Lec24 - Matter in our Galaxy emits different kinds of...

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Unformatted text preview: Matter in our Galaxy emits different kinds of radiation, depending on what stage of star–gas–star cycle it is in. Ionization Nebulae • Found around high-mass stars (OB associations) • O & B stars (T > 25,000K) make enough UV photons to ionize hydrogen in the nebula • Gas re-emits Hα line (red) Reflection Nebulae • Light from central star is reflected and scattered by dust • Blue light is scattered more easily than red • Similar to our blue sky lit up by yellow Sun Galactic disk, bulge & halo... Halo vs. Disk • Stars in disk relatively young. • fraction of heavy elements same as or greater than the Sun • plenty of high- and low-mass stars, blue and red • Stars in halo are old. • fraction of heavy elements much less than in Sun • mostly low-mass, red stars • Stars in halo must have formed early in Milky Way’s history. • when few heavy elements existed • no ISM in the halo • star formation in halo stopped long ago when all gas flattened into the disk Stellar Orbits • All Stars orbit Galactic center. • Stars in disk orbit: • • • • in same direction in same plane (like planets) just “bobble” up and down like merry-go-round • Stars in bulge & halo orbit: • in different directions • with various inclinations • like swarming bees Mass of our Galaxy • Use Kepler’s 3rd Law to estimate mass: M = a3 /P2 • Sun’s distance from center: ~28,000 l.y. = 8 kpc ~= 2 x 109 AU • Sun’s orbital period: ~200 million years = 2 x 108 yr • Sun’s orbital speed: ~2π 10 au/yr ~= 300 km/s • ⇒ Mass within Sun’s orbit is ~ 2 x 1011 M • Total mass of MW Galaxy : 1012 M Velocity form of Kepler's 3rd Law • From Doppler shift, can infer orbital speed V • Can then infer combined mass of stars from velocity form of Kepler’s 3rd law • Using that Earth’s orbit speed is: 2πAU/yr = 30 km/s = VE ⎛a⎞ ⎜ ⎝ AU ⎟ ⎠ M = 2 M ⎛ P⎞ ⎜ yr ⎟ ⎝⎠ 3 2 ⎛V⎞ ⎛ a ⎞ =⎜ ⎟ ⎜ ⎟ ⎝ VE ⎠ ⎝ AU ⎠ Centrifugal acceleration a = v2/ r Gravity M r g ~ M/r2 Acceleration ~ Mass/distance squared Galactic rotation Spectral Slit across Galaxy Spectrum from Slit across Galaxy Galactic rotation velocity Spiral Structure • Galactic disk has “spiral arms” – much as seen in other galaxies • Arms are not like fins of a fan. • Rather are moving patterns – like “wave” at baseball game • Compression waves propagate around disk • Note how spiral arms appear bluer than bulge or gaps between arms – due to lots of young, hot, blue stars M 51 Spiral Arms • • • • Compression to higher density leads to concentraton of molecular clouds source of mass for new stars short-lived hot, massive stars outline arms and make them blue & bright long-lived low-mass stars pass through several spiral arms in their orbits around disk Center of the Galaxy in Sagittarius Infrared Visual Center of the Galaxy Radio Although dark in visual light, there are bright radio, IR, and X-ray sources at the center of the Galaxy, known as Sgr A*. X-ray Center of the Galaxy • We measure the orbits of fast-moving stars near the Galactic center. • these measurements must be made in the infrared • in particular, this star passed within 1 light-day of Sgr A* • using Kepler’s Law, we infer a mass of 2.6 million M for Sgr A* • What can be so small, yet be so massive? ...
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