All Questions
529 questions
104
votes
7
answers
43k
views
What is the speed of sound in space?
Given that space is not a perfect vacuum, what is the speed of sound therein? Google was not very helpful in this regard, as the only answer I found was $300\,{\rm km}\,{\rm s}^{-1}$, from Astronomy ...
- 1,113
52
votes
3
answers
11k
views
How would we tell antimatter galaxies apart?
Given that antimatter galaxies are theoretically possible,
how would they be distinguishable from regular matter galaxies?
That is, antimatter is equal in atomic weight and all properties, except for ...
- 823
50
votes
8
answers
7k
views
Why isn't dark matter just ordinary matter?
There's more gravitational force in our galaxy (and others) than can be explained by counting stars made of ordinary matter. So why not lots of dark planetary systems (i.e., without stars) made of ...
- 603
61
votes
3
answers
41k
views
Why the galaxies form 2D planes (or spiral-like) instead of 3D balls (or spherical-like)?
Question: As we know, (1) the macroscopic spatial dimension of our universe is 3 dimension, and (2) gravity attracts massive objects together and the gravitational force is isotropic without ...
- 7,928
46
votes
3
answers
59k
views
What is the origin of elements heavier than iron?
In all the discussions about how the heavy elements in the universe are forged in the guts of stars and especially during a star's death, I usually hear that once the star begins fusing lighter atoms ...
- 573
53
votes
10
answers
41k
views
Can Jupiter be ignited?
Our solar system itself contains two candidate "Earths"
One is Jupiter's moon Europa and another is Saturn's moon Titan. Both of them have the problem of having at low temperature as Sun's heat ...
- 1,271
37
votes
4
answers
14k
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 ...
- 4,463
38
votes
7
answers
8k
views
How fast a (relatively) small black hole will consume the Earth?
This question appeared quite a time ago and was inspired, of course, by all the fuss around "LHC will destroy the Earth".
Consider a small black hole, that is somehow got inside the Earth. Under "...
- 20.2k
30
votes
4
answers
7k
views
What nonlinear deformations will a fast rotating planet exhibit?
It is common knowledge among the educated that the Earth is not exactly spherical, and some of this comes from tidal forces and inhomogeneities but some of it comes from the rotation of the planet ...
- 21.3k
23
votes
2
answers
10k
views
How can Quasars emit anything if they're black holes?
We've heard it many times, nothing can escape the gravity of a black hole, even light once it's past the event horizon. If this is true, how can a black hole emit anything? Quasars are massive black ...
Hunter
31
votes
2
answers
17k
views
What would happen to a teaspoon of neutron star material if released on Earth?
I've read on NASA's page on neutron star that one teaspoonful of that star would weigh over 20 billion tonnes on Earth. If it was somehow possible to bring it to earth would it:
Burn and disappear ...
- 421
31
votes
2
answers
1k
views
Experimental observation of matter/antimatter in the universe
Ordinary matter and antimatter have the same physical properties when it comes to, for example, spectroscopy. Hydrogen and antihydrogen atoms produce the same spectroscopy when excited, and adsorb the ...
- 3,950
18
votes
3
answers
6k
views
Why are stars, planets and larger moons (approximately) spherical in shape (like, the Sun, the Moon, the Earth, and other planets)?
Why are stars, planets and larger moons (approximately) spherical in shape (like, the Sun, the Moon, the Earth, and other planets)?
- 373
15
votes
3
answers
2k
views
Why don't absorption and emission lines cancel out in our Sun?
I was looking at this answer on why absorption lines and emission lines don't cancel out:
An experiment shining light on the material and looking at the
reflected spectrum will see absorption ...
- 2,005
20
votes
2
answers
3k
views
What is the safe distance to a supernova explosion?
In other words, what stars near the Sun may have an impact on the Solar system equilibrium or the Earth life if they become supernova ? Is SN 1987 A too far ?
user46925
38
votes
1
answer
9k
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 ...
- 8,338
17
votes
4
answers
2k
views
Can black holes actually merge?
If time stops at the event horizon, can we ever detect two black holes merging? In other words, if you are a short distance away, would you encounter a spherically symmetric gravitational field, or a ...
Ze Kraggash
57
votes
4
answers
13k
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 ...
- 3,107
32
votes
2
answers
5k
views
Is there a limit as to how fast a black hole can grow?
Astronomers find ancient black hole 12 billion times the size of the Sun.
According to the article above, we observe this supermassive black hole as it was 900 million years after the formation of ...
- 7,455
19
votes
1
answer
6k
views
Does the number density of photons $n_\gamma\approx 10^8 \:\mathrm m^{-3}$ refer to CMB photons only?
When we talk about the present-day number density of photons ($n_\gamma\approx 10^8 \:\mathrm m^{-3}$) in the universe do we mean the number density of CMB photons? I mean there are other sources of ...
- 27.2k
3
votes
1
answer
2k
views
Deriving relativistic Doppler shift in terms of wavelength? [closed]
Consider a star moving with velocity $v$ at an angle $\theta$ with respect to its line of sight to Earth. Show that the relativistic Doppler shift is
$$\lambda_{obs} = \frac{1 - \frac{v}{c} cos(\theta)...
14
votes
5
answers
8k
views
Why does Venus spin in the opposite direction?
Given: Law of Conservation of Angular Momentum.
Reverse spinning with dense atmosphere (92 times > Earth & CO2 dominant sulphur based).
Surface same degree of aging all over.
Hypothetical large ...
- 167
99
votes
7
answers
22k
views
How can a black hole produce sound?
I was reading this article from NASA -- it's NASA -- and literally found myself perplexed. The article describes the discovery that black holes emit a "note" that has physical ramifications ...
- 1,079
36
votes
4
answers
5k
views
Why doesn't the solar wind disrupt the planets?
The sun creates this heliosphere by sending a constant flow of particles and a magnetic field out into space at over 670,000 miles per hour, which is also known as solar wind. If the speed of the wind ...
- 543
17
votes
5
answers
5k
views
Age of the Earth and the star that preceded the Sun
One of the great unheralded advances made in the history of science was the ability to determine the age of Earth based on the decay of isotopic uranium. Based on the apparent abundance of uranium in ...
- 2,234
12
votes
2
answers
9k
views
What is the electric charge of the Sun and its corona?
What is the net electric charge (in magnitude and sign) of the Sun and its corona?
- 471
1
vote
3
answers
2k
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 ...
- 825
24
votes
4
answers
24k
views
Explanation for negative specific heat capacities in stars?
I've just found out that a negative specific heat capacity is possible. But I have been trying to find an explanation for this with no success.
Negative heat capacity would mean that when a system ...
- 3,934
11
votes
2
answers
3k
views
Do stars remain electrically neutral? [duplicate]
How electrically neutral do stars remain through their lifetime? As an example, I could imagine processes such as coronal mass ejections leaving the Sun in a slightly charged state. Are there such ...
- 1,631
6
votes
2
answers
2k
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 hydrogen into helium and ...
- 125
5
votes
3
answers
1k
views
How do shock waves form in space?
Today my aerodynamics professor mentioned that the equations we were learning for atmospheric shock waves can also be applied by astrophysicists to study black hole-related shock waves in space.
How ...
- 161
3
votes
1
answer
2k
views
Universe Expansion as an absolute time reference
Why we call "constant" to the Hubble constant?,
if the universe were really expanding then the Hubble "constant" should change, being variable, smaller and smaller..with "time".
Other example/view ...
- 2,937
26
votes
5
answers
37k
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. ...
- 261
25
votes
2
answers
2k
views
How do spiral arms form?
Why aren't all spinning galaxies shaped as discs as my young mind would expect? I understand how the innermost parts of a galaxy spin faster than the outer parts, and that could explain why some ...
- 1,137
19
votes
5
answers
3k
views
Anti-Matter Black Holes
Assuming for a second that there were a pocket of anti matter somewhere sufficiently large to form all the type of object we can see forming from normal matter - then one of these objects would be a ...
- 823
19
votes
6
answers
50k
views
What are good books for graduates/undergraduates in Astrophysics?
There are no book recommendations for Astrophysics here. I will write my own answer, but I am also interested in what are others' views on the question (I will NOT mark my own answer as the best one).
8
votes
2
answers
5k
views
Do black holes have infinite areas and volumes?
How to calculate the area / volume of a black hole?
Is there a corresponding mathematical function such as rotating $1/x$ around the $x$-axis or likewise to find the volume?
- 1,053
8
votes
2
answers
760
views
What causes the dimensions of a star increase when its hydrogen fuel is exhausted?
What causes the dimensions of a star increase when its hydrogen fuel is exhausted? For example, the Sun is expected to increase its radius 250 times. What causes this if its temperature is expected to ...
- 11.3k
6
votes
2
answers
876
views
Dark age of universe when all fusion process ceases?
Some say we live in the golden age of the universe because there exits countless number of stars that shines in the dark universe. As the supply of gas for star formation is steadily being exhausted, ...
- 245
5
votes
3
answers
2k
views
Size and density of neutron stars
Most of the books which I looked at give approximately 10 km as the radius of a neutron star. Just yesterday I looked at a book by Dave Goldberg titled The Universe In the Rearview Mirror (2013) which ...
4
votes
5
answers
1k
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 ...
18
votes
1
answer
2k
views
Why can a neutron star implode? [duplicate]
In a book* I was reading, it said that a neutron star is formed when the pressure is so large that the electrons in a white dwarf interact with the protons, forming neutrons. The neutrons then ...
- 572
16
votes
2
answers
2k
views
How many of which particles are in Hawking radiation?
My understanding is that a black hole radiates ~like an ideal black body, and that both photons and massive particles are emitted by Hawking radiation. So for a low temperature black hole, photons are ...
- 161
16
votes
4
answers
12k
views
Why does each celestial object spin on its own axis?
AFAIK all the celestial objects have a spin motion around its axis. What is the reason for this? If it must rotate by some theory, what decides it's direction and speed of rotation?
Is there any ...
- 1,394
15
votes
2
answers
4k
views
With what probability does nuclear fusion occur at energies far below the Coulomb barrier?
Even at the core of the sun, the temperature of $\sim 10^7$ K only results in $kT\sim1$ keV, which is about a thousand times less than the electrical potential energy of $\sim1$ MeV needed in order to ...
user4552
11
votes
1
answer
3k
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 ...
- 716
8
votes
1
answer
368
views
Where do the bipolar jets of black holes come from?
How are they formed? And why are they so bright?
- 3,662
8
votes
2
answers
1k
views
How does dark matter halo outside a galaxy help to explain galaxy rotation curve?
How does a dark matter halo outside a galaxy help to explain a galaxy rotation curve?
Suppose for simplicity we use a model of a star rotating about a more massive star in a fixed circular orbit. For ...
- 1,057
37
votes
6
answers
6k
views
How many times has the matter in our Solar System been recycled from previous stars?
I've got a basic understanding of these facts:
The Universe is a little over 13 billion years old. Our Galaxy is almost that old.
Our Solar System is roughly 4.6 billion years old.
The heavier ...
- 473
35
votes
4
answers
10k
views
Is the speed of sound almost as high as the speed of light in neutron stars?
Have you ever wondered about the elastic properties of neutron stars?
Such stars, being immensely dense, in which neutrons are bound together by the strong nuclear force on top of the strong gravity ...
- 3,870