Why are some gas clouds super hot?

[gazza]

Hi all, newbie here.hoping someone can answer my question.

 

I was reading an interesting article about dark matter. It mentioned about clouds of gas in deeps space being millions of degrees kelvin therfore giving off x-rays which in turn makes them visible to x-ray telescopes.

I couldn't quite get my head round the idea of gas floating in the vastness of space at near absolute zero being at such high temperatures.

I can kind of understand gas being hot from an exploding star/nova or being dense enough itself to form a star,but not why in such freezing temperatures it remains hot. Probably something obvious I'm not thinking of!

I kind of imagine the gas being not too dense and loosley packed. perhaps my assumption is wrong.

This led onto another thought. Firing off in a hypathetical space ship into seemingly "empty " space,one might encounter an invisible gas cloud,which presumably being millions of degrees K would destroy said ship?

many thanks in advance for any replies

[Klaynos]

I don't know for certain but I'd speculate the clouds are collapsing this produces alot of energy (converted from gravitational potential to KE which then is lost from the particle via collisions) which in turn raises the temperature of the cloud...

[Gilded]

There's just so much energy in a nova that it will take a very long time to cool down even though it's spreading to every direction. Also, if a gas cloud happens to be near a very massive object (black holes, neutron stars etc.) it will heat up as it falls towards the object. Overall there's a lot of plasma to go around in the universe as gases are heated up by various processes. Even parts of the intergalactic medium are permeated by strands of plasma that can have temperatures of millions of kelvins.

[big314mp]

You also need to consider that the gas can only really cool down by radiative emission. Space isn't "cold" the way your freezer is cold, as in your freezer, there are things to conduct away heat directly. In space, there is no such thing, so the vacuum sort of acts like a thermos.

 

As to the last bit, the gas is so undense (terms?) that it wouldn't be able to transfer too much heat to the ship. The radiation itself would be a bigger problem.

[Arch2008]

http://cse.ssl.berkeley.edu/chips_epo/hotgas_tour_slide4.htm

This says that shockwaves caused by supernovae do it.

With apologies to Star Trek fans, space travel would be a really dangerous business.

[Martin]

I'm seeing a pattern of thoughtful informed answers to questions volunteered by people who know what they are talking about even tho not designated official staff. It's a nonhierarchical feature of the SFN culture that is a real plus.

 

I feel like Gazza's question was adequately answered already by the combination of Klaynos, Gilded, and Amp, so anything I add is just extra.

 

I'd say think about the solar wind----and how temperature relates to particles' kinetic energy. Let's look up solar wind on Wikpee, even tho it's not completely reliable it's very easy to use.

 

"Components

 

The solar wind is divided into two components, respectively termed the slow solar wind and the fast solar wind. The slow solar wind has a velocity of about 400 km/s, a temperature of 1.4–1.6 million K and a composition that is a close match to the corona..."

 

So here it says the sun is blowing off a wind of particles with a temp on the order of a million Kelvin.

 

Another thing I'd check is what does a million Kelvin mean in terms of electron volts and in terms of the speed of a proton. And how does it compare with the escape velocity of one of these big clusters of galaxies.

 

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Personally I like a combination of what other people said

1. there could be several sources of million Kelvin intergalactic cloud

2. one source could be supernovae, another could be the normal life-cycle blow-off from very massive stars---like the solar wind but much much more.

3. the hot partially-ionized gas wouldn't have a lot of ways to cool down

 

collisions and near collisions between charged particles will cause radiation, any time a charged particle veers off course or accelerates in any direction there is radiation--------so even in a diffuse cloud with a long time between collisions you would still be getting radiation.

 

radiating off energy would be a way for it to cool down, but as both Gilded and Amp said, in a very diffuse undense gas this process of radiative cooling is quite slow. interactions between particles are so rare that particles don't often have a chance to veer, accelerate, decelerate etc. so they don't have much opportunity to dump their kinetic energy into radiation

 

Gazza's question is really interesting. I don't have much knowledge about this, can only speculate about it. How about the intergalactic magnetic field? Can that be helping to trap the hot plasma and keep it confined inside the cluster? Why wouldn't the hot particles be leaking out of the cluster? Wouldn't they have escape velocity? What is the escape velocity of a cluster of galaxies. My guess is around 300 to 500 km/s, rough order of magnitude.

 

Fun question. Score a polnt for Gazza

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HEY I FOUND SOMETHING!

I looked up "intergalactic medium" on Wikpee and got referred to a short article "intergalactic space" which had this:

 

"...The reason the IGM is thought to be mostly ionized gas is that its temperature is thought to be quite high by terrestrial standards (though some parts of it are only "warm" by astrophysical standards). As gas falls into the Intergalactic Medium from the voids, it heats up to temperatures of 10^5 K to 10^7 K, which is high enough for the bound electrons to escape from the hydrogen nuclei upon collisions. At these temperatures, it is called the Warm-Hot Intergalactic Medium (WHIM). Computer simulations indicate that on the order of half the atomic matter in the universe might exist in this warm-hot, rarefied state. When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of 10^8 K and above..."

 

so these million K temps are part of the structure formation scenario, according to (not always reliable) Wp.

the gas is primordial gas, originally distributed more or less evenly, but with some underdense and some overdense regions.

and it starts to pull together in the overdense, and coagulate into clusters, and fliament structures etc. (cobwebs)

 

and the million K energy comes from gas FALLING from the underdense regions into the denser cobwebby regions

 

OK, we don't automatically accept Wp as an authority, but still, that is an interesting idea to add to the mix of what we have already.

 

individual hydrogen atoms would be falling in from all directions and they would collide and get ionized and their kinetic would get randomized, so the gravitational energy would wind up as temperature. OK, but I bet that supernovae and wolf-rayet stars help.

 

This Wikipedia entry supports what KLAYNOS said!

POINT FOR KLAYNOS!!!! yay team!

Edited September 25, 2008 by Martin