What is the interstellar medium (ISM) and what is it composed of?
• We call all this gas and nebulosity the interstellar medium, the stuff “between the stars”, like interstate means between states. We abbreviate interstellar medium as ISM. • From spectra of the nebulae, we know they are made of gas, just like the photospheres of stars like our Sun. • The gas in the emission nebulae often have compositions very similar to the Sun’s: mostly hydrogen and helium, with traces of other elements like carbon, nitrogen, oxygen, and iron. In fact, it is as if they came from stars (…hmmmmm…). • There is also dust in the ISM, small grains about 1 micron (10^-6 m) in size. About 1% of the ISM is in dust grains. • Scattering of light off of dust grains is what reddens the light of stars seen through dust clouds. The short wavelength blue photons are more easily scattered than the longer wavelength red photons. This same effect reddens the Sun at sunset – and after dust storms or volcano eruptions or big fires sunsets are spectacularly red. • The dust is made o
