Astronomy 102 October 25th.docx

Interstellar Medium: this is the place where stars are formed Interstellar medium is very tenous The ISM is around .1 atom/cm^3 1% of the ISM material is in what we call interstellar dust Dust is iron, silicon, carbon, start to stick together in cold, dense regions It has to be a cold region so that they move slowly and gravity can make them stick together Dust blocks light from stars, galaxies, etc., this is called Interstellar Extinction Certain wavelengths of light will interact with the dust in ISM Short wavelengths will suffer heavily from Interstellar Extinction Cool red giant stars have a lot of dust around them The ISM is also composed of gas Dust is about 300nm in size Interstellar reddening is when we think that a star is redder than it actually is because of the way that dust distorts the light from stars Dust has its own emissions from 10-300K and emits in infrared The ISM is not uniform everywhere, not the same temperature or density The ISM has three types of gases: hot gas, cold gas, and warm gas Half of the volume of inter-stellar space is made of intercloud gas h alpha is a recombination line: protons and electrons combine, H atom is left in an excited state, the atom drops down emitting h alpha h2 regions: hydrogen is heated and ionized by ultraviolet light from hot, luminous stars h2 regions tells you where stars are being formed These come from O and B stars these regions are signposts of where active star formation is happening neutral gas: to go from one state to another, an atom emits a 21cm line it takes a long time for atoms to do it at lower temperatures, hydrogen is in single, neutral atoms this gas emits radio waves with a wavelength of 21cm most cold gas is in cool, interstellar clouds, with hotter intercloud gas between them temperatures are around 10 K, with densities as high as 10^10 atoms/cm^3 Molecular clouds are cold and dense Some places in the clouds are denser than average self-gravity makes these regions collapse The theory of cloud formation assumes that clouds spin, and because of the angular momentum law, it goes faster and shrinks into itself molecular cores collapse under their own gravity center shrinks fastest, outer layers later this produces a dense photostar spin of core produces a disk of material around the protostar material