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.
19
votes
1answer
658 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 ...
13
votes
6answers
834 views
Why isn't dark matter just matter?
There's more gravitational force in our galaxy (and others) than can be explained by counting stars. So why not lots of dark planetery systems (ie without stars) ? Why must we assume some undiscovered ...
17
votes
2answers
346 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 ...
10
votes
3answers
874 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 ...
7
votes
4answers
381 views
Is the Fine Stucture constant constant?
I have read that the fine structure constant may well not be a constant. Now, if this were to be true, what would be the effect of a higher or lower value? (and why?)
7
votes
3answers
672 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 ...
9
votes
5answers
784 views
Hubble's law and conservation of energy
If all distances are constantly increasing, as Hubble's law say, then lots of potential energies of form ~$\frac{1}{r}$ changes, so how is the total energy of the Universe conserved with Hubble's ...
7
votes
2answers
2k views
Why does Venus rotate the opposite direction as other planets?
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.
Theoretical large ...
2
votes
2answers
168 views
Are there formulae for calculating stellar luminosity and effective temperature as a function of age?
Is there a manageable formula or set of formulas or simple algorithms that approximate stellar luminosity and effective temperature (or radius) as a function of stellar age?
I'm aware that accurate ...
7
votes
1answer
728 views
Why do galaxies and water going down a plug hole spin?
We all experience things spinning, whether it's water down a drain, the earth on its axis, planets round the sun, or stars in a galaxy - even electrons round an atom.
But why is spin so common in ...
50
votes
7answers
3k 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 on the ...
20
votes
6answers
2k 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 ...
8
votes
6answers
1k 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 ...
8
votes
3answers
462 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 ...
2
votes
2answers
780 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).
...
2
votes
5answers
785 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 ...
1
vote
3answers
266 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 ...
17
votes
8answers
980 views
Why are L4 and L5 lagrangian points stable?
This diagram from wikipedia shows the gravitational potential energy of the sun-earth two body system, and demonstrates clearly the semi-stability of the L1, L2, and L3 lagrangian points. The blue ...
4
votes
5answers
475 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
2answers
274 views
Why is Larry Niven's Ringworld Unstable?
In his 1970 science fiction novel Ringworld, author Larry Niven describes the eponymous Ringworld, a gigantic structure shaped as a ring with a radius of around 1 AU, rotating around a star in the ...
7
votes
4answers
489 views
How is it possible for astronomers to see something 13B light years away?
In a NPR News story from a few years back:
"A gamma-ray burst from about 13
billion light years away has become
the most distant object in the known
universe."
I'm a layman when it comes ...
4
votes
4answers
260 views
Are galactic stars spiraling inwards?
Are the stars in our galaxy spiraling inwards towards the center, or are they in a permanent orbit?
And if we are heading towards the center then what is the rate of this process?
I started ...
4
votes
2answers
654 views
Origin of elements heavier than Iron (Fe)
In all the discussions about how the heavy elements in the universe are forged in the guts of stars and especially during a stars death, I usually hear that once the star begins fusing lighter atoms ...
1
vote
1answer
2k 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 ...
1
vote
0answers
67 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 ...
25
votes
1answer
459 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
1k views
How do you calculate the anomalous precession of Mercury?
One of the three classic tests of general relativity is the calculation of the precession of the perihelion of Mercury's orbit.
This precession rate had been precisely measured using data collected ...
3
votes
2answers
264 views
What is the temperature of the surface and core of a neutron star formed 12 billion years ago now equal to?
In what part of the spectrum is it radiating? In the infrared, in the microwave? Or is not radiating anymore at all?
In russian:
Чему сейчас равна температура поверхности и ядра нейтронной звезды, ...
1
vote
0answers
52 views
Could Voyager 1 have entered a solar radiation belt?
We currently believe that the Sun has no radiation belts because the unstable magnetic field, which turns every 11 years, is not stable enough to sustain a solar radiation belt. But observations from ...
12
votes
3answers
421 views
Why are most astronomy things spherical in the shape (like, the Sun, the Moon, the Earth, and other planets)?
What is the reason for all the astronomy things being spherical in the shape (like, the Sun, the Moon, the Earth, and other planets)?
12
votes
9answers
168 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 ...
12
votes
7answers
15k views
Is Melancholia's orbit impossible?
In the recent movie "Melancholia", a planet, also called Melancholia, enters the solar system and hits the Earth. I want to leave aside the (also unreasonable) aspect that planet "hides behind the ...
10
votes
5answers
676 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 ...
10
votes
2answers
1k 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 ...
7
votes
5answers
699 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 ...
3
votes
1answer
115 views
models for astrophysical relativistic jets from compact objects
what is the simplest way to understand the physics of relativistic jets? we know that they have axial symmetry with very tight angular spread, presumably aligned with the axis of rotation of the ...
3
votes
3answers
464 views
Energy of the electron-muon reaction
Lets see the reaction:
$e^- \mu^- \to e^- \pi^- \nu_\mu \;\;\;\;\;\;\;\;\;\;\; {(1)}$
I suppose, that this reaction occurs as follows
$e^- \mu^- \to e^- \mu^- \pi^+ \pi^- \to e^- \pi^- \nu_\mu$
Is ...
2
votes
1answer
189 views
2
votes
1answer
182 views
If Fine Structure Constant in not uniform in space what would that imply for cosmic background?
Recently J.Webb submitted paper which has extraordinary claim - that Fine Structure Constant is different in different directions in space! He (with others) measured $\alpha$ using quasar spectrum ...
2
votes
3answers
266 views
Why is it thought that normal physics doesn't exist inside the event horizon of a black hole?
A black hole is so dense that a sphere around it called the event horizon has a greater escape velocity than the speed of light, making it black. So why do astronomers think that there is anything ...
1
vote
1answer
296 views
Blandford-Znajek process: Why/how does the current flow along the magnetic field lines
Related: How would a black hole power plant work?
I have put a bit of commentary enumerating my confusions in parentheses
I read in Black Holes and Time Warps (Kip Thorne), that quasars can generate ...
1
vote
1answer
335 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 ...
12
votes
3answers
91 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 ...
7
votes
3answers
72 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?
5
votes
3answers
214 views
We're all star dust?
OK so we've all heard of this from Carl Sagan, Lawrence Krauss and others and we know the argumentation, I don't refute that. There are other examples, for instance I once calculated (this was before ...
5
votes
2answers
76 views
How does Sol's magnetic field continue to exist at such high temperatures?
The temperature at the surface of Sol is apparently well above 5000C; I'm assuming the layers beneath the surface may be even hotter.
At school we learnt heating a metal beyond a certain temperature ...
5
votes
3answers
280 views
Do neutron stars reflect light?
The setup is very simple: you have a regular ($1.35$ to $2$ solar masses) evolved neutron star, and you shine plane electromagnetic waves on it with given $\lambda$. Very roughly, what shall be the ...
4
votes
3answers
187 views
stability of hypothetical lunar atmosphere
Assume that by some means, the moon could be given an atmosphere, of the same density and pressure at the surface as the earth's. Obviously in a stable atmosphere there are temperature variations from ...
3
votes
0answers
49 views
squeezed radiation astronomy
Squeezed electromagnetic vacuum does have a renormalized energy density smaller than the vacuum. So it makes it in my opinion a inconspicuous candidate for a dark energy carrier.
Are there ...
3
votes
2answers
140 views
Pressures Necessary for Carbon Detonation
Carbon detonation is a characteristic event of Type 1a Supernova (EDIT: where an accreting white dwarf near the Chandrashankar limit of 1.4 solar masses explodes), an extremely important standard ...
