The application of physical theory to celestial systems such as stars, planets, galaxies, supernovae, and black holes. Astrophysics proper is concerned with explaining phenomena more so than making observations, the latter falling under the purview of astronomy.

learn more… | top users | synonyms

42
votes
2answers
3k views

Neutrinos vs. Photons: Who wins the race across the galaxy?

Inspired by the wording of this answer, a thought occurred to me. If a photon and a neutrino were to race along a significant stretch of our actual galaxy, which would win the race? Now, neutrinos ...
14
votes
4answers
463 views

What mechanism is responsible for the creation of these dunes on Comet 67P/Churyumov-Gerasimenko?

What mechanism is responsible for the creation of these dunes on Comet 67P/Churyumov-Gerasimenko? The following high resolution picture from ESA's Rosetta mission shows the dunes: At a distance ...
8
votes
1answer
376 views

Has Voyager 1 entered a solar radiation belt?

The Voyager 1 probe was sent out in 1977 to go where no man made object has gone before, after more than 35 years it’s still going strong. It’s now 124 AU away from Earth and many are wondering when ...
18
votes
3answers
2k views

What stabilizes neutrons against beta decay in a neutron star?

Free neutrons are known to undergo beta decay with a half-life of slightly above 10 minutes. Binding with other nucleons stabilizes the neutrons in an atomic nucleus, but only if the fraction of ...
13
votes
5answers
7k views

The final death of a black hole

What are the different death scenarios for a black hole? I know they can evaporate through Hawking radiation - but is there any other way? What if you just kept shoveling more and more mass and ...
11
votes
3answers
631 views

Evidence for black hole event horizons

I know that there's a lot of evidence for extremely compact bodies. But is there any observation from which we can infer the existence of an actual horizon? Even if we are able to someday resolve ...
9
votes
4answers
8k views

Size of universe after inflation?

Wikipedia states the period of inflation was from $10^{-36}$sec to around $10^{-33}$sec or $10^{-32}$sec after Big Bang, but it doesn't say what the size of the universe was when inflation ended. ...
5
votes
7answers
1k views

Is there a black hole in the centre of the Milky Way?

Is it true that the whole galaxy is actually revolving, and powered by a black hole? Has it been proven, and if it is true, how can our solar systems actually keep up the momentum to withstand the ...
15
votes
4answers
553 views

Are Neutron stars transparent?

Neutrons have no charge so they would not, I think, interact with photons. Would a neutron star be transparent?
12
votes
1answer
366 views

Do celestial objects experience drag from the near vacuum of space/does the near vacuum have a mean velocity?

For instance do the planets around the sun experience drag from the near vacuum of space? Or do the (hydrogen) atoms in interplanetary space have a mean velocity near orbital speeds, such that object ...
11
votes
5answers
450 views

Interstellar dust/matter distribution

It is known that one of the main problems of interstellar flight is a presence of matter between stars in form of very fine dust and huge asteroids. Which can slowly (or fast) destroy any ship. What ...
4
votes
5answers
666 views

If the size of universe doubled

My question is silly formulated, but I want to know if there is some sensible physical question buried in it: Suppose an exact copy of our universe is made, but where spatial distances and sizes are ...
2
votes
1answer
261 views

When would the proposed black hole at the centre of Milky Way gulp in our solar system? [duplicate]

I've heard and read that our solar system lies near to the peripheral region of the Galaxy. Then accordingly we would have a greater probability of sustaining to eventual gulping down by the ...
1
vote
1answer
186 views

Galactic Rotation Speeds - Ehrenfest Paradox, Gravitational time dilation, Dark Matter - all of the above?

The observed paths and speeds of objects, part of some distant galaxy, do not match up with speed vs distance curves it seems - the observed speeds are not falling off in fact they're trending as ...
1
vote
3answers
312 views

Effect of expansion of space on CMB

Is it true that the expansion of space time cause the CMB to become microwaves from a shorter wavelength. If it is has the amplitude been increased? Seeing as the amplitude has decreased; why hasn't ...
21
votes
1answer
597 views

Causes of hexagonal shape of Saturn's jet stream

NASA has just shown a more detailed picture of the hexagonal vortex/storm on Saturn: http://www.ibtimes.com/nasa-releases-images-saturns-hexagon-mega-storm-may-have-been-swirling-centuries-1496218 ...
15
votes
1answer
2k views

When will the Moon reach escape velocity?

From what I know, the Moon is accelerating away from the Earth. Do we know when it will reach escape velocity? How do we calculate this?
12
votes
1answer
710 views

What is the percentage of stars that are massive enough to end their lives in a supernova?

I have been searching for the percentage of stars that are massive enough to end their lives as a supernova but couldn't get any result. As far as I know, a star has to be at least 8 times more ...
12
votes
1answer
332 views

Near-Earth supernova

There are 51 stars within 17 light years of the Earth (source). If one of these stars was to become a supernova, how would they effect the Earth? I have read the Wikipedia article Near-Earth ...
8
votes
5answers
978 views

Two orbiting planets in perpendicular planes

Inspired by this question. Can a 3 body problem, starting with two planets orbiting a larger one (so massive it may be taken to stand still) in perpendicular planes, be stable? Is there known an ...
6
votes
1answer
578 views

Mass density of dark matter in solar system near us

What is the approximate mass density of dark matter in our solar system at the radius of the Earth's orbit? I would like some idea of the mass of dark matter going through each cubic meter of ...
5
votes
1answer
267 views

How to calculate the kinetic energy of supernova ejecta?

I believed I could use $0.5\times M_{ej} V_{ej}^2$, with $M_{ej}$ being the ejected mass and $V_{ej}$ being the velocity of the ejected mass. But I noticed in this and this that the the mean velocity ...
3
votes
5answers
134 views

Can a Coronal Mass Ejection (CME) cause a blackout on Earth and why?

Can a Coronal Mass Ejection (CME) cause a blackout on Earth and why is it so what's the relation between electro magnetic radiations and electrical and electronic appliance.what exactly does it do to ...
2
votes
0answers
95 views

Electric and magnetic field in a black hole

I have many questions about this topic: Does the electric field of a charged black hole look like this? I mean how can it have an electric field if nothing can escape from a black hole, and what is ...
2
votes
2answers
2k views

Differences between astronomy, astrophysics and cosmology? [closed]

What is the main difference between Astronomy, Astrophysics, and Cosmology? I have the impression that astronomy is a subject that runs parallel to physics but it is outside the physics field. This ...
1
vote
1answer
51 views

Do Massive Twin Quasar Jets Condense into separate Spiral Galaxies?

I was looking at a quasar, radio image of “Cygnus A”. Do the massive twin quasar jets condense into two separate galaxies? Do the average twin quasar jet emissions, plasma, condense, turn into ...
1
vote
3answers
204 views

Photons straight into black hole

What happens to a photon shot straight into a black hole? Does it gain infinite momentum before it crosses the horizon? If it has a finite momentum going in, then it would seem that a photon of the ...
12
votes
3answers
218 views

If neutrinos travel faster than light, how much lead time would we have over detecting supernovas?

In light of the recent story that neutrinos travel faster than photons, I realize the news about this is sensationalistic and many tests still remain, but let's ASSUME neutrinos are eventually proven ...
8
votes
1answer
199 views

Why are polar jets emitted along the axis of rotation?

Why are streams of energy-matter emitted along the axis of rotation of compact objects? (or) what is the reason that they are emitted along the axis of rotation of a compact object?
8
votes
3answers
135 views

What's dark matter and who discovered it?

I have heard about dark matter that's called the Master Of The Universe. What's this and is the dark matter the reason galaxies exist?
7
votes
1answer
117 views

Where do the bipolar jets of black holes come from?

How are they formed? And why are they so bright?
7
votes
2answers
523 views

What elements can be created in the fusion process of different types of stars?

As I understand it fusion inside a sun can produce heavier and heavier elements until some sort of "nucleus size limit" is reached. As far as I understand, the limit is thought to be reached with the ...
4
votes
1answer
47 views

No tremendous neutrino-flux for SNIa?

Why do neutrino account for 99% of the energy release for a SN II, while it is not expected to be the case for SN Ia? Is it because the densities are not high enough to induce inverse beta-decay? ...
4
votes
1answer
414 views

How much faster is the fusion we make on earth compared to the fusion that happens in the sun?

I have come across many websites that states that the proton-proton fusion which is the dominant type of fusion that powers the suns, is extremely slow and that is why the sun is still burning to that ...
3
votes
1answer
219 views

Why do galaxies “disappear?”

So, this bit of information confused me lately. Before, I figured galaxies were no longer visible by us because their luminosity decreased in an inverse square manner. However, while watching a movie ...
3
votes
1answer
174 views

How do the “tidal forces warming moons” theories hold when apart from heating from expansion, there may be also cooling from contraction?

I can understand a temporary heating, from the tital forces exerted on the moon but wouldn't there be cooling as well eventually when particles "give in" to contraction? Wouldn't they eventually net a ...
3
votes
2answers
116 views

Cosmic radiation cutoff at LOW energies?

The energy spectrum of the cosmic radiation (not CMB) is limited to both sides. I know about the GZK-cutoff at high energies. Basically the interaction probability for photons of energies above 10^20 ...
2
votes
2answers
291 views

Why does a star beyond a certain mass limit (Chandrasekhar limit) only become a black hole?

Why does a star beyond a certain mass limit (Chandrasekhar limit) only become a black hole? A star is first made of hydrogen, it undergoes nuclear fussion reaction combining into helium and ...
0
votes
1answer
7k views

Negative and positive energy and Hawking [closed]

I don't have any physics background (except the material we did in high school-long time ago). I was watching a documentary with Stephen Hawking about whether God created the Universe and I could not ...
31
votes
3answers
3k views

Why does a supernova explode

This is really bugging me. When you look up some educational text about stars life, this is what you find out: Gravity creates the temperature and pressure to start fusion reactions. The fusion ...
8
votes
1answer
238 views

$\Delta^+$ decay in GZK process

The dominant channels in the GZK process are $$p+\gamma_{\rm CMB}\to\Delta^+\to p+\pi^0,$$ $$p+\gamma_{\rm CMB}\to\Delta^+\to n+\pi^+.$$ According to the pdg, $\Delta\to N+\pi$ makes up essentially ...
27
votes
8answers
3k views

Why is absolute zero considered to be asymptotical? Wouldn't regions such as massive gaps between galaxy clusters have temperatures of absolute zero?

Why is absolute zero considered to be asymptotical? Wouldn't regions such as massive gaps between galaxy clusters have temperatures of absolute zero? I just do not see why our model must work the way ...
22
votes
3answers
1k views

Are neutrino stars theoretically possible?

Since neutrinos have a small mass and are affected by gravity, wouldn't it be theoretically possible to have such a large quantity of them so close to each other, that they would form a kind of a ...
13
votes
1answer
1k views

What is the theoretical lower mass limit for a gravitationally stable neutron star?

I ask here intentionally not for the size of the smallest possible observed size of neutron stars, which corresponds approximately to the well-known Chandrasekhar-limit for the upper limit of the ...
12
votes
2answers
2k views

Are we living in a false vacuum? Is there any way to tell?

I was thinking of the noted 1980 paper by Sidney Coleman and Frank de Luccia--"Gravitational effects of and on vacuum decay"-- about metastable vacuum states that could tunnel to a lower energy "true ...
26
votes
4answers
4k views

Why are stars white?

That is may be a easy question, but I am not a professional. The sun is a star and when I look at the sun is usually yellow. Why stars in the night are white? I suppose is for the distance. What is ...
10
votes
2answers
396 views

Does the Milky Way have dark matter satellite galaxies?

This recent paper by Weinberg et al. discusses that one potential problem with our current model of Cold Dark Matter (CDM) is that is predicts a greater number of satellite galaxies for the Milky Way ...
16
votes
10answers
286 views

In astronomy what phenomena have theory predicted before observations?

As far as I know, astronomy is generally an observational science. We see something and then try to explain why it is happening. The one exception that I know of is black holes: first it was thought ...
9
votes
2answers
743 views

Entropy of the Sun

Is it possible to measure or calculate the total entropy of the Sun? Assuming it changes over time, what are its current first and second derivatives w.r.t. time? What is our prediction on its ...
1
vote
1answer
4k views

Why does the moon or the earth revolve anti-clockwise?

The question is very simple: Why does the moon or the earth or any another planet revolve anti-clockwise? And can any planet (or satellite) revolve clockwise? What is the physical law for this ...