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.

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Why is planet CFBDSIR2149-0403 hot?

According to a BBC report Astronomers have spotted a "rogue planet" - wandering the cosmos without a star to orbit - 100 light-years away. ... The proximity of the new rogue planet has ...
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1answer
80 views

Why does $H_2$ form on such a long time scale?

If we were trying to figure out the time scale for a gas-phase reaction between two hydrogen atoms in a molecular cloud (which has density $~10^4/$cm$^3$), apparently the reaction would happen on a ...
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4answers
132 views

How do black holes accrete mass?

Thanks to time dilation, a distant observer watching a man fall in to a black hole will only see him asymptotically approach the event horizon. So how do black holes ever get bigger?
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1answer
181 views

What day/night cycles, climate and seasons would experience Alpha Centauri Bb inhabitants?

Alpha Centauri Bb is an exoplanet orbiting Alpha Centauri B. It is asserted that given the close distance to the star the planet should be tidally locked. The orbiting period of the planet is about ...
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1answer
248 views

Trying to link Eulerian and Lagrangian perturbations

I'm trying to make sense of the link between a (linear) Eulerian (i.e., at a given point) and Lagrangian (following a fluid element) perturbation. I will here express not only where I'm stuck, but ...
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1answer
1k views

Why are the orbits of the planets in our solar system along the same basic plane? [duplicate]

Possible Duplicate: Why are our planets in the solar system all on the same disc/plane/layer? After watching this video I realized that the orbits of the planets in our solar system ...
5
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1answer
182 views

When and How far out will the Voyager twins come to a full-stop?

So space is mostly, but not entirely, empty. Every few (dunno the order) cubic metres of space there is some cosmic dust. Assuming that Cosmic dust exerts some friction on passing bodies, The ...
5
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2answers
116 views

Why is the gas halo of the Milky Way so hot?

I have read on the webpage of NASA that there is a massive hot gas halo around our galaxy. Its temperature is between 100,000 and 1 million Kelvins or more. I do not understand why is it so hot. The ...
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2answers
794 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 ...
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1answer
114 views

What would happen in the final days of the universe?

I would like to know the stages of how the universe would end and what would happen and what the possible scenarios are. I understand that eventually all the stars would burn out and that would ...
3
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1answer
178 views

Why are black hole singularities stable?

The Friedmann equations says that huge matter densities lead to huge expansion rates. In Newtonian gravity, two massive point particles separated by an infinitesimal distance will experience an ...
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2answers
653 views

What is the physical size of a black hole?

Something that's always confused me. How large is a black hole's physical size - not mass? From descriptions, it would seem that the 'singularity' is a single point, but is it really? Say for ...
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1answer
823 views

What would it be like if we had a planet very close to us?

My desktop background rotates through a bunch of space and nature scenes, and this one came up. What would be the effect on people walking around down here, if another earth was looming overhead ...
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2answers
117 views

Why is NASA interested in Gamma ray bursts?

Gamma-ray bursts(GRBs) are flashes of gamma rays associated with extremely energetic explosions why nasa interested to Gamma ray burst?
3
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1answer
83 views

Does the spectrum of Sol's emission change as it ages?

A follow-up to my earlier question How would one navigate interstellar space? that just occurred to me; albeit on a different tack. Sol is probably in a state of continuous flux. The change of state ...
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1answer
121 views

How is retrograde motion explained in Tycho's model?

Tycho proposed a model of the solar system where all planets but Earth move around the sun while the sun and the moon move around the earth. I wonder how this model could explain the retrograde ...
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2answers
91 views

Is the magnetic field of a white-dwarf merely residual?

Follow-up to my other question How does Sol's magnetic field continue to exist at such high temperatures? Assuming Sol's magnetic field is generated by convective currents in it's plasma, how is ...
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2answers
97 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 ...
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8answers
2k 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 ...
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1answer
161 views

Is the fuel burn for a satellite launch affected by the position of the moon relative to the launch site?

The gross mass of a satellite rocket is tiny compared to that of Earth, and Luna. Between them, however, the two bodies set up tides in bodies of water which itself is again considerable mass. At ...
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3answers
190 views

Why and how will we be able to image extraterrestrial planets in the next two decades?

According to a recently published article, a well-established Cambridge astrophysicist stated that: the “origin of life, where it exists, and whether aliens exist, is going to be crucial over ...
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6answers
1k views

Why is there an escape velocity?

I've been trying for days, but I just can't understand why escape velocities exist. I've searched the web and even this site, and although I've read many explanations, I haven't been able to truly ...
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2answers
138 views

What is the Schechter luminosity function's domain of support?

I'm trying to fit a Schechter luminosity function to some data points, but it's not clear from this definition what the domain of support of the PDF should be. I'm familiar with the standard Pareto ...
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1answer
132 views

Official definition of astronomical units

As it is relatively easy to find an official value for a large number of physical constants, (thanks to CODATA), it is not so easy for some units widely used in astronomy (Wikipedia, Google and IAU ...
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6answers
2k views

How close can spaceship get to the Sun

If you want to fly a spaceship with human passengers as close to the Sun as possible, then what effects would the spaceship have to be designed to counteract in order to keep the passengers alive and ...
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0answers
95 views

Mass loss rate of planetary nebulae

The “interacting wind” model of planetary nebulae is based on the idea that the white dwarf phase of stellar evolution is preceded by a red giant phase. A fast wind from the hot white dwarf overtakes ...
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1answer
47 views

Distinguishing Gamma-rays and stars from each other in nebulas

How do you tell the difference between a gamma-ray burst and a star just from a picture of a nebula, in which it cannot flash on and off here and there?
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0answers
107 views

gauss-bonnet gravity constraints from string theory

recently there has been advances in observational constraints of gravity theories that contains scalars coupled to the gauss-bonnet topological term: http://arxiv.org/abs/0704.0175 ...
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1answer
203 views

How much does electromagnetic radiation contribute to dark matter?

EM radiation has a relativistic mass (see for instance, Does a photon exert a gravitational pull?), and therefore exerts a gravitational pull. Intuitively it makes sense to include EM radiation ...
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4answers
395 views

Explosion in space

I'm curious about what happens if an explosive substance detonates in space. On Earth, I guess a good chunk of the energy released is carried away by shock waves in the atmosphere. But in space, the ...
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0answers
62 views

Thermal gravitational radiation and its detection

To my poor knowledge on the topic, the gravitational waves that are most likely to be detected by LIGO or other experiments do not have thermal spectrum. But I'm not certain. I know that Hawking's ...
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2answers
88 views

What allows new born suns to travel away from each other?

What allows massive new born suns to move away from each other, as they have been observed. I would think that their massive gravity would prevent this and cause them to slam into each other.
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1answer
365 views

What would happen if a hydrogen bomb were to explode in Saturn's atmosphere?

Purely hypothetical since any kind of testing in atmosphere/space is banned by international legislation/agreement. The humans have already bombed Luna so ... what could be expected to happen on ...
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4answers
4k 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. ...
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2answers
403 views

Can $10^{23}$ stars be treated with methods of statistical mechanics?

Statistical mechanics is used to describe systems with large number of particles ~$10^{23}$. The observable universe contains between $10^{22}$ to $10^{24}$ stars. Can we treat those many stars as a ...
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2answers
331 views

How convincing is the evidence for dark matter annihilation at 130 GeV in the galactic center from the Fermi Satellite data?

I listened to Christoph Weniger present his results at SLAC today. See his paper is here: http://arxiv.org/abs/1204.2797 and also see a different analysis here: http://arxiv.org/abs/1205.1045. The ...
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1answer
76 views

Does a celestial system exhibit a collective magnetic field?

Sol exhibits a magnetic field, most of the planets in orbit around Sol exhibit a magnetic field - strong and weak both. Does the solar system as a whole exhibit a magnetic field? Does the paradigm ...
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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 ...
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0answers
45 views

Experimental tests of Cluster Decmposition

How tight are experimental and astrophysical tests on whether Cluster Decomposition is satisfied at various space-like separations? Is there a review paper or a standard reference on the question? I ...
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1answer
89 views

Why are galactic centers always brighter than the edges?

As you can see this image http://i.stack.imgur.com/YOt8C.jpg and other galaxy images, the centers generally much brighter. Why is that? Is there a very big star? A very big gravitational field?
3
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1answer
170 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 ...
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0answers
56 views

How is the universe expanding? [duplicate]

Possible Duplicate: spacetime expansion and universe expansion? Is the space between planets is growing or the space between stars is growing or the space between galaxies is growing?
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1answer
4k 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 ...
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2answers
111 views

Precessional motion of active galactic nuclei

I want to set a simulation for jet which has a precessional motion. The symmetry axis of jet is $z$ axis, i.e. jet is propagating along $z$ direction making angle $\theta$. I set the velocity ...
0
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1answer
120 views

Does diffraction contribute to the Black Drop effect?

Two reasons are given to explain the black drop effect here, but I think I came up with a third. Consider a two-dimensional cross section of the situation just after second contact and just before ...
0
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1answer
122 views

Radiative transfer equation solution

The solution of the radiative transfer equation for spherical ionized blob : \begin{equation} \frac{dI_{\nu}}{ds} = j_{\nu}-\alpha I_{\nu} \end{equation} and solution is (Ref: ...
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1answer
842 views

Do materials cool down in the vacuum of space?

Do materials cool down in the vacuum of space? If yes, how does it really work?
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2answers
197 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 ...
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2answers
376 views

speed of light,breaking the barrier

when the sound barrier is broken,a series of concentric waves of sound is produced.Does it mean when the speed of light barrier is broken,a ripple of photons are created in the space-time fabric?
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1answer
90 views

Is there any way to produce food without sun, synthetically?

Is there any way to produce food without sun, synthetically? I mean if we face solar winter. look at: Is there any way to survive solarwinter like in Sunshine - movie?