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