interstellar gas cloud

mee

http://science.nasa.gov/headlines/y2....htm?aol667632

At the above link, it appears there is a high temperature cloud of gas has been colliding with the solar system. What could keep a cloud of gas at such a high temperature in the interstellar medium?

Chronos

The temperature of a gas is a measure of the speed of random motions of molecules in a gas. The higher the speed, the higher the temperature. When molecules collide, they emit radiation. The relative intensities of spectral lines in these emissions are used to calculate the gas temperature [a Wein's Law thing].

hellfire

I think one must note that the local bubble (in which the solar system is located since aprox. 4 million years) is a cold environment if one compares with the interstellar medium. The temperature of this cloud the solar system is entering seams reasonable to me if one considers the assumption of the creation of the the local bubble due to a few supernovae from the Sco-Cen OB-association (emission nebulae surrounding O or B stars have usually 8000 - 10.000 K). This supernovae generate a cavity within the interstellar medium (our bubble is called the local bubble or local chimney) in which the density as well as the temperature decrease. The walls of the cavity are hotter and there are also couldlets near the walls, which should be also hotter.

Nereid

Another (ESA) report of these findings. To extend Chronos' excellent answer to this question, you can turn it on its head - what could possibly cool such a cloud of gas? As Chronos noted, it can only cool by colliding with something colder (such as a dense, cold interstelar cloud; planets are far, far too small to cool the ISM) or through radiative cooling. The interesting thing is that the gas (as opposed to the plasma) behaves just like the air around us - you can apply the classical physics gas laws and come up with all the right answers!

mee

Well. I'm not much educated about some of these things, it seems people are always saying stuff like " the terrible cold of 'space'" or something. Not only that, but when one gets far away from the sun, so I assumed for any star or starlike source, it gets colder: like Pluto and Charon.

Nereid

Hmm, how to explain this.

Let's take a lump of iron, say 1 kg (or 1 pound, if you're from the US), at 20C (or 70F if you're in the US). We magically instantly move it to a stable orbit about 50 au from the Sun (that's some way beyond Pluto's orbit). What's its equilibrium temperature?

1) radiative equilibrium: the lump absorbs photons, principally from the distant Sun, and emits photons, more or less as blackbody radiation. If this were the only thing, the lump would indeed become quite cold, ~40K (depends somewhat on whether it's got a nice coat of white, shiny paint on it, or a thin layer of soot).

2) equilibrium with the local gas (not plasma): even if the gas temperature is 6000K, it will have only the tiniest effect on the temperature of the lump! Why? The lump has a great many atoms (how many? google on Avogadro's number); a volume of the surrounding gas equal to the volume of the lump, very few (<10?). Even if thousands, indeed millions, of these gas molecules (or atoms) collided with the lump every second, the heat they'd transfer to it would be trivial, and soon radiated away.

3) equilibrium with the local plasma and cosmic rays: similar situation to 2.

Now, suppose we ripped the lump into atoms, and made them into a very thin gas, of density approx equal to the surrounding gas way out beyond Pluto's orbit (how much volume would that 'lump-gas' have?). Now the situation is very different; atoms in the dispersed lump - now a thin gas - will collide with the gas (how often?), and through those collisions, come into equilibrium with it (what pressure? what temperature?).

mee

Thank you very much for your patient and well-crafted answer.