Astro 201: Sept. 30, 2010

1. Astro 201: Sept. 30, 2010 • Pick up Midterm #1 from piles along the wall. Correct answers are printed on the scantrons, • I will post keys also and correct mistake on Green Key • New Homework 4 on web site, due next Thursday • Next on-line quiz will be posted after Tuesday’s lecture • Read Hester, Chapter 15 • Today: • STARS: Ages of Clusters Redux, Star-formation • After this: Evolution of the Sun, Stellar endpoints: white dwarfs, neutron stars and black holes

2. H-R Diagram Hertzsprung- Russell Diagram Plot Luminosity versus Surface Temperature (or equivalently, Luminosity versus spectral classification)

3. Stellar Lifetimes on the Main Sequence: More Massive Stars are more luminous, and are burning hydrogen more efficiently. They therefore have shorter lifetimes on the Main Sequence before they burn up the Hydrogen in their core Shine bright, die young

4. After the hydrogen fuel in the core of the main sequence star is used up, There is no longer enough thermal pressure in the core to balance gravitational collapse. No more hydrostatic equilibrium What happens next? Star rearranges itself outer layers expand and cool Star becomes a red giant or supergiant Eventually more processes happen and the red giant becomes a supernova or planetary nebula, and then a white Dwarf, neutron star or black hole – more on this later

5. STAR CLUSTERS All the stars in a cluster are (1) at the same distance, and (2) were formed together, so are the same age. Open Clusters: Young (Less than a billion years old) found in the disk of the Milky Way typically 100's - 1000's of stars often have gas and dust Globular Clusters: Contain oldest stars in the Milky Way -- 12-13 billion years old stars in orbit around center of cluster, gravitationally bound Typically 100,000 - million stars never have gas and dust

6. PLEIADES Open Cluster

7. Omega Cen: Globular Cluster

8. Ages of Star Clusters and the HR Diagram In Old clusters, some of the stars have "left" the Main Sequence – and become Red Giants, white dwarfs, etc. The age of the cluster = the lifetime of the stars at the "main sequence turnoff" in the H-R diagram.

11. Homework #4: Step 1: Find the Turn-off MASS, ignore “Blue Stragglers” Step 2: Use the plot on the right to figure out the main sequence lifetime (GYRS) Of the Turn-off Mass star. This is the age of the cluster.

12. Pulsating Variable Stars: Cepheid Variables, RR Lyrae

13. Pulsating Stars

15. Period of pulsation is correlated with absolute magnitude. Thus, given the apparent magnitude and period of pulsation, you can derive the DISTANCE to the star.

16. STAR-FORMATION Key Concepts: * Stars form by the gravitational collapse of dense clumps of interstellar gas, molecules and dust. * The collapsing protostar forms a disk and jet. * The collapse stops when nuclear fusion begins in the protostar core. * When a giant molecular cloud of interstellar gas forms stars it fragments into several protostars, and eventually a cluster of stars forms. * If the clump is too massive it splits into a binary star system. * Eventually the protostellar disk turns into a system of planets orbiting the star.

17. Schematic of Star-Formation

19. A real protostellar disk and jet, with dark dust lane running through the disk:

20. Two important physical principleswhich govern what happens when gas clouds collapse to form stars • ADIABATIC Contraction and Expansion • (no source of sink of energy) • Gas contracts  HEATS up • Gas expands  COOLS • CONSERVATION OF ANGULAR MOMENTUM • When gas cloud collapses, conservation of angular • momentum makes the cloud spin faster and faster in one • plane, and collapse in the other dimensions

21. Conservation of Angular Momentum caused the gas to form a spinning disk.

22. The Spinning Skater

25. Images of Star-Formation in the Milky Way Galaxy

26. Bow shock by a young stellar object in Orion

28. Disk in Orion

37. Big star-formation region in the Large Magellanic Cloud: 30 Doradus When a cluster of stars form, the massive stars (which are short-lived) produce UV photons which blast the surrounding gas and dust. The UV photons excite the electrons to high energy orbitals In the hydrogen atoms --> H-alpha emission, red in these pictures. Black areas are clouds of dust. The green is another emission line of oxygen.

48. Spitzer Space Telescope IR image

50. Pillars of Creation HST Picture of part Of the Eagle Nebula