Chapter 19 Our Galaxy All-Sky View
The Milky Way galaxy appears in our sky as a faint band of light Dusty gas clouds obscure our view because they absorb visible light
Edge-on view, primary features: disk, bulge, halo, globular clusters
From above the Milky Way has spiral arms
We’re about 28,000 ly from center, in one of the spiral arms
Stars in disk all orbit in same direction (with a little up & down motion) Stars in the bulge and halo have random orbits
Star-gas-star cycle Stars make new elements by fusion Dying stars expel gas and new elements, producing hot bubbles (~106 K) Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K) Further cooling permits molecules to form, making molecular clouds (~30 K) Gravity forms new stars (and planets) in molecular clouds Gas Cools
Observations of Milky Way’s disk using many different wavelengths of light
Where are the star forming regions
Ionization nebulae - found around high-mass stars, means active star formation Orion nebula
Reflection nebulae - scatter the light from stars Why do reflection nebulae look bluer than nearby stars? They scatter blue light the most (same as sky!)Chamaeleon 1 complex (VLT UT1+FORS1) V, R, and I bands http://www.eso.org/outreach/gallery/vlt/images/Top20/Top20/top8.html
Halo: No ionization nebulae, no blue stars no star formation Disk: Ionization nebulae, blue stars star formation
Most star formation in disk happens in spiral arms. Ionization Nebulae Blue Stars Gas Clouds Whirlpool Galaxy
Gas clouds get squeezed as they move into spiral arms Squeezing of clouds triggers star formation Young stars flow out of spiral arms
How did our galaxy form?
Models of formation assume 2 things: Matter originally distributed almost uniformly Gravity of denser regions pulled in surrounding matter
Denser regions contracted, forming protogalactic clouds H and He gases in these clouds formed the first stars
Halo stars formed first as gravity caused cloud to contract
Supernova explosions from first stars slowed collapse & kept much of the gas from forming stars Leftover gas settled into spinning disk
Stars continuously form in disk as galaxy grows older
This cosmological simulation follows the development of a single disk galaxy over about 13.5 billion years, from shortly after the Big Bang to the present time. Colors indicate old stars (red), young stars (white and bright blue) and the distribution of gas density (pale blue); the view is 300,000 light-years
What lies in the center of our galaxy? Infrared light from center (left) , radio emission from center (right) Swirling gas near center Orbiting star near center
Stars are orbiting something massive but invisible. Orbits indicatethe object has a mass of about 4 million MSun Download a great movie of star motions from: http://www.astro.ucla.edu/~jlu/gc/images/orbits_pause.gif
Stellar Orbits Around Galaxy Center