Transcript chap_11
Chapter 11 Galaxies and the Foundation of Modern Cosmology The Milky Way A band of light making a circle around the celestial sphere. What is it? Our view into the plane of our galaxy. If we could view the Milky Way from above the disk, we would see its spiral arms. We see our galaxy edge-on. Primary features: disk, bulge, halo, globular clusters We observe the star–gas–star cycle operating in Milky Way’s disk using many different wavelengths of light. How is gas recycled in our galaxy? Insert TCP 6e Figure 19.6 X rays from hot gas in supernova remnants reveal newly made heavy elements. Where do stars tend to form in our galaxy? Much of the star formation in the disk happens in the spiral arms. Ionization nebulae Blue stars Gas clouds Whirlpool Galaxy Molecular clouds in Orion Composition: • Mostly H2 • About 28% He • About 1% CO • Many other molecules Orion Nebula Proplyds Gravity forms stars out of the gas in molecular clouds, completing the star–gas– star cycle. Radiation from newly formed stars is eroding these starforming clouds. How did our galaxy form? Our galaxy formed from a cloud of intergalactic gas. Halo stars formed first as gravity caused gas to contract. Remaining gas settled into a spinning disk. Stars continuously form in disk as galaxy grows older. Insert TCP 6e Figure 19.19 Detailed studies show that halo stars formed in clumps that later merged. Insert TCP 6e Figure 20.4 Barred spiral galaxy: has a bar of stars across the bulge Lenticular galaxy: has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical) Elliptical galaxy: all spheroidal component, virtually no disk component Red-yellow color indicates older star population. Irregular galaxy Blue-white color indicates ongoing star formation. Spheroid dominates Hubble’s galaxy classes Disk dominates The collisions we observe nearby trigger bursts of star formation. Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical.