# What's the nature of intraction between hydrogen atoms in the interstellar medium?

Interstellar densities are about 1 atom/cm$^3$. Do these atoms even move? If they do, why do they move? What would be their average speed? (Free expansion)

• Of course they are moving around. The mean free path is on the order of astronomical units, i.e. these atoms interact very rarely. – CuriousOne Feb 23 '16 at 9:39
• Why the close votes? The question is about the collision dynamics of interstellar hydrogen. CuriousOne's comment shows it can be answered. – John Rennie Feb 24 '16 at 6:30
• ( still, it would be a lot clearer to have "interstellar medium" instead of "space density" in the title and question. ) – Fabrice NEYRET Feb 24 '16 at 10:33

The interstellar medium (ISM) has many components that vary in densities (see the table in the link to the ISM Wiki page). For the regions with densities in the $n_\text{ISM}\sim10^0\text{ cm}^{-3}$ range, the temperatures vary from between $(50,\,10^4)\text{ K}$. Using simple Boltzmann statistics, the mean thermal velocities of particles in the ISM could be $$v_\text{ISM}=\sqrt{\frac{3k_BT}{\mu}}\simeq(3,\,80)\text{ km/s}$$ which seems pretty high, but recall that the solar system is moving at about 250 km/s around the galaxy.

ISM gas can also be pushed by astrophysical shock waves and pulled by gravitational collapse, both of which will cause motion in less-constant manners (depends on parameters of the shockwave or collapse). Further, if there are disordered magnetic fields in the ISM, the ISM can also be affected by these fields.

• My math may be wrong. If it is, someone let me know and I'll try fixing it tonight. – Kyle Kanos Feb 24 '16 at 11:27
• @Hammar: Well the intergalactic medium (IGM) isn't 2.7 K (which is the temperature of the cosmic microwave background), the IGM is about $10^6$ K. And as it's less dense (but more hot), the velocities are about the same. – Kyle Kanos Feb 24 '16 at 17:48
• In the vast gulf between stars and galaxies, the temperature of gaseous matter routinely drops to 3 degrees K. science.nasa.gov/science-news/science-at-nasa/2014/… Do you mean that less dense space will always be more hot and less hot will always be more dense ? – Hammar Feb 24 '16 at 18:22
• I suppose there could be some 2.7 K regions, but there's the warm/hot IGM (or also this page) – Kyle Kanos Feb 24 '16 at 18:22
• The Coolest Spot in the Universe youtube.com/watch?v=J9_LmSTtpkI This video answered my question about atoms motion in 2.7 k. – Hammar Feb 24 '16 at 18:34

They move, at first because they once moved :-) . How do you stop them ?

Anyway the interstellar is not always cold, and it is even ultra hot at places. One of the reason is UVs travelling through, and thus bringing kinetics energy to atoms.

• Would you say that the main influences on their motion are stellar and galactic gravitational fields, and shockwaves? (e.g. from supernovas) – Mitchell Porter Feb 24 '16 at 10:52
• This is so vague, I can't imagine how it could help anyone. – Kyle Kanos Feb 24 '16 at 10:55
• Man, you missed the point. Look closely at the OP's question. The FIRST thing to do in presence of such remarks is to make the OP understand it would be very artificial and difficult to have atoms "motionless" (thinking of what it really means). THEN once this is understood you can go to the next step and explain the various forces and phenomenons in the ISM. Don't confuse peoples with your students. – Fabrice NEYRET Feb 24 '16 at 11:47