possible.

Is there any causal mechanism (hypothetically or imaginatively possible) that could cause the sun to heat up faster than it is at present? This could be internal (something collapses and the increase in core pressure speeds up the reaction rate of hydrogen fusion) or external (something impacts the sun with enough extra mass to increase net g, thus increasing pressure in the core).

If not a causal mechanism, maybe something you can think of that current science/ scientists could have missed/ aren't taking into account in their models that, if true, would mean the sun is going to be much hotter, much sooner than we thought.

Thank you so much for your help! I am pretty passionate about writing and I am only an amateur in the subject of astronomy. I'm hoping some brilliant minds can help me flesh out this idea to make it more concrete.

The Solar Neutrino Problem

  The solar neutrino problem concerned a large discrepancy between the
  flux of solar neutrinos as predicted from the Sun's luminosity and
  measured directly. The discrepancy was first observed in the mid-1960s
  and finally resolved around 2002.

  The flux of neutrinos at Earth is several tens of billions per square
  centimetre per second, mostly from the Sun's core. They are
  nevertheless hard to detect, because they interact very weakly with
  matter, traversing the whole Earth as light does thin air. Of the
  three types (flavors) of neutrinos known in the Standard Model of
  particle physics, the Sun produces only electron neutrinos. When
  neutrino detectors became sensitive enough to measure the flow of
  electron neutrinos from the Sun, the number detected was much lower
  than predicted. In various experiments, the number deficit was between
  one half and two thirds.

The problem is practically solved today. It wasn't so until $\approx$ 2002. However, a little chance that we know something not enough well, still exists, but it is little.

The currently accepted solution of the problem is this: yes, we know the Sun enough well, but the neutrinos can change flavor on the way they reach us. Thus, for example, if an electron-neutrino becomes muon-neutrino, then the detectors capable to see only electron-neutrinos, won't detect it.

It is today a hot research topic in the particle physics, thus some surprising result in the future isn't yet closed out. The main problem is that the neutrino practically don't interact with anything (a light year of lead would stop half of them), so there are only very little possibilities to measure their properties.

The KATRIN neutrino experiment

  KATRIN is a German acronym (Karlsruhe Tritium Neutrino Experiment) for
  an undertaking to measure the mass of the electron antineutrino with
  sub-eV precision by examining the spectrum of electrons emitted from
  the beta decay of tritium. The core of the apparatus is a 200-ton
  spectrometer. In 2015, the commissioning measurements on this
  spectrometer were completed, successfully verifying its basic vacuum,
  transmission and background properties. The experiment began
  running tests in October 2016. Regular measurements started
  2018-06-11, with a projected duration of 5 years.

As far I know, too much surprising results aren't expected from this - most likely, the electron neutrino mass of some tenths of eV will be found, or possible the result will be that the experiment is not enough precise to find anything. (In this case, we will still get an "upper limit").

However, there is a little chance that some surprising result will happen.

For example, such a neutrino mass could be found, which can be explained only by some hyphotetical effect (like sterile neutrinos, which might also contribute to the solution of the dark matter problem).

The idea

Go back to the 80ties, where a possible solution to the solar neutrino problem was a different working and internal structure of the Sun, which looks the same for us, but generates lesser neutrinos. By analogy, a similarly different Sun should be expected, which goes into a red giant phase soon. (Or, at least its luminosity will increase enough in the near future to make the Earth inhabitable).

Note, we also know a lot of other stars today, there are very good models running on big computers, and these are enough sophisticated to close this possibility out with a good probability. Thus, the Sun should be somehow exceptional on these results.


What will be the end of the universe? I am looking at this video about the timeline of the future.
It says that the universe will stop expanding in about 1 googol years and become a cold bubble but I am not sure that will be true. There are several theories that can be true, so I am not sure which one. Anyone have some proof that one will be true? Thanks!

There are several theories about the future of the universe, but I don't think we can be completely certain about any of them. The favourite at the moment, and probably the one with the most evidence to support it, is the dark energy universe which will go on expanding for ever, with the galaxies getting further and further apart and more isolated until they burn themselves out into total darkness. This raises problems about a unique, one-off beginning, with no cause, and vast quantities of matter and energy created magically out of nothing, thus violating the First Law of Thermodynamics.

My preferred option is the Big Bang-Big Crunch-Big Bounce universe in which the expanding universe is brought to a halt by gravity and collapses on itself to form a blue-shifted, shrinking universe which ends in a fireball and singularity similar to the one it was created from. Except it doesn't end. It erupts again (the Big Bounce) in a gigantic white hole which we call the Big Bang, and this cycle is repeated endlessly. It had no beginning, it will never end, and it doesn't violate the Fist Law of Thermodynamics.


Could a Jovian or Saturnian moon keep its atmosphere if the system was within the habitable zone? Currently the atmosphere of Titan is 1.5x thicker than Earth's atmosphere, but it's also much much colder. It's been said that an astronomical body can keep it's atmosphere if the escape velocity is more than six times the average speed of the molecules.

Does this mean if a moon like Titan was moved to the habitable zone and heated up to livable temperatures, then it's atmosphere would escape?

What is the minimum gravity necessary for a moon to hold onto an earth-like breathable atmosphere within the Habitable Zone?

To your first question... from this paper, we can see that there are 3 types of atmospheric escape:

Jeans Escape: Temperature and escape velocity factors determine the gasses and the amount that's lost. Using the chart provided in the comments, and moving Titan along the x-axis to around below Earth's position, we can see Titan will lose most gasses except Xenon.
Charge exchange: Solar radiation creates electrons and positively charged ions in upper atmospheres by tearing electrons off atoms or molecules. Titan is protected ~95% of the time by Saturn's magnetic field, and the other 5% of the time Titan's atmosphere is magnetized, creating an artificially induced magnetic field for a short period of time. This prevents a massive atmospheric loss. Thus, if Titan is placed in an orbit similar to Earth's Titan will lose a lot of atmosphere due to the solar wind.
Vertical atmospheric escape/impact erosion: Energetic objects that strike a planet erodes its atmosphere by creating a plume of heated gas. If Titan is struck, if will lose a considerable amount of its atmosphere. However, this has little to do with your scenario, so let's not consider this.

So we can see if Titan is moved into the Habitable zone, it will lose a considerable portion, or possibly almost all, of its atmosphere.

Regarding your second question, the major factor to atmospheric loss in that scenario, for a small(relatively speaking) celestial body in the Sun's habitable zone is the solar wind. If we look at the inner planets, we can see that they all have some form of magnetic field. Mercury has a relatively strong one; Venus has an weak, induced one; Earth has a strong magnetic field; Mars has remnants of its dissipated one. The magnetic field provides protection to the planet's atmosphere.

Titan does not have a magnetic field. If placed in the Sun's habitable zone, a celestial body would have a hard time satisfying your criteria. So then, let's suppose the moon we put in the Sun's habitable zone has a magnetic field likewise to that of Earth's.

In the habitable zone, we can say that the average temperature of the body would be around 300 K, similar to Earth's. Then, let's define that an Earth-like atmosphere would be comprised of at least nitrogen, oxygen, H2O vapor, and CO2. Using the same chart, we can start by moving Titan to below Earth, and then moving it up to where H2O vapor can be retained. We find that a celestial body with an Earth-like magnetic field can retain an Earth-like atmosphere if its escape velocity is around 6~8 km/s.

No, it doesn't mean that its atmosphere would escape, though some of it would because of Titan's lower gravity. Titan is unlikely to move into the habitable zone, but if it did it would keep most of its atmosphere for billions of years. Earth has kept most of its nitrogen for billions of years, and though the escape velocity of Titan is less it could hold on to nitrogen for a very long time. The trace of H2 and He4 in its atmosphere would not last long.

The habitable zone is not just Earth's orbit. Mars was once habitable apart from lack of oxygen, and so was Venus with the probable exception of its equatorial zone, which would have been too hot. Both planets have held on to their CO2, and so would Titan. There would be some modifications for a while if Titan moved into a Martian orbit. The methane would soon be reduced, the now underground ocean of ice would melt, subterranean deposits of frozen CO2 would evaporate and the resulting greenhouse effect would make Titan warmer than Mars, just as ancient Mars was warmer than the Mars of today. Water vapour would become an important atmospheric constituent and increase the greenhouse effect.

Similar considerations apply to Ganymede and Calisto, except that they would be very short of nitrogen. What use all this speculation is, I'm not sure, seeing that it is never going to happen.


What keeps galaxies together? The super massive black hole at the centre of Milky Way has a mass of about 4 million times that of our sun. Is this enough to keep the entire galaxy together, is it this black hole which keeps the galaxy together? I did read that there are also galaxies that don't have a super massive blackhole, what keeps those galaxies together?
I did ask this question at space stackexchange but it got closed because it was posted at wrong place.

The galaxy is kept together by the combined mass of the matter in the galaxy, of which the supermassive black hole is a negligible part. There are galaxies that don't have a central black hole (such as the Triangulum galaxy), and they are also held together by their combined mass.
In particular, the dark matter of the galaxy is what provides most of the mass that holds the visible matter together. Galaxies usually have far more dark matter than visible matter. This mass is distributed in a roughly spherical shape, with the greatest density towards the centre of the galaxy, and dropping off further from the centre. This "dark matter halo" is substantially larger than the visible disc.

Galaxies are actually moving, space between galaxies increment proportionally to the distance beteween them, the universe is in expansion and it may be by effect of dark matter. Some phisics say that.


What would change if Venus rotated at a speed and direction similar to Earth? I am interested in two aspects of this question. Imagine that Venus had a similar rotation speed and the same direction of rotation that we have here on Earth.

What would change for Venus itself? I'm interested about changes in the core and mantle behaviour, a potential magnetosphere, changes in the atmosphere and tectonic plates.

What would be different for the other planets near the Sun? I'm interested about changes in gravitational resonance and potential orbit changes.

I'm interested about changes in the core and mantle behavior, a
  potential magnetosphere, changes in the atmosphere and tectonic
  plates.

A lot of this is very theoretical.   Venus' atmosphere is nearly 100 times as massive Earth's atmosphere.  It's also nearly all CO2, a greenhouse gas, which means it's good at trapping heat.   That wouldn't change if Venus' rotation rate was faster.

"Core and mantle behavior" is a bit of an unusual statement, cores rotate with the planet and they slowly cool, there's some convection vs conduction going on, but it's difficult to study the cores of other planets.  It's hard enough studying the core of Earth.   But what we know of the core of Earth, or think we know is that Earth's core and Earth's crust rotate at slightly different rates.  This may be due to Earth's crust slowing down over time and the core, not slowing down as fast.   There's a slight variation in rotation rate.   Also, Earth's inner core is solid, Earth's outer core is somewhat liquid (though extremely viscus), and as it cools, the inner core slowly grows, the outer core shrinks and in the process heat is released.   This process creates a flow, which may be partially influenced by the Moons orbit, and together the circulations are thought to create Earth's magnetic field.   This isn't certain, but the flow of the outer core is thought to be the cause and Earth's rotation may play a role in that.

So if Venus rotated much faster, in combination with it's proximity to the Sun and the solar tidal forces, it's possible a faster rotating Venus would have a permanent magnetic field.   It's far from certain, but that's a possible outcome.

Plate tectonics is more tricky.  Various studies suggest that planetary size has a lot to do with plate tectonics, and abundant water plays a role too, oceans making plate tectonics easier.   Venus is on the small size and it has almost no water, so it's not a good candidate for plate tectonics.   Venus also has very little granite.    This is geology more than astronomy, but abundant granite (IMHO) plays a role in plate tectonics.   The rocky bodies in our solar-system.   Mercury, Venus, Mars, the Moon, are all, basically basalt like on their surfaces.   Earth formed large granite land masses that float above the more dense basalt like layers under the oceans.   Without granite land masses and without oceans, I don't think Venus is a good candidate for plate tectonics.

Did Venus ever have plate tectonics, like billions of years ago?   Maybe.   I have no idea, but maybe.   Would it have plate tectonics if it rotated faster?   I doubt it.  A magnetic field - maybe.  Plate tectonics, I don't think so.

This question is probably too general for stack exchange and it may be closed, but I don't believe that just spinning Venus faster would change all that much apart from maybe giving it a magnetic field, and that wouldn't change all that much on Venus. It has an induced magnetic field anyway.

If you could give Venus an ocean, maybe make it more massive and move it further away from the sun, then some interesting things might happen like plate tectonics, which could have lead the planet down a different path, perhaps more similar to Earth.

Earth was able to absorb CO2 into it's vast oceans and take in CO2 by photosynthesis and rocks were able to absorb CO2 by plate tectonics, and Earth was far enough from the sun to not have it's surface water evaporated and lost like what probably happened to Venus.

I think planetary formation, based on size, type of star, temperature, initial materials, etc, will be a very interesting field of study when/(if) big enough telescopes are set up to actually get a look at distant planets.   It's very difficult to even observe, much less, study a planet that's light-years away.   The so far twice or three ties delayed James Webb Space Telescope might get the ball rolling on this subject with better observations of exo-planet atmospheres, but it's a long ways away from being well understood.  The James Webb ST is an enormously difficult undertaking, so we should be patient.  With luck, it'll be up and taking in images within a few years.

Finally, would Venus' rotation, if it was faster, affect other planets?    Basically not at all.   Jupiter's rotation doesn't affect Earth.   Rotation rate doesn't affect other objects at great distances.

I don't think anything substantial would have changed. Four billion years ago, Venus was similar to Earth but hotter. It had seas, and like Earth, a very substantial atmosphere composed mainly of CO2. The rotation rate was probably not quite the same as it is today, but for your scenario let's suppose it was once every 24 hours. Owing to its greater proximity to the sun and the greenhouse effect of a CO2 atmosphere with a strong flavouring of methane, regardless of a 24 hour rotation rate the seas would have eventually evaporated away, increasing the greenhouse effect. Meanwhile, outgassing from volcanoes would have added more CO2 and sulphur dioxide to the toxic mix, forming sulphurous acid  in combination with the water vapour. At a guess, I would think this took about a billion years, perhaps slightly faster than an earth-like rotation would have done. Venus's 8-month rotation rate would have meant that one side of the planet was exposed far longer to the sun's rays than was the case on Earth, so it also had very long nights.

The situation today is that despite the very long days and very long nights, the atmosphere is so thick that there is no significant difference in temperature between the night side of the planet and the day side. Had it been rotating every 24 hours, there might have been a temperature difference between night and day for the first few million years, but this would have decreased as the greenhouse atmosphere built up to become a more effective blanket. In other words, had Earth been 30 million miles closer to the sun, the same sort of thing would have happened here, so the answer to your question is that a more rapid rate of rotation wouldn't have made much difference.


How do we know that our galaxy is a spiral galaxy? I know that our galaxy is spiral in shape, but I'm wondering how the scientists found out that our galaxy has a spiral shape.

I don't think we can see the entire galaxy from telescopes on Earth, right?

I think it makes sense that they say that Andromeda has a spiral shape because we can see the whole galaxy, but how do scientist know about our galaxy?

To elaborate on Conrad Turner's excellent list:

The disk structure is even more easily seen in the infrared, where dust extinction is much less than in the optical. See for instance this image from the 2MASS infrared survey:

Stars are not really (as Milan van Dijck suggests) used for mapping the spiral arms. Rather the gas clouds of gas in between stars are used, more specifically neutral hydrogen observed through its  21 cm observation. The figure below (from Oort 1959) shows the resulting map. As you can see, its not really as well-defined as seen in many artist's-conception pictures.

Neutral hydrogen is fuel for stars. Massive stars ionize the gas around them, and since star formation mainly takes place in the spiral arms, the gas clouds are accompanied by pockets of ionized hydrogen, as seen in this map from Georgelin & Georgelin (1976):

The physical properties of the Milky Way (stellar population, gas fraction, color, kinematics, etc.) are also similar to other spirals.

You can see that the milky way appears as a stream across the night sky. We can calculate the distance between us and the stars in this stream, therefor we also know the distance between those stars (by using the Pythagorean algorithm for example). Because we now know the distance between them, we know their relative position of each other (by triangulation).

If you would then construct a 3D model by using these relative positions it would look like a spiral.

Edit: Here's a Kahn Academy video explaining how distance from earth to a star is calculated: Stellar distance using parallax


Does matter accelerate to the speed of light as it approaches the singularity? $$v_\mathrm e=\sqrt{\frac{2GM}r}$$

If we understand the escape velocity to be the speed needed to escape from the 'surface' of a gravitational object described by the above equation.

It is equally true that the escape velocity is the speed an object pulled from rest will reach when it hits a certain point in the gravitational field.

Since the event horizon is defined as when the escape velocity reaches the speed of light for a black hole, would matter accelerate near the speed of light as it approaches the singularity? Accretion disks are known to move at relativistic speeds near the event horizon, but perhaps oddities occur beyond this point?

The event horizon can be derived from setting the escape velocity of an object to the speed of light, it is a point of no return. I understand there are better definitions, but this is not really germane to the question at hand. How about from the perspective of another in-falling observer?

There is indeed a sense that infalling objects accelerate to the speed of light as it reaches the event horizon, but you need to be careful what you mean by the speed of the object because the speed is observer dependent.

This is explained in detail on the Physics site in my answer to Will an object always fall at an infinite speed in a black hole? An observer watching from far from the black hole would see the falling object initially accelerate towards the black hole but then decelerate to a halt at the event horizon. However an observer hovering a distance $d$ above the event horizon would see the falling object pass them at a speed:

$$ v = c \sqrt{\frac{r_s}{r_s + d}} \tag{1} $$

where $r_s$ is the radius of the event horizon. As the distance above the horizon $d$ goes to zero the speed calculated from equation (1) goes to the speed of light $c$.

Does matter accelerate to the speed of light as it approaches the singularity?

In a way. But let's get rid of that word "singularity", and replace it with "black hole". Yes, falling bodies fall faster and faster. As you know, falling bodies do not slow down. Taken to the limit, the falling body would be falling at the speed of light.

  Since the event horizon is defined as when the escape velocity reaches the speed of light for a black hole

Where did you get that definition from? I don't recognise it. Let's just agree that the event horizon is the location from which the upward light beam cannot escape.

  Would matter accelerate near the speed of light as it approaches the singularity?

Yes, but again let's replace singularity with black hole. As the falling body approaches the black hole, it falls faster and faster. Eventually it approaches the speed of light. But there's a catch, and it is a whopper.

  Accretion disks are known to move t relativistic speeds near the event horizon, but perhaps oddities occur beyond this point?

Perhaps. However we don't know what happens on the other side of the event horizon. But we do know that oddities happen on this side of the event horizon. Such as gamma ray bursts.

  The event horizon can be derived from setting the escape velocity of an object to the speed of light, it is a point of no return...I understand there are better definitions, but this is not really germane to the question at hand. How about from the perspective of another in-falling observer?

See what Einstein said in 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”. The falling body is falling into the black hole because the speed of light is reducing. It falls faster and faster, until at some point it's falling at the local speed of light. Then it erupts into a gamma ray burst.

For some strange reason not many people know about this. But see the 2013 AMPS paper an apologia for firewalls. Tucked away in the conclusion is footnote 31, containing a reference 87 to Friedwardt Winterberg’s 2001 paper gamma ray bursters and Lorentzian relativity. Winterberg talks about the direct conversion of an entire stellar rest mass into gamma ray energy. See the Wikipedia gamma ray burst article and note that “a typical burst releases as much energy in a few seconds as the Sun will in its entire 10-billion-year lifetime”. I think this is the hard scientific evidence that Winterburg is correct, it ties in with what Einstein said about the speed of light, and it clears up the issue wherein falling bodies are said to accelerate to the speed of light and decelerate to a halt.


Need help finding a star I've tried utilizing "Google Sky", however, once I've found the location of said star and am in the process of zooming in, the site craps out.
Anyway... if there's someone out there with a little more experience in this field, here are the coordinates of the star i'm searching for:
10h 10m 11.1s, +4 3' 12.3"

I'd appreciate the effort, and thank's in advance.

As far as I can see, this is just a faint star. Here is some data from the USNO B1.0 catalogue:

USNO B1.0 star

B1.0 ID: 0940-0181715

RA: 10h10m11.13s +/- 69 milliarcseconds

declination: + 4 03' 12.3" +/- 195 milliarcseconds

Epoch: 1977.9

Proper motion in RA:  12 +/-       2 milliarcseconds/year

Proper motion in dec: 6 +/-       6 milliarcseconds/year

Photometry:

Band mag   Cal Survey  Emulsi field  S/G   xi    eta

B1: 14.36   1  POSS-I  103a-O 550     4   -0.05    0.03

R1: 13.33   1  POSS-I  103a-E 550     1    0.05    0.01

B2: 14.07   2  POSS-II IIIa-J 782     9    0.09    0.26

R2: 12.79   2  POSS-II IIIa-F 782     8   -0.07   -0.02

I:  12.37   2  POSS-II IV-N   781         -0.04   -0.30

"Cal" = calibration: 0 means calibrated via bright standard stars on
the plate; 1 = via faint standards on the plate; 2 = faint standards on
adjacent plate; 3 = faint standards two plates away,  etc.
  "S/G" = star/galaxy index. 11=object looked starlike, to 0=fuzzy/galaxy-like.
  xi, eta = displacement of the star's position on this plate,  in
arcseconds,  relative to the mean solution.

This USNO-B1.0 data was downloaded from a VizieR server.

Up to my understanding these are 2 angles, One is vertical one(Where parallel to ground is zero, perpendicular towards sky is 90) and One is horizontal one (Compass, where north will be considered as zero/360).

10h 10m 11.1s, +4 3' 12.3"(Vertical, Horizontal)

First one , 10h 10m 11.1s. It's hour min sec format. Where 1 hour is 360(total degrees)/24(number of hours in a day)=15 degrees. for 1 minute, 15(degrees per hour)/60 (no of minutes in an hour). same for the second.
So that this angle will come around ~150+2.5+ (second's part).

second one, +4 3' 12.3". This one is degree min sec. Most probably this is compass angle. where north might be zero and clockwise is +ve. representation may vary, but if you'll check the scale you'll be able to figure out that.

Hope this will help you.if not feel free to ask more.


Why type Ia supernova reaches its peak absolute magnitude(~ -19.3mag) at around 15-20 days after the explosion? I'm not sure why type Ia supernova reaches its peak magnitude 15-20 days after the explosion. Is this luminosity generated only by the radioactive decay of nickel-56 to cobalt-56?(considering a half-life, I don't think the decay to iron-56 can be involved with it) Or is there any combination that produces the peak luminosity?

The early light curve of a Type Ia SN (i.e., the fact that the brightness increases to a peak, then declines) is the result of the combination of two things, both of which are decreasing over time: energy from the decay of Ni-56 to Co-56 (and Co-56 to Fe-56), and the opacity of the explosion as it expands.

Just after the actual explosion, the expanding fireball is dense and opaque. Only light from the very outermost layer escapes to the outside.[*] Since the Ni-56 created in the thermonuclear burning that caused the explosion is in the interior regions, the initially visible part of the expanding fireball has little or no Ni-56, and thus no energy from its decay.

As the fireball expands, it gets less dense, and thus less opaque, to the point where light from the layers heated by radioactive decay can actually escape. Over time, more and more of the inner part of the fireball becomes visible, and so we see more and more of the regions where Ni-56 (and its daughter product Co-56) is decaying. Roughly speaking, the peak of the light curve is when the opacity is low enough that all the light from the Ni-56 + Co-56 decay can escape.[**]

But since there are fewer and fewer radioactive nuclei as time goes by, the amount of energy produced by the decay decreases with time. This is why the light curve turns over: now we're seeing all the light produced by Ni-56 and Co-56 decay, but that just keeps getting fainter over time.

[*] At this point, the SN is too faint to be seen. Even the initial thermonuclear explosion is too faint to be seen under most circumstances.

[**] Ni-56 and Co-56 actually produce gamma rays (and energetic positrons) when they decay, so what "the light from Ni-56 + Co-56 decay" means is thermal radiation from the gas heated by the gamma rays. Also, as I understand it, what we're seeing is really the combination of energy from current decay and residual heat from previous radioactive decay.

It has to do with the Chandrasekhar limit, and how all type 1a supernova are created from the destruction of a white dwarf. It takes the same amount of time to hit absolute magnitude because they convert the same amount of mass.


Why is this moon red? I wish I could attach a good quality photograph of this red color moon I'm looking at right now.

I searched on internet but there was no lunar eclipse scheduled to occur on 23rd December 2018

in  HYDERABAD, INDIAN SUBCONTINENT around 7:20pm
Please tell me why the moon is red tonight:) ?

First of all there could not be an eclipse on December 23rd because that is one day later than the full moon.

There can be a couple of reasons for the moon looking red or orange:

It is near the horizon, and being viewed through the atmosphere over a large city.  Air pollution (for example from cars, trucks, fires etc) push small sooty particles in to the air.  These particles scatter blue light more than red, so there is less blue light coming towards you directly from the moon.
Sometimes sand from deserts can get blown up into the atmosphere, and it can get carried a long way.  The sand has a similar effect to the soot.

During a total lunar eclipse, Earth completely blocks direct sunlight from reaching the Moon. The only light reflected from the lunar surface has been refracted by Earth's atmosphere. This light appears reddish for the same reason that a sunset or sunrise does: the Rayleigh scattering of bluer light.


Spiralling in Galaxy evolution Could some stars be orbiting galaxies in outward spirals instead of perfectly round circulation? Patterns of matter movement relate strongly to the galaxy centre. A special relationship between the galaxy and the central object suggest the centre may be an origin point in 'outflow'. The BH central object behaves strangely by instigating the release outwardly of massive amounts of matter. This may form stars that spiral out across the galaxy. The grand design of features in galaxies may related to general outward spiralling.

Basically, yes, it's possible.  Our moon is slowly moving away due to energy transfers from Earth related to tidal drag.  This is very slow spiral.

I do not know whether a star can experience sufficient tidal drag relative to the center of its galaxy to cause a change in orbit - and there are a few billion other objects in the galaxy producing gravitational force in other directions, making the analysis kind of messy.


Relation between phase and magnitude Given the phase of a planet or satellite, I can find the area illuminated visible to us, if we consider a 2D surface. But how do I find the percentage of illuminated area visible considering it a 3D surface and thus finding the magnitude of the partially illuminated planet/satellite.

In general, you'd have to project the light from the sun (Assuming we're talking about objects in our solar system) onto the 3D surface and project it onto a 2D surface. This can be accomplished via a ray tracing algorithm.

This procedure will give you the fraction of the surface illuminated.

Now if you are concerned about the fraction of the illuminated surface that is reflected in a specific directly -- so that it is "viewable" from that direction -- you have to do the ray tracing in 3D from the source (Sun) to the object and on to the viewer. There packages you can google that will do this type of ray tracing if you can model the objects in 3D...


What is the size of the largest asteroid orbiting the Earth, other than the Moon? It seems odd to me that no other asteroid around the Earth is even close to the size of the Moon. Anyone have any idea about the size and orbit radius of Earth's "second moon" ?

There are no other known natural satellites orbiting the earth. Most of the objects orbiting the earth are man-made and largest object other than the Moon orbiting the earth is the International Space Station which orbits the earth at a height of about 400km from the surface and is about 100m wide.

It may be odd to you that there are no other natural satellites orbiting the Earth. We are not sure but it maybe that the probability of an object ending up in a stable orbit around the Earth is not that high. There are roughly 3 different categories of natural satellites - Large moons in roughly circular orbit like the Moon, small rocks in low orbits formed as a result of collision and nearby asteroids that a planet catches in its gravitational well.

It maybe that there are Earth is not massive enough to capture fast moving asteroids in orbit. Or the Moon which is quite large relative to the earth destabilizes a satellite's orbit around the Earth and there are not many stable orbits that a random natural object can get itself in.

To the best of my knowledge there are no natural satellites of Earth other than the moon. If you know of one, it has only just been discovered, in which case it should feature on the TV news tonight, but I think more likely you are confused by something you've read in science fiction. Any newly discovered second moon would have to be very small, very far away and have a low albedo to have escaped detection for all these years.


Formation of elements in the Sun other than helium I just learned that the Fraunhofer lines of the Sun's spectrum indicate that the Sun contains various elements other than just hydrogen and helium (for example, Na and Fe) but don't the Sun's p-p chain reactions only involve hydrogen and helium? So where do these other elements came from?

The Sun is currently turning hydrogen into helium. There are no other nuclear reactions taking place at any significant rate in the Sun. The Sun will not start to make heavier elements until it reaches the tip of the red giant branch in about 7 billion years time.
The elements heavier than helium that are present in the Sun were almost all made inside other stars. These stars lived and died before the Sun was born 4.5 billion years ago. The nucleosynthesis products from these previous stars were recycled in stellar winds, supernovae, kilonovae and novae, and then mixed into the turbulent interstellar medium.

The act of fusion transfers part of atom into energy and the part left over is fused together with Hydrogen to make heavier and heavier elements.  Heavier elements create more pressure in the middle of the star which creates more and more heavier elements fusing the many nuclei together.  Once the sun starts making Iron(Fe), it dies and collapses which creates more elements.  Or explodes if a supernova and makes gold and such.  Don't know what a blackhole makes?  Maybe fuses time and matter together so it can't be separated from each other.


Sagittarius star cloud? I was using SkySafari with my phone, but the compass didn't quite work.
So as I knew where the cardinal points were, I think I have shot the Sagittarius star cloud, at least a part of it.

Can you guys tell me if I am wrong?

It was the 5th of July at 00:10 local time. I was at N 45.61°, E 7.35°.

The image scale is a bit small, but I can see the constellation of Lyra just above center. The bright star Deneb (in the constellation of Cygnus) is left of center, and the star Altair (in the constellation of Aquila) is at the bottom (just right of center). Thus, your photo does not include Sagittarius. Sagittarius is off to the bottom right. The Scutum star cloud is the bright patch toward the right corner.

It would be helpful if you include the time of the photograph and location (latitude and longitude).

You've definitely got the Milky Way here. The galactic centre in Sagittarius is slightly thickened and noticeably brighter than the rest of the Milky way, so I think you might have got it in the bottom right hand corner of your photograph.


What will happen to the shape of a galaxy when a super massive black hole lying in its center dies(evaporates out)? What's at the Center of the Milky Way?
 In this article it is said that a supermassive black hole lies in the center of milky way galaxy.

  At its center, surrounded by 200-400 billion stars and undetectable to the human eye and by direct measurements, lies a supermassive black hole called Sagittarius A*, or Sgr A* for short.
  The Milky Way has the shape of a spiral and rotates around its center, with long curling arms surrounding a slightly bulging disk. It's on one of these arms close to the center that the sun and Earth are located. Scientists estimate that the galactic center and Sgr A* are around 25,000 to 28,000 light-years away from us. The entire galaxy is around 100,000 light-years across.

We revolve around the center every 250 million years.Presumbably we rotate beacuse of the BH.

When the black hole dies in our galaxy will we be thrown out of the revolving orbit?

The shape of the galaxy is expected to change right?It will be some irregular shape not spherical?

Presumbably we rotate beacuse of the BH.

No. The galaxy is being held in one piece due to its own total gravity. The black hole is only a small fraction of that. Basically, the BH doesn't matter.

  When the black hole dies in our galaxy

The BH will probably be the last thing left of our galaxy at the end. And even then it will take some incredibly long time for it to evaporate. BH evaporation for very large BHs is basically the slowest process you could imagine.

  It will be some irregular shape not spherical?

The galaxy is not spherical. Its shape is rather more like a round disk (with some irregularities and some features like arms, etc).

The evaporation of the super massive black holes will take a billions of years and thus the gravitational attraction will get weaken in a very long course of time . This will result in  expansion of the the galaxy and all the stars system and gases will spread in the universe .
 But Hawking radiation is very slow process , even it is possible that till that time all the fuel of the stars will get burnt up (hydrogen) resulting into total darkness .


Whats the farthest distance a spacecraft has been detected (except by radio) As far as I understand it, one can not hide spacecraft, because the heat from the engine would be very visible.

Now I tried searching for the farthest distance an actual spacecraft has been detected besides its radio signals that are intentionally sent to us. Unfortunately I could not find any data on this.

OSIRIS-Rex has been spotted on its approach to Earth, at a distance of approximately 7 million miles (12 million kilometers) away, with a brightness of approximately 25th magnitude. The Large Binocular Telescope was used for this observation, this has a pair of 8.4-meter mirrors.

This is unfortunately a vague question.   If, for example, the spacecraft were reasonably large, you could detect it indirectly due to its gravitational pull on other (putative) nearby objects,  or even gravitational lensing.

So far as heat-sensing or not,  the spacecraft would have to be very cold indeed to not radiate in the microwave range at levels inconsistent with the cosmic background. How sensitive a radiotelescope would be needed is a separate issue.

And finally, if you had a really powerful lidar you'd easily "ping" the spacecraft -- perhaps vaporizing it in the process.


What is the effect of this expanding universe on the energy density of it? We know that our universe is expanding i.e.  volume of the universe is getting larger with time with total energy being conserved. My question is whether the energy density is getting smaller with time.
If this is true then what is the effect of this phenomenon on all the celestial bodies?

I am no expert and I am sure there will be a more rigorous answer later, but here my 2 cents:

It appears to me that the term "energy" and energy conservation becomes a bit more complicated in general relativity and cosmology, taking into account more abstract forms as the energy of the gravitational field or space curvature. As you seem to be more interested in the implications I just want to focus on the common forms of energy, i.e., radiation, matter and dark energy.

For a given expanding volume of space only the energy from matter (dark and normal) is dilutes as expected. You could say that with respect to expansion its energy is conserved. This means its energy density decreases with $\sim a(t)^{-3}$, where a is the scale factor (or just the diameter) of the universe.

Radiation additionally looses energy via cosmological red-shift, so its energy density decreases even faster with  $\sim a(t)^{-4}$.

Finally you have the dark energy density. What makes it special is, that its density does not depend on $a(t)$. One can think of it as the energy associated with space itself. So if the universe expands new space and dark energy "appears" and its density stays constant.

So over time the average (matter and radiation) energy density decrease and approaches the energy of empty space itself (or vacuum energy).

Note, however, that this is only true for the average densities. In regions of space, that are held together by gravity, as our local group, the matter density will not decrease. On the contrary, these region are still contraction due to gravity.

So to answer your question on the effect on all the celestial bodies:
Those which are gravitationally bound, won't notice much but others will drift apart never to see each other again.

The experts say that as fast as the universe expands the increased space fills instantly with countless trillions of virtual particles which arrive from nowhere. Not only does this contravene the 1st Law of Thermodynamics (mass/energy can neither be created nor destroyed),but there is a discrepancy of 40 orders of magnitude (yes,40 orders of magnitude!) between the vacuum energy density required by Quantum Mechanics and cosmological observations. It has no effect on celestial bodies, though it should have, which is where the conflict with cosmological observations comes in. Supposedly, the 1st Law of Thermodynamics doesn't apply on a cosmological scale.


How big would the asteroid belt planet be? As I understand it, the asteroid belt exists because the gravitational force of Jupiter prevents the asteroids from accreting (is that a word?) into a planet.

If, however, Jupiter didn't exist and they did create a planet, how big would that planet be?

The mass of the asteroid main belt is estimated at 4% the mass of our moon according to Wikipedia so any object formed from the aggregation of that mass would not be a planet.

It would be the size of a very small moon.

Even if all the asteroids in the solar system were combined, the total mass would be below a third of the moon's mass.

Just for clarification Ceres is not considered an asteroid, but a dwarf planet like Pluto. I think people confuse Ceres and call it an asteroid due to its orbit running in or bordering the asteroid belt.


How does a Super-Massive Black Hole 'flare'? This week we have read of evidence of the Super Massive Black Hole in the centre of the Galaxy 'flaring'.

My understanding is that light cannot escape from a black hole.

My question is: How does a Super-Massive Black Hole 'flare'?

The flare happens just outside the black hole.  Matter which aproaches a black hole but not perfectly straight on doesn't fall into it immediately. It gets pulled around the black hole, just as a satellite is pulled around the Earth.

If the object is a single small particle, that's almost the end of the story. Gravitational radiation and the precise effects of GR will cause its orbit to decay until it eventually falls in, but if it's small enough not much of anything is emitted in the process.

If there are lots of objects in the neighbourhood, or if your obect is something relatively large and diffuse (like a gas cloud, or a planet or star) on the other hand, different things happen. As gravity pulls on different parts of it with different strengths (tides) the object gets stretched and then pulled to pieces, and multiple objects collide and interact in all sorts of ways. Magnetic fields also start to play a role. Some of that matter escapes completely, but much of it ends up in a disk (a little like Saturn's rings, but less well organised) around the hole. By this point all the stretching and collisions have made it very hot, so it shines.

Now even within the disk there are collisions and friction, and they have the overall effect of heating the matter in the disk and pushing some of it out of orbit, so that it falls into the hole, getting even hotter in the process.

So finally we come to a flare. There are probably many kinds of flare-up in this basically unstable setup, but the most obvious is when a particularly large object or cloud of gas falls into the accretion disk, or when some instability in the disk causes a large amount of matter to fall out of the disk. Either way, the larger amount of hotter matter shines more, and we see a flare.

A supermassive black hole usually has an accretion disc composed of gas, dust and other debris attracted into close orbit. These materials gradually fall toward the black hole, becoming heated by compression and friction as they go. When a large mass of this matter is drawn to the inner edge of the accretion disc just before it disappears for ever into the black hole, the result is a flare up or rapid but temporary brightening of the accretion disc. The light doesn't escape from the black hole itself, but from regions just outside the black hole. There are also high energy particle jets accelerated by the black hole's magnetic field in precisely opposite directions by the black hole's magnetic poles, but again they are emitted before they enter the event horizon, not afterwards.


Is the newly discovered exoplanet 'Teegarden b' habitable? A new exoplanet has been just discovered around the star Teegarden, at only 12 light years away.

TLDR version: at this point there are too many unknowns to determine whether this planet candidate is habitable.

The discovery paper for this system, Zechmeister et al. (2019) "The CARMENES search for exoplanets around M dwarfs. Two temperate Earth-mass planet candidates around Teegarden's Star" is available on the arXiv. Table 1 lists the properties of the star and table 2 lists the properties of the planets.

The planetary quantities we have reliable estimates for are the minimum masses, the orbital periods and the distances. The eccentricities are consistent with zero so I'm going to assume for the moment that the orbits are circular. The reason that minimum mass is relevant is that this is an example of a radial velocity detection, which results in a degeneracy between the inclination of the planet's orbit to the line-of-sight and the mass of the planet.

Habitable zone

According to figure 13 in the discovery paper, Teegarden b is too close to the star to be in the conservative estimate of the habitable zone, though it is in the optimistic habitable zone. The Earth Similarity Index (ESI) appears to be a fairly misleading metric (I am of the opinion that ESI is a bad metric that should be abandoned), from the discovery paper:

  In addition, planet b receives nearly the same stellar flux
  as the Earth and therefore has a nearly identical equilibrium temperature, but is outside the conservative HZ. As discussed by
  Kopparapu et al. (2014), this is caused by a runaway greenhouse
  effect that is due to water vapour starting at lower insolation for
  low-mass stars. This effect is neglected in the ESI calculation
  and leads to the curious fact that planet b has a higher ESI value
  than planets inside the conservative HZ.

The position of Teegarden b therefore suggests it may have undergone a runaway greenhouse effect, though this might be avoided if it can build up enough reflective cloud cover on the daylight hemisphere, or if it is a desert planet without enough water to destabilise the climate.

Even if a planet is located in the habitable zone, it is not necessarily habitable. An obvious counterexample would be the Moon.

Terrestrial or not?

The next question: are these planets terrestrial? Unfortunately we don't know: Teegarden b does not transit the star, and there has so far been insufficient coverage of Teegarden c to say whether it transits. As a result, we do not know the radii of the planets so we cannot tell whether or not the planets have densities compatible with being terrestrial. The minimum masses are low enough that these planets do likely fall below the transition region between rocky and non-rocky planets suggested by Kepler data, see for example Rogers (2015) "Most 1.6 Earth-Radius Planets are not Rocky" - using the mass-radius relationships of Zeng et al. (2016) "Mass-Radius Relation for Rocky Planets based on PREM" the transition is somewhere around 3–5 Earth masses.

Another point of comparison here is the other known system of low-mass planets around an ultracool dwarf star: TRAPPIST-1. In this case, despite the small size of the planets, the inferred masses indicate the presence of substantial amounts of volatiles, e.g. Dorn et al. (2018) "Interior characterization in multiplanetary systems: TRAPPIST-1" where the volatile contents of the planets are estimated to correspond to a few percent by mass, compared with 0.02% by mass for the Earth. A planet like this would be very different to the Earth, in an optimistic scenario it might result in an ocean planet. The habitable zone for ocean planets would be different to that for Earthlike planets, since the negative feedback of continental weathering would not occur.

Red dwarf stars

Red dwarf stars have a variety of problems when it comes to habitability. One issue is that they have an extended high-luminosity pre-main sequence phase which might result in runaway greenhouse effects and/or severe water-loss on close-in planets, see Ramirez & Kaltenegger (2014) "The Habitable Zones of Pre-Main-Sequence Stars".

An additional problem is that red dwarf stars have severe stellar activity that would erode an Earth-like atmosphere in a very short amount of time, as an example Garcia-Sage et al. (2017) "On the Magnetic Protection of the Atmosphere of Proxima Centauri b" estimate an Earthlike atmosphere at Proxima Centauri would be lost in less than 400 million years. Teegarden's Star is fairly quiet for its spectral type, but it is an old star and was likely far more active in its youth.

These issues are not necessarily fatal, outgassing from volcanism might be able to replenish the atmosphere to some extent after the stellar activity dies down, or if the planet was born with a large amount of volatiles it might be able to emerge as a habitable planet. Unfortunately we don't have sufficient data on habitable zone planets around red dwarf stars to know for sure.

A further issue is that the atmospheric chemistry on planets around stars like Teegarden's Star would likely result in substantial quantities of carbon monoxide accumulating in the atmosphere, see Schwieterman et al. (2019) "A Limited Habitable Zone for Complex Life". This would be lethally unpleasant for any human visitors (or other organisms that use haemoglobin as their oxygen carrier) caught outside without an air supply. Note that haemoglobin is not the only possible oxygen carrier: some groups of life on Earth use alternatives (haemerythrin, haemocyanin) that are less vulnerable to carbon monoxide poisoning, as noted by Howell (2019) "Comment on "A Limited Habitable Zone for Complex Life" (Schwartzman et al. 2019 ApJ 878, 19)":

  In short, there is a notable diversity of extant invertebrates on this planet that would be quite comfortable outside of the range of CO concentrations proposed by Schwieterman et al. to constrain the habitable zone. There is no a priori reason to rule out the possibility that these lineages could have given rise to intelligent life during the history of life on this planet. A significantly higher atmospheric CO concentration would have prevented the evolution of hemoglobin, but not necessarily prevented astronomically observable biosignatures from Earth.

Summary

It is too early to be able to say whether the planets around Teegarden's Star are habitable. There are plenty of reasons to be sceptical but worlds like Teegarden b are still promising enough to be worth investigating in more detail. It is unfortunate that it is not a transiting planet, but similar systems like TRAPPIST-1 should be helpful in constraining what the possibilities are.

It is potentially habitable.

Teegarden b has a minimum mass almost identical to Earth and it orbits within the star's habitable zone.

The exoplanet has a 60% chance of having a temperate surface environment where
temperature should be closer to 28°C assuming a similar terrestrial atmosphere.

Moreover, it is the exoplanet with the highest Earth Similarity Index discovered so far: 95%. This means that it has the closest mass and insolation to terrestrial values.


Are the stars in constellation located in a plane or they are in different distances Are the stars in constellation located in a plane or they are in different distances from each other and when we observe them from the Earth we think they are all in a plane?

Constellations and asterisms are generally not proximate in space, but rather happen to be nearby only when viewed from Earth. From Wikipedia's article on asterisms:

  Like constellations, asterisms are in most cases composed of stars which, while they are visible in the same general direction, are not physically related, often being at significantly different distances from Earth.

A recognizable example of this would be the brightest star in Taurus, Aldebaran.  Aldebaran appears in the V-shaped bull's head, mostly formed by stars in the Hyades star cluster. However, while those stars are actually close to one another, Aldebaran is less than half as far away.

  Since the star is located (by chance) in the line of sight between the Earth and the Hyades, it has the appearance of being the brightest member of the more scattered Hyades open star cluster that makes up the bull's-head-shaped asterism; however, the star cluster is actually more than twice as far away, at about 150 light years.

Similarly, the stars of Orion's belt are actually hugely separated in space.

In addition, it's worth noting that the stars have different velocities across the sky, so the constellations will look increasingly different over long periods of time. Here is one prediction for 50,000 years time, and the effect will get proportionately larger as time goes on.

Star are far away from each other : many light years separate them but they appear to be in the same plane because from Earth, many light years away, we see them as if they are besides each other.

For example, take a person standing at 80m in front of you and another man 100m.
You see that both are standing besides each other and the one which is the farthest from you appears to be smaller than one nearby.
This same logic applies for the stars.


Why does the Hubble flow does not carry photons with it? I have this naive question. We know that the expansion of the universe "takes" or "carries" the galaxies with it, separating them, but this doesn't apply to photons. My guess is that it has to do with the fact that photons are massless (i.e. they always move with c), but I am not sure and would like a more physical explanation or insight.

Expansion isn't some mysterious force that "carries" galaxies. There's a very common misconception that it is, but it's not true. The expansion is just the recessional motion of galaxies and other matter.
Light does participate in the expansion. Even though it moves at a constant speed (and therefore can't move with the galaxies as you correctly observed), it still thins out with time. This is counterintuitive, but it happens even in a special relativistic cosmological model (the Milne model, which is a special case of FLRW cosmology). If you think of the big bang as an explosion of light bulbs, the bulbs that you see from a particular era in the universe's history have higher and higher relative speeds as time goes on (since the light from nearer, slower bulbs has already reached and passed you), and therefore are more redshifted, and therefore emit photons at a lower rate (since redshift affects the perceived rate of every process). So the relic photons from a particular era of the universe's history get more redshifted and less dense as time goes on.


Center of the universe Big bang is not an explosion, but an expansion of space and time. Universe had almost infinite density... wait a moment. If it had ALMOST infinite density, it had a certain volume, and thus, space already existed.
Am I commiting a mistake?

I had this question after reading this post.

Krotanix : nobody really knows the answer to this. We have good evidence that the universe is expanding, and we extrapolate this back to the big bang. Then some people say the universe was once the size of a grapefruit. See for example this. Since WMAP evidence in 2013 suggested the universe is flat, some people have started saying the observable universe was once the size of a grapefruit. And that the universe was always infinite. But this is a non-sequitur, and a non-answer.

Yes,  it looks as if 13.8 billion years space had a much higher density and started expanding. And it may have had a certain volume, so space may have already existed. But nobody knows for sure.

Re your follow-on question, nobody has discovered hollowness or emptiness anywhere in the centre of the universe. The expansion is thought to be like that of a raisin cake rising in an oven. There's no big hole in the middle.

You're right; space did exist. In fact, the universe was infinite in size at the moment of the Big Bang, and it still is. This is because a common misconception is wrong: The Big Bang did not happen at a point. Thinking of it like an explosion isn't correct.

You asked in a comment about the multiverse. This is speculative, to say the least. There are some theories involving objects called branes that posit that we are on the surface of some object floating through a higher dimensional "bulk". However, these theories of so-called large extra dimensions have yet to be supported by experimental evidence.


Is it possible that a satellite creates a forever eclipse? So, to add a bit of context, I’m doing the NaNoWriMo challenge, which consists of writing a novel of 50k words in one month. My whole idea is based around the following question: What would happen if the sun were to suddenly disappear?

To make it short, the story is about a colony on a faraway planet, which is settled for few generations already. And one day or let’s say, in less than a month, the sun disappear. So far, so good. Except I need a valid reason as to why the sun disappear… I have found some ideas (dust from volcanic eruption, change of orbit, dying sun, etc…) but none of them looked good enough. (I’m open for suggestion if any.)

I’m now investigating the possibility that the planet would gain a satellite big enough to hide the sun on the whole planet (like an eclipse but lasting forever). This topic lead me to think that if the satellite is in geostationary orbit, it is possible. But there is still some questions I can’t get my head around.

How fast a new satellite can “appear”?
Is it possible that a bigger but “lighter” (with less gravitational force) satellite can orbit around a smaller but “heavier” planet?
This topic reminded me that no sun means no heat. Can the geothermic activity of a planet be enough to prevent the surface from being impossible to live on? (Off topic.)

So, basically, the question is, if it's possible to have a forever eclipse, how ?

I’m sorry in advance if the questions are not clear enough, even though I understand the basics, I don’t necessarily have all the technical terms.

Thank you !!

The only potentially valid way to cause a long-lasting eclipse that I can think of is to have your moon orbit at the planet's L1 Lagrange point between the planet and the star (and the moon has to be the right size). If it had just the right orbit and was the right size, it could then cause a long-lasting eclipse. Note I'm not saying permanent because such a system wouldn't be permanent. I'm not going to run through the math$^1$ to determine if such a system is stable or for how long. I'd guess it may be stable for a few orbits at least.

The main problem is getting your moon there in the first place. One could conceivable say that by some freak chance a third fairly massive body came close to your planet/moon (either from outside the solar system or near the edge) and gravitationally perturbed your Moon's orbit such that it ended up in the L1 point. But the chances of such an event are so slim that it would be incredulous if it actually happened.

And just to add a technicality, once your moon moved to the L1 point, it would now be orbiting the Sun and wouldn't be a moon anymore according to the definition of a moon.

$^1$ Out of curiosity, I ran through some of the math very loosely and found that if you put the Moon in Earth's L1 position, the Moon survives for about 3-5 months before perturbations knock it out of the L1 position. Of course our Moon isn't the right size to cause an eclipse at that distance, but this gives a very rough estimate for how "permanent" the scenario I described above really is.

I'm afraid this hypothetical object blocking the sun would have to be artificial. If the object is in synchronous orbit, and if it is to perpetually eclipse the sun, then the planet would have to be tidally locked to the sun. If that's the case, then the object blocking the sun would have to be at the sun itself.
If you want to put an object in orbit around the sun that's large enough to block the sun's view from the planet, the object would have to have a delta-v lower than the amount required to sustain that orbit. Because of this, the object blocking the sun would require extra energy to sustain it's orbit in between the planet and the sun.

To answer the three questions,

I guess it depends on the civilizations technological ability to detect it. Theoretically, nothing can truly be hidden in space. There should always be some method to detect anything, up until the inverse square law diminishes the information to nothing.
Yes. Mercury is smaller than Callisto, but has more mass than two Callisto's combined.
Depends on the kind of planet. If it is very volcanic, maybe? Anyhow, I wouldn't think it would last long. There's a lot of surface area on a planet to radiate heat out into space. It would get really cold, really fast.

Edit: I forgot about the Langrange points. The object could orbit on the point between the star and the planet, but I'd suspect the orbit would not be very stable for such a large object. At least, on a cosmic timescale.


How do I see the milky way? I've travelled to farms at night, and away from civilation. I don't really know alot about astronomy, but my goal is to see something like this:

Stars, lots of them, but that "thick purple cluster of gas right in the center". I want to see that.

I assume that on a very very dark day, we would be able to see this, but sometimes I only see stars, just lots and lots of stars, something like this:

I want to see this "thick cloud cluster, which I assume is the milky way". How do I see this? How on some days do I only see just stars and stars, when I really want to see this main beauty?

"thick purple cluster of gas right in the center". I want to see that.

First off, the human eye is a wonderful device, but it's no match for modern cameras. We can't see colors at such low levels of light. We're all colorblind in those conditions. Only cameras continue to see color when illumination is low.

As a result, when looking at objects such as nebulae or galaxies, including our own, all you see is a black-and-white image. Even with telescopes, the human eye still doesn't see colors in these objects.

There is exactly one single exception - the Orion nebula, which is by far the brightest of all. Some observers, after a bit of practice, can begin to distinguish a few shades of color there.

Planets, and some stars, are different. These objects are much more bright. The human eye has no trouble seeing color where color exists here. E.g. Jupiter is a slight shade of yellow, Betelgeuse is quite clearly red, etc.

  purple

I'm pretty sure the Milky Way is not purple. That's probably just a processing artifact. Go on Astrobin and search for "milky way" for a lot more examples - they tend to not look very purple usually.

https://www.astrobin.com/

  I want to see this "thick cloud cluster, which I assume is the milky way". How do I see this? How on some days do I only see just stars and stars, when I really want to see this main beauty?

There are two main obstacles to doing astronomy at the bottom of an ocean of air: atmospheric turbulence (a.k.a. "seeing") and light pollution.

Seeing affects observing high resolution objects such as planets and double stars when seen in telescopes. It's caused by air eddies and vortices that muddy the image. It's of no interest here so we can ignore it for this discussion.

Light pollution affects observing faint objects such as galaxies (including our own), nebulae, and star clusters. It's caused by light from artificial sources (city lights, industrial lights, cars, etc) going up, being reflected on particles in the air, then going back down. It makes the sky seem to "glow". It drowns out those objects which are not very bright - and galaxies tend to not be bright.

There's only one way to avoid light pollution: go far away from human settlements and facilities. The desert and the open sea are not light polluted.

Google "light pollution map" for some websites that will help you choose a dark site. Here's one example out of many:

http://www.jshine.net/astronomy/dark_sky/

Recently I've visited Death Valley. The surrounding area has essentially zero light pollution. Even driving 30 minutes out of Ridgecrest you're already under a very dark sky. I took the opportunity to show my kids how a dark sky really looks like.

The Milky Way is very clearly visible. Wherever you look, there are stars upon stars. Take a pair of binoculars, point them at random places in the sky, and even more stars are visible. There are star clusters that you could chance upon just by sweeping the sky with the binoculars, and they look like someone threw a handful of diamonds on black velvet. Little shooting stars are visible often. It's extraordinary.

It should be noted that the Milky Way is not visible during the winter months (December – February). It's best to try to watch it in June – August. Even later in the year (September – November) it's still visible in the West, just before it sets, a short time after nightfall. If you're an early riser, you can see it before daybreak in the spring (March – May).

You need to wait until the sky goes completely dark after sunset. That means 1.5 hours after sunset, or more. To determine the precise moment of sunset, go on Google or Wolfram Alpha and search for "sunset today". Then add 90 minutes to the result.

You will also need to time your visit to a night when the Moon is not visible. The Moon must be below horizon, otherwise it would cause some "light pollution" of its own. A few days before and after the new moon are the best time for this.

You could go on Wolfram Alpha and search for "next new moon" or even "new moon in september" and it will give you the date. Several days plus or minus that date should be fine.

http://www.wolframalpha.com/

TLDR: A moonless night in the summer, in a desert or some other unpopulated area, 1.5 hours after sunset or later.

Darkest skies I've seen were in the outback of Australia, perhaps 20km from the nearest (small) town.  Still couldn't see purple clouds though.  The naked eye view was more like your 2nd image.


Types of Multiverses Can anyone recommend me books or other online resources on the concept of the Multiverse, it types, and about higher mathematical dimensions of space-time. I have been interested in such topics for a while but have not found any accessible book that could explain them in a way that's comprehensible to me. (I'm new to the subject of astrophysics, just getting started).

What is the numerical value of $c$, the SR does not say that. It is a global parameter of our Universe, we need to measure it and substitute it into the SR formulas.

There are also more trivial global parameters. Like the dimension of the spacetime. With (space) dimensions more or less than 3, the formulas of the classical mechanics would be the same.

All the essays talking about "multiverses" or "alternate Universes", are playing with the consequences of the current theories, but substituted with different global parameters than we have now. We could imagine, they are playing with alternative Universes.

However, here we have two major problems.

The logical problem. What does "Universe" mean? The word has latin root, "uni versum" means "to contain everything". The "Universe" is the thing what contains everything. How can multiple be existing then?

For example, the sci-fi topos on which a parallel 3D spacetime referring as "another Universe". No, it is not "another Universe". If there would be proof that such a thing exists, then "our Universe" and this "alternate Universe" would be the "Universe" on its original meaning. We could refer the alternative as an "other spacetime" or "parallel space" or so.

We have also a major problem, what is science. If we can access the parallel 3D space, i.e. if we can interact with it, then it is not "another Universe", it is just a newly found region of ours. But what is no interaction is possible between them and us? Then they are not existing. They might be existing like that our thoughts exist. In any case, it is not physics/astronomy any more, it is phylosophy in the best case.

Professional scientists generally get itchy if they need to answer not science questions. It is because you are assuming rhat they know it, while they know that it is not even science. For example, how hot is a 10 solar masses supernova, that is their field. Thus, your question will be likely closed as unclear, because it is.

What you can find:

You can find popular science texts about playing the current theories, but with different global constants. For example, "what if speed of light would be different" will give you likely useful google hits.
General relativity has some interesting topological extensions. For example, the Kerr-solution looks as if rotating black holes would bind different Minkowski-spacetimes together and so.
There might be interesting hits for googling for "string theory landscape".

In any case, make it more clear, what do you understand on "multiverse" or so, use science terminologies for that. You can find these terminologies by googling forums.

Probably the most popular google keywords will change some decade later.


Compared with similar stars, does the sun itself have properties that make life on Earth possible? We often hear our solar system has properties for life. For example, it is far away from the center of Galaxy. It has Jupiter far away to protect us from asteroids. It has optimum size of territorial planets, plate activity on Earth, and so on.

But besides that, compared with similar stars, does our sun itself also have properties that make life possible? For example, does it have extra stable brightness? Or does it emit less high energy radiation?

One thing that is, if not unique, unusual about the Sun is the size of it.
It belongs to the top 5% brightest stars, meaning that 19 out of 20 stars are smaller than the Sun.

Having a Jupiter size planet is certainly not unique, and plate tectonics we know nothing about for exoplanets.

Well our sun was not unique 11 billion years ago, Because almost every star in the Universe were all created at once so they all went through Stellar evolution all at once also.Our Sun is 4 billion years old and in a childhood stage compared to the rest of the Stars in the Universe.Our Star systems was one of the last formed from the Big Bangin so only a few stars are in a similar stage as ours as of right now. There still is a few that haven't formed yet, but right now our Sun is a rarity because of it's age and stage giving it a uniqueness in the present Universe.


Blowing up an asteroid/comet really potentially worse? Often on television shows and in articles I see it mentioned that it'd always be bad to blow up an asteroid or comet because then the energy would just be spread out and cause even more damage.

According to some estimates I've seen around 100 tons (or more) of meteoroids hit the Earth each day. If all of this were combined together into a single asteroid, it could destroy an entire, large city.

Given that alone, it makes it seem to me that it be more logical to take the chance and blow up an asteroid and thereby trim its weight down, causing a lot more to be more easily burnt up on entry, so that when it did hit, it would cause less damage.

Is this logical at all? If my science/math/physics is incorrect, I want to understand why having more burn up by spreading out is worse compared to having it more concentrated and vastly more dangerous.

Well there are some things to consider. Initially if you could make sure that after you blow up an asteroid you will end up with numerous but small enough pieces so that they will either: one, burn up in the atmosphere or two, be headed away from Earth (and not hitting us five years later) then we are OK, and blowing up the asteroid with a missile would be a fair solution.

The problem here lies in the fact that we know little about the internal composition of asteroids in general, and presumably even less about a particular one, so it is very hard to predict exactly where the pieces of the asteroid generated by an impact are or aren't going to end up or, be headed towards or even its size.

Another scenario could be that if you effectively smashed the asteroid into small pieces that could then burn into the atmosphere, and if those pieces were coincidentally to end up being consumed by Earth's atmosphere, it would heat up provoking presumably an unpleasant day on Earth of course depending on the mass of the object.

But there is a much better solution than that Armageddon-Hollywood inspired one. It is call gravitational tethering. There is something we know, and we know very well about asteroids, and that would be their trajectory paths or orbits. Even when a new asteroid is discovered, its orbit can be computed pretty quick and with great accuracy (because we know the solar system's gravity very well). So if an asteroid is to impact Earth, it is likely that we will know with years, probably decades in advance. And so we can just send a space vehicle (called gravity tractor), with enough mass and time in advance, and place it just beside the asteroid, hence allowing us to tilt its orbit by just a tiny amount, due to the gravitational pull between the two objects. Now when you consider the effect of that tiny amount in the long run, it effectively deflects the path of the given asteroid from that of the Earth so that it won't hit us 20 or 30 years later.

And this is something we have control over, and something we can predict with great accuracy. It is the (safe) way to go.

If you are still not happy with my answer, you can listen to Neil de Grasse Tyson himself explaining it in this 5 min video.

Also check out this talk from the American Museum of Natural History on "Defending Earth from Asteroids" LINK

Further reference here.

The OP is obviously correct.  If you knew an asteroid was headed for Earth and had the choice of either doing nothing (and letting it strike) or cutting it in half and letting both halves strike, you would split it.
Nuking it into a million pieces would spread out the impact, which would make the collision less dangerous.  Would you rather get shot in the chest with a bullet or 100 BBs?


Does more ocean on an Earth analog produce a different rotation period? Would an exoplanet that has more ocean than Earth rotate at a different speed as a result of this? Would the amount of water impact the weight, gravitational pull, and/or tidal forces and cause a difference in the exoplanet's rotation period?  (For simplicity's sake, say, if it was earth-like in every respect other than having more ocean than Earth does at about 70% of the surface area.)

Would an exoplanet that has more ocean than Earth rotate at a
  different speed as a result of this?

Basically no.   Rotation speed, or angular velocity doesn't measurably change with your proposed example.

To explain this in more detail, there's two important concepts.  First, is Angular Momentum, (Short explanation or longer) and the 2nd concept is Moment of Inertia.

The basic formula is that Angular velocity (time for 1 full rotation) = Angular Momentum divided by Moment of Inertia.  Formulas and explanations are in the link(s) above, and if I was to explain it, it would get wordy, but your question seemed to be more general, less about doing the mathematical calculations, so I'll skip the formulas.

In nutshell, talking about a planet, the state of matter doesn't affect the angular momentum.   Angular momentum is conserved and that, divided by moment of inertia determines rotation speed.  Now if you get something spinning fast enough the angular rotation can overwhelm the gravity and when this happens, the planet can begin to fly apart, which happens more easily with water than a rocky surface which has some cohesion, but ignoring crazy super fast rotations, a water world, a deep ocean world, a shallow ocean world and a rocky world all obey the the angular velocity law and composition doesn't matter.  The angular momentum is conserved.  The moment of inertia of a planet can change but for the most part, doesn't change much.

The Earth's moment of Inertia, for example, changes as glaciers grow or shrink, or when there's an earthquake.   Even, every time we a tall building is built the Earth's moment of inertia increases a teeny tiny bit, similar to a skater extending their arms to slow down coming out of a spiral.

When there's an earthquake which, for the most part, settles the Earth, there's a small increases in the Earth's rotational speed.   The total angular momentum and total mass remaining the same but shifting of material changes the moment of inertia.   (Granted space dust and tidal effects change the Earth's moment of Inertia, but quite slowly).

  Would the amount of water impact the weight, gravitational pull,
  and/or tidal forces and cause a difference in the exoplanet's rotation
  period?

This is harder question to answer precisely because adding water changes the mass of the planet and changing the mass changes the moment of inertia, but sticking to the principal of your question, there's no measurable effect.

Lets take a somewhat simpler example without changing the planet's mass.  Ice ages.  When the Earth is in an ice age there's less liquid oceans and more ice at the poles but the total mass is unchanged.   More mass at the poles and less mass in the oceans decreases the Earth's moment of inertia because the bulk of the Earth's moment of inertia is around the equator,  so, as a result, the Earth rotates slightly faster during an ice age and slightly slower after an ice age.   Over time, the Earth's crust has a tendency adjust for this effect but that takes tens of thousands of years.  Parts of the Earth's crust is still rebounding from the last ice age.

Gravitational pull isn't relevant.  Neutron Stars with enormous gravitational pull can rotate very fast and the planet with the fastest rotation in our solar-system is Jupiter and the one with the slowest rotation is Mercury.   Angular velocity has no direct correlation to mass or gravity though there is an indirect correlation.   As a star, for example condenses it's rotation speeds up, because the angular momentum is conserved but as it settles the moment of inertia decreases.   That's why young stars, White Dwarfs and Neutron stars can spin very fast.

Tidal forces can create drag on rotation but the effect is slow, taking millions or hundreds of millions of years.  With enough time, tidal forces cause a planet or moon to stop rotating and become tidally locked but there's no short term affect.  (I'll say a bit more on this later).

So, a planet with large oceans wouldn't rotate any slower than a planet with no oceans because liquid or solid can have equal angular velocity, but over time, tides will slow a planet with oceans more quickly than a planet without them.

Because the Earth has oceans, the Moon's gravitation on the Earth's tidal bulge does slow down the Earth's rotation, but this has been happening for 4 billion years and the Earth still rotates every 24 hours - one of the faster planets.   If the Earth had more water the Moon's tidal tug would slow the earth down a bit faster, but it would still be very gradual.

Jupiter, which is basically a ball of gas, is the fastest rotating planet and Mercury, basically a rock, the slowest, so those are 2 examples of composition not being a factor, though Mercury's slow rotation is in large part due to the strong tidal forces it receives from the Sun.

Now, I Understand the logical approach to your question, as there's something apparent about water resisting rotation - touched on in this question, but the fact that water doesn't spin with a glass when you spin a glass is an example of conservation of angular momentum, not an argument against it.  On a planet, the oceans are rotating with the planet and the angular momentum is already there.

Hope that wasn't too long, but that's the gist of it.   I can try to clean up or clarify if needed.

I would assume so seeing as how it would change the density

If it is exactly like earth but has a crap ton more of an ocean- it is perceivable that it's (average) density would be lower,
Also, minutely, it would compress this water more and heat it and make something of an atmosphere given that it doesn't escape. Trapping more and more energetic particles into a thick foggy atmosphere  can change the rotation so slightly it will probably never be noticed.....

Also, if it has a moon, tides/ waves due to gravity can also minutely push the planet in a different state of rotation. Very slightly

I read it wrong, but I'm keeping that in case it helps.

It really depends on how much more water there is. If it was 100% coverage, it would be more "tidy" and the ocean would certainly be more compact (same mass as earth, but add A HUGE amount of water to that to get from 70% to 100% coverage of the area)-- tides may not be as wild. I suppose it would effect the rotation period, but not much.

Note, it might be easier to post this on physics stack exchange, you get better answers, much sooner. This seems like a good question to get up votes off of...


What is the highest angle the moon ever makes above the horizon at the North Pole? The question is in the title. I'd like to find out the maximum angle the moon ever makes above the horizon at the North Pole. By "the horizon at the North Pole," I mean the tangent plane to the earth at that point.

Note that I'm not asking for the highest angle that the moon ever appears to make above the horizon. Appearances are distorted by refraction, etc., although I'm not sure by how many degrees. Though I'd be interested in the exact distortion: is there a data repository where I can look up the highest observed angle?

I would make the following prediction. The moon's orbital plane is inclined to the earth's equatorial plane at about $23.5+5\approx 28.5$ degrees. Let $R_e$ denote the radius of the earth and $R$ denote the radius of the moon's orbit around the moon. A little geometry for the highest point suggests

$$\theta_{\text{max}}=\tan^{-1}\left(\frac{R\sin(28.5^{\circ})-R_e}{R\cos(28.5^{\circ})}\right)$$

But because $R>>R_e$ (about 385,000 compared to 6300 km), $\theta_{\text{max}}$ should be just a little less than $28.5^{\circ}$.

Is this right?

${R_e}/{R}$ is about 0.017 radian or 1°.
Using your trigonometry and these values:

Lunar orbit inclination = 5.15° (varies between 5.0° and 5.3°)
Lunar perigee = 362600 km, apogee = 405400 km
Earth polar radius = 6357 km
Ecliptic obliquity = 23.44°

I get 27.7° at perigee and 27.8° at apogee.
At that altitude, atmospheric refraction is only about 0.03°.
The highest apparent altitude I found with JPL HORIZONS was 27.91° on 1987-09-15 around 17:15 UT.

Due to nodal precession,
the Moon doesn't reach ±28.6° geocentric declination every year, only near a major lunar standstill every 18 or 19 years, e.g. in 2006 or 2025.
Near a minor lunar standstill, e.g. in 2015, the Moon's declination is limited to ±18.3°.

One should consider the problem as relative position between EARTH and MOON only, irrespective of EARTH's tilt w.r.t. ecliptic. I am not able to put up a sketch here but would explain as--
Thus if we consider the geometry between EARTH and MOON, then as per my calculation as :
The C to C distance  = 383,000 km @ 5.145 deg inclined, Horizontal distance will be = 383000 x COS( 5.145) = 381,452 km. AND the vertical distance above north pole = 383,000 x SIN ( 5.145)- 6317 (radius of EARTH)- 1737 (radius of MOON) = 26,338 km.
Thus highest inclination above NORTH POLE tangent plane  is
Arc TAN ( 26338 / 381452 ) = 3.944 deg.


When will all eight planets in our solar system align? Ignoring expansion of the universe, entropy, decaying orbits, and interference from any bodies colliding with or otherwise interfering with their orbits, will the eight planets known planets in our solar system ever align?

What is the "period" of the planets; how often would they align perfectly? And based on their current positions, how far off into the future is their next theoretical alignment?

This is a low accuracy - yet simple - answer

It allows you to calculate only radial alignment configuration of the planets.

If you would like an approximation, let's say, you approximate the position of the planets as hands in a clock, you could work the math out by something like this.

Assume $\theta_i$ is the initial angle for planet $i$ at time $t_0$ - measured from an arbitrary but fixed position, and $l_i$ is the length of the year - in days - for planet $i$.

Then it resumes to solving this system of equations:

$$
x \equiv \theta_i \left( \ mod\ l_i\right)
$$

From here you would then simply apply the Chinese Remainder Theorem.

Finding the minimum x, will give you the angle that the planet that at $t_0$ had angle $\theta_i = 0$ would have travelled until an alignment configuration was reached. Asuming you choose Earth as the mentioned planet, then divide that angle by a complete revolution ($360^{o}$) and you will get the number of years for that configuration to be reached - from the $t_0$ configuration.

The different $\theta_i$ in degrees for all the planets on 01 Jan 2014 - you can use this as your $t_0$:

\begin{align}
Mercury &\quad 285.55
\\Venus &\quad 94.13
\\Earth &\quad 100.46
\\Mars &\quad 155.60
\\Jupiter &\quad 104.92
\\Saturn &\quad 226.71
\\Uranus &\quad 11.93
\\Neptune &\quad 334.90
\end{align}

Source

The different $l_i$ in days for all the planets:

\begin{align}
Mercury &\quad 88
\\Venus &\quad 224.7
\\Earth &\quad 365.26
\\Mars &\quad 687
\\Jupiter &\quad 4332.6
\\Saturn &\quad 10759.2
\\Uranus &\quad 30685.4
\\Neptune &\quad 60189
\end{align}

Finally under an integer values aproximation and using this online solver for the system of equations the answer is $x = 4.0384877779832565 \times 10^{26}$ which divided by $360^{o}$ gives you roughly $$1.1218 \times 10^{24} \quad\text{years}$$

Edit 1

Just found this site you may like to play around with. It's an interactive flash application with the accurate position of the planets.

I also know that the all the information can be obtained from this NASA page and that is as accurate you can get, but it is just incomprehensible for me now. I will try to revise it later when I find time.

Also this book by Jean Meeus called Astronomical Algorithms covers all the fundamental euqations and formulas - it has nothing to do with programming algorithms though.

Edit 2

Seeing that you are a programmer, it could be worth for you checking out the NASA site I mentioned above, the data for all the planets can even be accessed via $\tt{telnet}$.
Or this Sourceforge site where they have implementations for many of the equations described in the book also mentioned above.

Technically the true way to find the period between alignment of all 8 planets is to find the LCM of all 8 of their year lengths.

LCM (88, 225, 365, 687, 4333, 10759, 30685, 60189) = 814252949520007202031000.
I understand that this is a rough estimate since these are rounded to the nearest integer, but it gives a good idea of the number of days it would take.

814252949520007202031000/365 = 2230829998684951238441. That's how many years.


Can life on earth be supported by only a Red Dwarf star or a White Dwarf(state still hot not cooled enough yet)?(Provided it is near to the star) Can a red dwarf star or a white dwarf have enough energy to support life on earth? How it will change the climate and the seasons of earth if it is our only star? (energy source)(Expecting it to change since earths orbital mechanics will also change)

Certainly a red-dwarf star can have enough energy for a planet around it to be in the goldilocks zone.   There are some difficulties with red-dwarf stars and Earth like planets.   The planet would need to be very close to the star and as a result, tidally locked.   The orbital period would be quite short, so there would be no seasons and one side of the planet would be in permanent sun the other side in permanent shade.

Red-Dwarf stars, unlike our sun, grow colder with time and they can be quite magnetically active, sometimes called flare stars, though not all red-dwarf stars are flare stars, many start out that way.   The proximity to that much solar-ejection could strip planets of their atmosphere early in the planet's life.

None of that makes it impossible.  It's certainly possible for a Red-dwarf star to host an Earth-like planet, but there's some difficulty.  Orange or Yellow stars are, on average, considered better candidates.

A white dwarf, however, likely not possible.   A white dwarf follows the red-giant stage of a star.  It would be too small and give off too little energy and any planets that survived the red-giant phase would likely be very distant.   Life and/or an Earth-like planet around a white dwarf is virtually impossible.

It is possible that the red dwarfs can support life, but we would need a strong magnetic field and a path that would take us closer and closer because that dwarf will cool overtime. A white dwarf would not be possible because they are the remnants of dead stars, so Earth would not have survived the blast (the other planets wouldn't have either).