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programming numerical algorithms for astrophysical applications.


12m
comment What type of energy is escaping from black-hole's poles?
@HDE226868 Cannot, for that question has been put on hold (no answers possible).
19m
comment Why don't planets give off their own light?
-1 You explain the source of energy, but not the reason for light emission. Moreover, Mercury does emit, but in the infrared, not the visible.
22h
comment Why don't planets give off their own light?
See my answer. The energy source of a stars is thermonuclear fusion in their core (requiring $T\gg10,000$K), but the light is emitted at the stellar surface as the black-body radiation with $T\sim5000$K.
1d
comment Why don't planets give off their own light?
-1 this is wrong. The light is not produced by fusion.
Apr
12
comment What is the name of that which exists beyond the Universe?
This question appears to be off-topic because it is about philosophy. What's more, it does seem to cause discussion rather than seek for an answer.
Apr
12
comment Satellite/Planetary Orbits
I think only two of the 5 Lagrange points support stable orbits. These are the non-co-linear ones. Asteroids trapped into such orbits are called Trojans and Greeks, respectively.
Feb
9
comment If the earth stopped rotating, would we really fly inertially at 1000mph?
You cannot stop earth rotating just like that. If you consider non-scientific non-sense (superman), then you shouldn't ask for a scientifically based answer either.
Feb
9
comment Do stars have exactly sphere shape?
nothing real is perfectly spherical.
Jan
28
comment When we see half moon, why is it always the lower half?
The moon has no "lower" or "upper" half. Up and down are local coordinates and point in different directions for people at different places on Earth.
Jan
19
comment What if the black hole in the center of the galaxy grew faster?
@AlexeyBobrick Nope. The fact that these beasts grow to $10^{6-9}$ Solar masses shows that somehow they find a way to overcome the first problem. But eventually the feedback form the hole is so strong that it drives all the gas out of its host galaxy. This is thought to occur when all the energy of the wind is dumped into the galactic ISM, i.e. when the swept-up ISM cannot cool efficiently (if it can, only the wind momentum drives the outflow). Most efficient cooling is due to inverse Compton scattering off photons from the AGN itself (the shocked gas is much hotter than the accretion disc).
Jan
18
comment What happens to the energy from a GRB?
@AlexeyBobrick I think the precise formation mechanism of GRBs is still under debate, so your statement can only reflect a possible model, but is no definite widely accepted paradigm.
Jan
18
comment Is it possible that we see our solar system as a star?
perhaps you should read some more (and more reliable) sources. Did you look at wikipedia? No galaxy expands. The space between distant galaxies expands, though. Also, "expanding faster than light" makes no sense: light doesn't expand.
Jan
18
comment SPH simulations
There many different SPH codes around the astronomy community (I have one myself), but AFAIK none except gadget are publicly available for download (simply because these codes are very specific and easy to break if you don't know what you're doing). The problem with gadget is that it implements an out-of-date SPH algorithm (>10 years old) and was never really meant for planet collisions (I bet you would need a particular equation of state for planet collisions).
Jan
18
comment Do all moons orbit their planets on the ecliptic?
no moon is exactly in the eccliptic and no planet (except the one that defines the eccliptic) either. Why should they?
Jan
10
comment Heavy star and redshift
@frodeborli At cosmological red-shift 1000 there are no stars.
Jan
9
comment Could something temporarily break the Moon's tidal lock?
@Envite You may be right here. The mass distribution of the Moon is not exactly spherical w.r.t. its centre of mass (the lowest order deviation is described by its quadrupole) and therefore, there will be a favourable orientation which minimizes the gravitational energy. However, it's not clear that the (global) minimum is very prominent: there may be several minima of comparable depth, when it shall be random which one is picked.
Jan
7
comment Two species of dark matter?
@AlexeyBobrick So, which theory naturally contains two different (one hot, one cold) species of DM particles in roughly equal proportions (so that no either dominates)? The other answer does not explain why such theories are not seriously considered. AFAIK, a mix of particles hot and cold particles cannot currently be ruled out, but Occam's razor is used.
Jan
6
comment Could something temporarily break the Moon's tidal lock?
with the same face towards us -- that is surely wrong: which face the moon will show must be a random effect.
Jan
6
comment How does a telescope measure parallax angle?
Perhaps it's worth adding that the mean position is usually not fixed, but (for single stars) moves (nearly) on a line. This motion is called proper motion and measured from the same type of data (stellar positions over several epochs) as the parallax.
Jan
6
comment Two species of dark matter?
@AlexeyBobrick Occam's razor says we should not start playing with more than one different DM particle type, unless there is convincing independent evidence or theory to the opposite. Here, a theory is not just a simple model (messing around), but a prediction for the relation between two DM species which naturally emerges from some deeper insight. So, if your "A+B" is a theory in this sense, then Occam's razor doesn't apply. However, AFAIK, no such DM theories with more than one species are currently seriously considered.