Milky Way (-like) Galaxy

Milky Way Galaxy WE ARE INSIDE IT Eye 1. Sun + Moon … EARTH AS A SPINNING PLANET 2. planets … SOLAR SYSTEM 3. nearby stars … SOLAR NEIGHBORHOOD 4. white haze of sky … MILKY WAY GALAXY Greek “galactos” = milk thickest in Sagittarius, through Cygnus, Orion dark bands obscuration star clusters nurseries Galileo first resolved into stars WE ARE INSIDE IT

Milky Way from Inside

Milky Way Galaxy Members stars 1011 to 1012 entire HR Diagram not stars white dwarfs, neutron stars, black holes brown dwarfs rocks solar systems, planets, asteroids, KBOs, comets gas/dust nebulae, ISM, GMCs life

Herschel’s Milky Way History pre-20th century … no idea there were “galaxies” Herschel star count model … pre-parallaxes, HR assumptions: (a) Sun at center (b) all stars identical (c) no obscuration result: much too small

Galactic Scales Galactic Scales Spiral Nebulae like Andromeda individual baby stars but, no parallax measured

Clues to the MW Mystery Variable Stars small fraction of all stars eclipsing … two stars novae/supernovae … sporadic pulsating … intrinsic, regular HR “Instability Strip” pulsate because of changing opacity

Distance Ladder Rung #4

Shapley Solves MW Mystery Globular RR Lyrae variables Clusters found: GCs at great distances spherically symmetric EUREKA! … center of GCs = center of MW

Milky Way Map

Milky Way Territories DISK BULGE HALO size details origin stars 30000 x 300 pc 100:1 arms last young Pop I blue/white 2% metals open clusters gas/dust/* formation merry-go round BULGE 4000 x 6000 pc football bar mixed mixed Pops yellow massive BH gas/dust ring random spiral HALO 50000+ pc spherical edge? first old Pop II red 0.02% metals globular clusters no gas/dust/planets electron-like disordered size details origin stars contents motion

Milky Way Formation HALO FIRST … DISK LAST Four Phases 1. early star formation in ENORMOUS contracting cloud GCs as substantial blobs halo stars old halo stars have random motion size of halo slight halo rotation 2. collapse of gas to disk … no gas/young stars 3. star formation in disk … young stars now 4. cannibalization of other galaxies/GCs … star streams HALO FIRST … DISK LAST

Formation

Star Motions

Andromeda Galaxy

Spiral Arms Tracers 21 cm emission of H atom CO in GMCs Scale Sun is 8 kpc from center of MW stars, GCs, arms, observed to radius of 15 kpc gas observed to radius of 25 kpc Formation? cohesive arms would wind up and disappear …

Spiral Arms Formation? cohesive arms would wind up and disappear … Density Waves gas moves into ripple compresses stars form Causes galaxy collisions? companion galaxy tides? bar-like asymmetry of rotating bulge? gas instability near the bulge? Observe patterns not mass movement!

Spiral Traffic

Milky Way Rotation

Missing Mass Rotation Curve orbital velocity vs. distance curve RISES even beyond where we see stars more mass than seen out to 40 kpc (at least)

Missing Mass Conclude > twice as much mass as can be seen dark matter dark at all wavelengths Candidates black holes … need lots of big stars white dwarfs … burned out regular stars red/brown dwarfs … need lots subatomics … hard to test mixture?

Milky Way Center center of GC distribution center of gas distribution 8 kpc away

Milky Way Center Contents 50000 stars/cubic parsec … million X local! stellar encounters and crashes molecular gas ring, diameter 400 parsecs Ground Zero bright radio source … Sgr A* X-rays, gamma rays … energetic hot gas at 10000 K … heating extended filaments … magnetic fields flickering … small fast moving gas+stars … MASSIVE

Milky Way Center see the movie!

SUPERMASSIVE BLACK HOLE Answer? 1 million Msun 0.02 AU = 4 Rsun SUPERMASSIVE BLACK HOLE

Milky Way Today THIS IS AN ARTIST’S CONCEPTION! .

Milky Way Today Milky Way Today THIS IS REAL!

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