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Gas too thin remains gas. If it gets too dense collapses into stars Jeans Mass -- gravitational attraction > thermal Equilibrium / cycle 1 Msun/yr new stars 0.5-0.8 Msun /yr returned SN Heavy Red Giants Element O stars Enriched
gas in spiral arms has a high density, crashed in and becomes more dense. Star formation in young stars -- O stars in spiral arms Basic energertics HEATED by supernova ejecta shockwaves COOLS by Bremstrahlung with lines n1kT1=n2kT2 HOT -- x-rays (soft x-ray background) WARM -- UV (can be seen in absorption, but not in emission) COLD -- IR
Supernova Remnanats SN explodes --> 2-10 Msun ejected at 3-10000 km/s Energy released Three Phases 1. Free expansion 2. Adiabatic sweeping up matter gas heats up 106-107K 3. Snowplough (momentum conserving) Radiates away its thermal energy through x-ray emission Shock front gets lost in cloud motion
Equations of Growth (n1 inside, n0 outside) Pressure Equilibrium x-ray sources: non thermal radio (trapped high energy particles)
Cyngus Loop The classic SNR 770 pc away ~20,000 years old Veil Nebula dense clouds optically emit, shocked by passage of waves Very bright x-ray source
Interstellar Bubbles 10-5 Msun / yr V~2000 km/s E after 10^6 years ~ 0.1 SN -- large!! Can affect ISM near star -- blow it away When SN goes off -- 1051 ergs released SNR control state of ISM: makes tunnels of very hot gas in warm and cold gas BUT SN come for OB stars OB stars come in clumps, therefore SN come in clumps
HII Regions O star photon flux near star very high ionization rate > recombination rate Radiates energy away through recombination lines Ha, Hb forbidden lines [OIII] Lya trapped, therefore IR source from dust Gas between stars galaxy started as gas some left over some reprocessed Known for hundreds of years
HII regions photon l < 912 angstrom, can remove electron for atom OB stars generate lots of l < 912 angstrom light since OB young stars, are usually still in clouds they formed from start with turn on star, ionizes gas around star Gas recombines, hits balance at some radius. Ha is very strong, results in a red appearance of HII region Next aspect of ISM was dust Dust -- next aspect of ISM was dust light does not need to be absorbed to make a star appear fainter. Like a sunset on a dusty day things appear redder. Blue is scattered more. Infrared is not scattered. IR is the only way to see center of Milky Way. Clearly activity is there -- IR will reveal it.
Molecular clouds Very high density regions -- stars formed. High density therefore radiates fast, therefore cold T falls to 50-100K Molecular form -- CO Seen in IR and radio Clouds in less dense regions exist too. 21 cm emission Can be seen by absorbtion spectroscopy Molecular hydrogen Hot gas --- OVI 300,000K gas Lots of elements Motion of gas -- 100 km/s
Supernova Remnants Explosion sends out ~10 Msun at 10,000 km/s. Blast wave -- 1051 ergs. Same equation as effects of atom bomb in atmosphere. Phase I: Expands freely, picks up all gas in its path ~100 years ~few parsecs Phase II: Energy conserving, momentum also conserved. Slows down, but heats up. 1-10 million K --> emits x-rays ~20,000 years ~50-100 parsecs Phase III: X-rays carry energy away ~1 million yearrs Waves roll through ISM, interact with other SNRs Phase IV: Breaks up and loses intensity
Structure of Milky Way This basic picture is hard to put together. Cant just count stars -- too may. We also cant see all of them because of interstellar dust. WE could look at other galaxies, but we didnt know what they were. Globular clusters are bright enough to be studied over dust. This raises the question of how far away they are. We can fit them on the HR diagram. Centered around point in Sag - 10 kpc away. This wa the first break in understandign the Milky Way. We can see local arms, these indicate that we are in a spiral galaxy.
Radio Astronomy No obscuration by dust Karl Jansky -- first radio telescope. The first radio source is Sag A. -- galactic center. 21 cm line -- tracer of neutral Hydrogen. We can deconvolve this and map. In the 1960s, for the first time, observers were abole to recognize spiral structure of galaxy. Tracing the rotation outward, we see that there is a lot of mass. IN fact, most of the mass is in giant, invisible halo. This raises aquestion of what the halo is made of... neutrinos? baseballs? blackholes?
WE CAN NOT SEE MOST OF OUR GALAXY Why is it spiral? but would wind up too much. Sun has time to go around 100 times, yet only several spirals -- it isnt that simple. Spiral arms -- density waves Matter slows down, gas piles up --> prefered star formation. It is OBA stars that light up spiral arms. Otherwise would hardly be able to see them. Stellar generations: galaxy formed first in middle. more generations in center --> higher heavy element abundance
UV Astronomy 912-3500 (3500 --> optical) Particularly 1200-2000 Angstrom Why study UV? Basic ground state transitions of the atoms all ionizing potential 3eV < IP < 25 eV most 4eV < IP < 14eV If you can observe at 4-14 eV, will get fundamental data most notably Lyman alpha -- main line o f main elements 1216 A Must be above atmosphere, but conventional optics work (just be careful) 1. Rockets Stellar Winds 1967 2. Copernicus High Resolution 1972-1980 3. IUE Spectrograph 1978-now 4. Space Telescope 1985 5. FUSE 900-1200 1990 Types of objects 1. O stars Photospheres a) Stellar Winds b) ISM in absorption 2. Late Stars -- Chronospheres 3. QSOs a) Emission Spectrum b) Intergalactic Gas
Galaxies Self gravitating Instabilities, perturbations excite spiral node. Computer simulations --> much heavier for spiral, low mass stars in halo?
Clustering Local group Magellanic Clouds Dwarf galaxies M31, M33 Local Group M31 Sb Milky Way SBc M33 Sc Magellanic Clouds 1% mass of Milky Way 66,000 pc distant Early type HI stream -- orbits behind 30 Doradus Dwarf Galaxies NGC 205, M32 orbit M31 Virgo cluster falling toward it x-ray source 10^8 K Bremsstrahlung M87 supermassive galaxy Missing Mass -- virial theorem vs mass/light rotation curves Super clusters hierarchy of clustering largest structure scal of universe x-rays
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