# Harvard Astronomy 201b

## CHAPTER: Topology of the ISM

In Book Chapter on February 6, 2013 at 9:57 pm

(updated for 2013)

A grab-bag of properties of the Milky Way

• HII scale height: 1 kpc
• CO scale height: 50-75 pc
• HI scale height: 130-400 pc
• Stellar scale height: 100 pc in spiral arm, 500 pc in disk
• Stellar mass: $5 \times 10^{10} M_\odot$
• Dark matter mass: $5 \times 10^{10} M_\odot$
• HI mass: $2.9 \times 10^9 M_\odot$
• H2 mass (inferred from CO): $0.84 \times 10^9 M_\odot$
• HII mass: $1.12 \times 10^9~M_\odot$
• -> total gas mass $= 6.7 \times 10^9~M_\odot$ (including He).
• Total MW mass within 15 kpc: $10^{11} M_\odot$ (using the Galaxy’s rotation curve). About 50% dark matter.

So the ISM is a relatively small constituent of the Galaxy (by mass).

The Milky Way is a very thin disk (think a CD with a ping-pong ball in the middle) In class (2011), we drew the following schematic of the “topology” of phases in the ISM.

A schematic of several structures in the ISM