ASTR 1020 1st Edition Lecture 17Outline of Last Lecture I. The Stellar Graveyard: Black HolesA. Surface of a Black HoleB. Space Travel Near a Black HoleC. Observational EvidenceD. Gamma-Ray Bursts E. Star OutcomeOutline of Current Lecture I. Milky Way GalaxyA. ComponentsB. Our PositionC. MassD. RecyclingE. Spiral ArmsF. Formation of GalaxyCurrent LectureI. Milky Way GalaxyA. Components- Disk – flattened distribution of mainly young stars and star formation regions – clouds of gas and dust, open clusters, and spiral arms (Sun resides in the disk) - Central Bulge – contains a mix of older and younger stars surrounding center - Halo – large spherical region containing some stars and globular clusters (very old stars; little gas or dust)B. Our Position- Harlow Shapley 1920 - Found the globular clusters form a nearly spherical distribution around a point in the direction of Sagittarius.- Stars in the disk all orbit in the same direction with a little up-and-down motion- Orbits of stars in the bulge and halo have random orientationsC. Mass- To find Mass of the Galaxy- Use Kepler’s Third Law: (M + m) P 2 = a 3 M = mass interior to Sun (solar masses) m = mass of Sun (solar masses) (1<<M)P = orbital period (years) = 230 x 106 yearsa = distance to Galactic center (AU) = 8600 pc x 206265 AU/pc = 1.8x109 AU M = a 3 / P 2 = 1.1 x 1011 solar masses (110 billion Suns!)D. Recycling - Stars make new elements by fusion - Dying stars expel gas and new elements, producing hot bubbles (~106 K) and cosmic rays (energetic electrons, protons and atomic nuclei) - 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- X-rays from hot gas in supernova remnants - New elements made by supernova mix into interstellar medium: gas enriched with heavy elements- Multiple supernovae create huge hot bubbles that can blow out of disk - Gas clouds cooling in the halo can rain back down on disk- Gravity forms stars out of the gas in molecular clouds, completing the star-gas-star cycleE. Spiral Arms- Much of star formation in disk happens in spiral arms- Spiral arms are waves of star formation 1. Gas clouds get squeezed as they move into spiral arms2. Squeezing of clouds triggers star formation 3. Young stars flow out of spiral armsF. Formation of Galaxy- Protogalaxy formed first condensations some 14 billion years ago: - initial collapse forms halo stars - remaining gas rains in, forms disk - inner part has highest heavy atom content from early, intense star formation- Our galaxy probably formed from a giant gas cloud- Halo stars formed in clumps that later merged- Remaining gas settled into spinning disk- Stars continuously form in disk as galaxy grows