# Harvard Astronomy 201b

## CHAPTER: A Sense of Scale

In Book Chapter on January 13, 2013 at 8:40 pm

(updated for 2013)

How dense (or not) is the ISM?

• Dense cores: $n \sim 10^5 ~{\rm cm}^{-3}$
• Typical ISM: $n \sim 1 ~{\rm cm}^{-3}$
• This room: 1 mol / 22.4L $\sim 3 \times 10^{19}~ {\rm cm}^{-3}$
• XVH (eXtremely High Vacuum) — best human-made vacuum: $n \sim 3 \times 10^{4}~ {\rm cm}^{-3}$
• Density of stars in the Milky Way: $2.8~{\rm stars/pc}^3 \approx 0.125~M_\odot/{\rm pc}^3 = 8.5 \times 10^{-24} ~{\rm g / cm}^3 \sim 5~{\rm cm}^{-3}$

In other words, most of the ISM is at a density far below the densities and pressures we can reproduce in the lab. Thus, the details of most of the microphysics in the ISM are still poorly understood. We also see that the density of stars in the Galaxy is quite small – only a few times the average particle density of the ISM.