1. Brittany Griner Robin McCollum INTERSTELLAR MEDIUM

2. Matter among the stars Made up of two components: gas and dust Gas atoms exists at a size of 10^-10 m and at density of one atom per cubic centimeter Dust particles exist at a size of 10^-7 m and at density 1000 per cubic kilometer Interstellar matter are mainly individual atoms and particles Wavelengths smaller than the matter from distant stars cannot penetrate the interstellar matter resulting in the dimming stars or extinction Because the blue, higher wavelengths are affected, stars are subject to the reddening effect

4. COMPOSITION 90% of the gas is atomic molecular hydrogen; 9% is helium; 1% consists of heavier elements Abundances of carbon, oxygen, silicon, magnesium, and iron are very low Infrared evidence suggests that it is composed of silicates, graphite, and iron which supports the theory the interstellar dust forms out of interstellar gas Dust probably also contains some “dirty ice” which is a frozen mixture of ordinary water ice contaminated by ammonia, methane, and other chemical compounds

6. PHYSICAL PROPERTIES Dust particles are not spherical Individual dust grains are apparently elongated or rod-like Can tell the shape because the light emitted by stars is dimmed and partially polarized by the dust Is heated by low-energy cosmic rays, photoionization, x-rays, photoelectric heating, chemical heating, and grain-gas heating Is cooled by permitted lines and fine structure cooling