General Astronomy The Interstellar Medium Credits: Much of this slideset is modified from lectures by Dr. Peter Newbury (UBC)

Interstellar Dust Detection: Temperature: Density: Content: K 1000 particles per cubic Km Small bits of carbon, silicates, nanodiamonds, iron. Some water, methane and ammonia ices. Dark nebula blocks light from background stars

Interstellar Dust Detection: Temperature: Density: Content: Subaru is Japanese for "Star Cluster" K 1000 particles per cubic Km Small bits of carbon, silicates, nanodiamonds, iron. Some water, methane and ammonia ices. Reflection nebula reflects the light of nearby stars

Interstellar Dust Grains Detection: Temperature: Density: Content: Frozen 100 to 1000 grains per cubic Km Grains of 10 6 to 10 9 atoms of carbon, silicates, nanodiamonds, iron. Interstellar grains block and scatter (redden) starlight

Interstellar Dust Grains Is there dust that we cannot see? Yes. Quite a bit in fact. The space between the stars is filled with thin clouds of dust. Its major effect is in what is known as extinction. When light from distant objects passes through these clouds, it is reddened (just like a sunrise or sunset)

Extinction Dimming of starlight at all wavelengths. –Extinction caused by scattering of light out of the line-of-sight--less light reaches us. In 1930, R.J. Trumpler plotted the angular diameter of clusters vs. distance to cluster. –Distance found from inverse square law of brightness. –He found a systematic increase of the linear size of the clusters with distance. –Unreasonable! It would mean that nature had put the Sun at a special place where the size of the clusters was the smallest. –More reasonable: the Sun is in a typical spot. It's simply that more distant clusters have more stuff between us and the clusters so that they appear fainter (farther away) than they really are.

Reddening Extinction depends on wavelength –Bluer wavelengths are scattered more than redder wavelengths. Behavior says that the dust size must be about the wavelength of light –Less blue light reaches us so object appears redder than it should. Trumpler showed that a given spectral type of star becomes increasingly redder with distance.

Interstellar Gas Clouds Neutral Hydrogen (H I) Ionized Hydrogen (H II) Ultrahot gas Giant molecular clouds

Neutral Hydrogen (H I) Detection: Temperature: Density: Content: Radio telescopes. This is the source of the 21cm band. 100 to 6000 K 1 to 50 atoms per cc Clouds of Hydrogen 3 to 30 lightyears in diameter. Also C, O, N, CO, CH, CN, H 2 Organic Chemicals! 21cm Milkyway Optical Milkyway

21-Cm Radio Line A hydrogen atom has a proton and an electron. In addition to mass and charge, they also have a property known as spin. There are two possible states for spin: Up and Down. That means there are 4 possible configurations. Up/Up, Up/Down, Down/Up and Down/Down (Proton Spin/Electron Spin). The less-energy states are where the spins are opposite. So the act of "jumping" from a higher state (Up/Up or Down/Down) to the lesser will emit a photon at radio wavelengths (21-cm)

Ionized Hydrogen (H II) Detection: Temperature: Density: Content: Ionized hydrogen is continually capturing and emitting photons. It is mostly the red H  line of Hydrogen Hot. 100,000 K 100 to 10,000 atoms per cc Glowing clouds of ionized Hydrogen near hot stars

H II Regions Fluorescence of hydrogen atoms. Ultraviolet light from hot O & B stars is absorbed by the Hydrogen gas and re-emitted mostly at visible wavelengths, primarily 6563 Å(red color). Each UV photon produces a visible photon. O & B stars only found in regions of star formation (know why?). H II region spectra much simpler than star spectra- easier to decipher. Distribution of H II regions is in spiral pattern. O & B are spiral tracers also.

Ultrahot Interstellar Gas Detection: Temperature: Density: Content: Ultrahot K 1000 atoms per cubic meter Particles, atoms and molecules are fired out into space at 1000s of km/sec; usually from Supernovae or other eruptives Ultrahot outflow heats up the surrounding gas

Giant Molecular Clouds Detection: Temperature: Density: Content: Complex brightline and darkline spectra. Varies from very cold 10K to very warm 10 5 to 10 6 K (depending on nearby stars) 100,000 atoms per cc Large clouds, 50 to 200 light years across. H, O, C, N, S, C 2 H 5 OH (ethanol), HC 3 N (cyanoacetylene), CH 3 CHO (acetaldehyde) Building blocks of DNA

Molecular Clouds Most of the molecules in the ISM are clumped together into clouds with masses anywhere from just a few solar masses to over a million solar masses with radii ranging from a few pc to over 100 pc. Milky Way has about 2.5 billion solar masses of molecular gas with about 70% of it in a ring at 4-8 kpc distance from the center. Not much molecular gas at 1-3 kpc distance from center. About 15% of total molecular gas mass is located close to galactic center within 1.5 kpc from the center. Most of the gas is clumped in the spiral arms within the disk