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Jul
2
awarded  Curious
Jun
16
answered How would the night sky look from inside a globular cluster?
Jun
15
comment How are black holes found?
You should definitely check the time it would take for a three solar mass black hole to evaporate.
Mar
19
comment Why is the interstellar medium so hot?
@chris: Cosmological expansion should affect the temperatures of IGM similarly as it affects the CMB temperature, that's why I am mentioning it. But then IGM would be too cold comparing to 10^4 K. It indeed seems plausible that it was reheated when it was reionized. Thanks again for your comments!
Mar
18
comment Why is the interstellar medium so hot?
@chris, very useful point, thank you again! Do you know if the primordial IGM cooling is mainly due to cosmological expansion or radiative cooling?
Mar
17
comment On analogies between gas and stellar systems
Indeed interesting, @chris, thank you!
Mar
1
comment Do telescopes harm while observing sun through them?
@Gerald, very illustrative, thank you!
Feb
28
comment Do telescopes harm while observing sun through them?
So, what exactly will happen if you point, say, VLT at the Sun?
Feb
23
comment If we lived in a multiverse, what would our universe most likely then be named?
@Rhys, very reasonable and good question. If questions whether pluto is a planet and alike are discussed at IAU meetings, why would it be unreasonable to speculate on the possible names for our part of the multiverse?
Feb
10
comment A clock travelling “faster than the speed of light”
Yes, you would age by four years and travel very very far and everyone you know would get 400000 years older. However, you, having stopped at that distant location would have to wait for about 400000 years for these events to unfold as you point your powerful telescope at the Earth. Remember that when you look far you see the past, but not the present.
Feb
10
comment Integral calculus for the Olympiad?
The question is opinion-based. I recomend that you talk to someone in person. There is not so much you need to know about calculus to solve these problems. Good link, @Gerald.
Feb
9
comment What would happen if a body were to fall into a neutron star?
@StanLiou: Given that the energy release per unit mass at the moment of impact is much larger than nuclear binding energy per unit mass of the impactor, ionisation and nuclear bonds are irrelevant. The outcome would be the same as if the object would be just a set of protons and neutrons.
Jan
25
comment Can we calculate the average temperature of the Heliosphere?
I am not entirely happy with the question to answer it. There is no clear specification for the temperature, which is asked: neither location, nor the component is clearly specified. Your answer is much more fun in this sense, it gives eventually a nice picture of what is going on.
Jan
24
comment Can we calculate the average temperature of the Heliosphere?
So given the above said, I don't think this is the correct answer to the question, if there is any. However, there is a great deal of interesting phenomena touched upon in your answer and comments.
Jan
24
comment Can we calculate the average temperature of the Heliosphere?
From the SOHO paper you pointed to, it seems likely that due to expansion the relaxation time for the plasma gets rather rapidly long comparable to the timescales it takes to reach the Earth and hence most likely the heliosphere. In this case plasma just gets decoupled from any thermodynamics happening inside heliosphere and any temperatures associated with it, until it starts schocking the ISM.
Jan
24
comment Can we calculate the average temperature of the Heliosphere?
It seems to me, that you don't differentiate between the bulk motion of the gas (plasma) and its thermodynamical properties, for example temperature and density. Solar wind surely expands till 100AU, as was pointed out already in the question, and it probably keeps the bulk velocity rather constant until gradually the shock from collision with ISM takes over. However, the temperature will go down almost immediately. You can check Kelvin-Helmholtz timescale of a mole of ideal gas in a reasonable volume. The wind will: 1) Cool down, 2) Possibly stop being in thermodynamical equilibrium.
Jan
23
comment Can we calculate the average temperature of the Heliosphere?
It is too optimistic to expect that the wind which had been travelling for about a year and expanding by at least a factor of $10^6$ in volume would neither cool adiabatically, nor radiate away most of the energy.
Jan
21
comment Length contraction of a star
Yes, so it will.
Jan
21
comment Do all the objects in the universe exert force on all other objects?
@astromax: Hubble constant has the dimensions of velocity over distance. So, to use your phrase "the universe expanded faster than the speed of light", which I have been commenting on so far, you have to have a distance measure between some two points. The choice of distance measure is arbitrary. So the concept of "superluminal expansion of the Universe" is also arbitrary. But yes, Hubble constant is surely well defined.
Jan
20
comment Calculate latitude and longitude based on date and sun
@Envite: Speaking of longitudes, you are right of course. For latitude the answer to your comment is yes and no. Yes, if you can wait and see how the Sun moves, and no in the opposite case. If you can wait, you can get the latitude from the position of the Sun at the noon. If you can only know the immediate position of the Sun, you would also need the local time to get the latitude.