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

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

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 Mass of the Star (M sun )Main Sequence Lifetime 110 billion years 5100 million years 1010 million years Shine bright, die young

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

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 's of stars often have gas and dust Globular Clusters: Contain oldest stars in the Milky Way billion years old stars in orbit around center of cluster, gravitationally bound Typically 100,000 - million stars never have gas and dust

PLEIADES Open Cluster

Omega Cen: Globular Cluster

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.

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.

Pulsating Variable Stars: Cepheid Variables, RR Lyrae

Pulsating Stars

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.

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.

Schematic of Star-Formation

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

Two important physical principles which govern what happens when gas clouds collapse to form stars 1.ADIABATIC Contraction and Expansion (no source of sink of energy) Gas contracts  HEATS up Gas expands  COOLS 2.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

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

The Spinning Skater

Images of Star-Formation in the Milky Way Galaxy

Bow shock by a young stellar object in Orion

Disk in Orion

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.

Spitzer Space Telescope IR image

Pillars of Creation HST Picture of part Of the Eagle Nebula

Protostars in the H-R diagram:

How long does it take a protostar to "reach" the main-sequence, i.e. start nuclear fusion? It depends on its mass:

What do stars look like inside when they just finish the proto-star stage and start their main-sequence phase?

Failed stars: If mass < 0.08 solar masses, nuclear fusion never starts, and we call the object a "brown dwarf". Gravitational contraction in brown dwarfs is halted by Electron DEGENERACY PRESSURE Electrons cannot be smushed so hard that two electrons occupy the same place at the same time. (Pauli Exclusion Principle)