# Harvard Astronomy 201b

## CHAPTER: Energy Density Comparison

In Book Chapter on February 26, 2013 at 3:04 am

(updated for 2013)

See Draine table 1.5. The primary sources of energy present in the ISM are:

1. The CMB ($T_{\rm CMB}=2.725~{\rm K}$
2. Thermal IR from dust
3. Starlight ($h\nu < 13.6 {\rm eV}$
4. Thermal kinetic energy (3/2 nkT)
5. Turbulent kinetic energy ($1/2 \rho \sigma_v^2$)
6. Magnetic fields ($B^2 / 8 \pi$)
7. Cosmic rays

All of these terms have energy densities within an order of magnitude of $1 ~{\rm eV ~ cm}^{-3}$. With the exception of the CMB, this is not a coincidence: because of the dynamic nature of the ISM, these processes are coupled together and thus exchange energy with one another.