Quiz 2 Study Guide --- Astronomy 1101 --- Planets to Cosmos Stellar Brightnesses: Luminosity Apparent Brightness Inverse Square Law of Brightness : B = L/(4 pi D^2) Stellar Spectra: Colors of stars and relation to Temperature Main Spectral Types: O B A F G K M L T The Hertzsprung-Russell Diagram: Plot of Luminosity vs. Temperature for stars. Main Sequence Stars Giant Stars Supergiant Stars White Dwarfs Luminosity-Radius-Temperature Relation: L = 4 pi R^2 sigma T^4 Stellar Structure & Evolution ----------------------------- The Internal Structure of Stars: Mass-Luminosity Relationship: on the Main Sequence : L ~ M^4 Hydrostatic Equilibrium : balance of pressure and gravity. Energy Generation in Stars Hydrostatic equilibrium Nuclear Fusion Energy The Solar Age crisis: Is the Sun powered by Chemical Reactions? Gravitational energy? Proton-Proton Chain Neutrinos CNO Cycle Hydrostatic Thermostat, thermal equilibrium Energy Transport: Radiation, Convection The Main Sequence Burn Hydrogen into Helium in their cores. In Hydrostatic & Thermal Equilibrium Mass-Luminosity Relationship for M-S stars The Main Sequence is a Mass Sequence Lower M-S: M < 1.2 Msun Burn H via p-p chain Upper M-S: M > 1.2 Msun Burn H via CNO cycle Dependence of M-S Lifetime on stellar Mass. Larger Mass = Shorter Life. Typical lifetimes of O-stars, M-stars, & the Sun t_MS ~ 1/M^3 Minimum and Maximum masses of stars Brown Dwarfs (M < 0.1 Msun) The Evolution of Low Mass stars (M < 4 Msun) Main Sequence phase through H exhaustion in core He core formation & H shell burning Ascent of the Red Giant Branch Helium Flash & the Triple-Alpha Process Descent to the Horizontal Branch He core burning & H shell burning C-O core formation Asymptotic Giant Branch star He and H burning Shells Onset of instability Envelope Ejection & Formation of a Planetary Nebula Core evolves into a White Dwarf star What are the timescales for these phases? Which way do stars move on the HR diagram? Stages of Evolution of High Mass O & B Stars (M > 4 Msun) Stars with 4 < M < 8 Msun Burn Hydrogen, then Helium, then Carbon Blow off their envelope after exhaustion of Carbin Burning Core becomes an O-Ne-Mg White Dwarf Stars with M > 8 Msun Burn Hydrogen up through Carbon, Neon, Oxygen & Silicon What are the timescales for these burning phases? Iron Core Formation & burning shells Catastrophic collapse of Iron Core leading to Iron core bounce & supernova explosion ejecting envelope core collapses to a neutron star or black hole BHs may come from stars with M > 20-30 Msun, but uncertain Supernovae Nucleosynthesis in Supernovae (main source of heavy elements) Role of supernovae in seeding interstellar space with heavy elements Role of supernovae in producing neutron stars Star formation & the main sequence raw material for stars are giant molecular clouds cold and dense blobs of gas in galaxies steps in star formation: gravitational collapse establish hydrostatic equilibrium (proto-star phase) establish thermal equilibrium (pre-main sequence phase) land on the main sequence planets-forming debris disks around newly formed stars what energy source powers a protostar? what is the Kelvin-Helmholz timescale? which stars reach the main sequence the fastest? are low or high-mass stars intrinsically rarer? what are the minimum and maximum masses of stars? White Dwarfs: Remnant cores of low-mass stars (M < 8 Msun) About the size of the Earth R ~ 5000 km Held up by Electron Degeneracy Pressure Different from the Ideal Gas Law Equation of State: does not depend on temperature. Allows bodies that are cold to remain in hydrostatic equilibrium. Maximum Mass ~1.4 Msun (Chandrasehkar Mass) White dwarf just cools off over a long time. Can accrete from a binary companion, or run into another (collision, merger) white dwarf to produce a Type Ia supernova; thermonuclear detonation; run-away C burning. Produces a lot of Iron, most of the Iron in the universe. Neutron Stars: Remnant cores of massive stars (M > 8 Msun) About the size of a city: R ~ 10 km. Held up by Neutron Degeneracy Pressure: does not depend on temperature, allows cold neutron stars to stay in hydrostatic equilibrium Pulsar = rapidly spinning neutron star Neutron stars cannot support themselves above 2-3 Msun. Neutron degeneracy pressure fails. Gravity wins. Black hole formation Black Holes: Black Holes are totally collapsed objects gravity so strong not even light can escape predicted by General Relativity theory remnant cores of very massive stars (M > 20-30 Msun?) Singularity Schwarzschild Radius & Event Horizon Find them by their Gravity X-ray Binary Stars & Black Hole Candidates Hawking Radiation & Black Hole Evaporation Tests of Stellar Evolution: H-R Diagrams of Star Clusters Why clusters are good for testing stellar evolution Changes in the H-R diagram of a star cluster as it ages Estimating cluster ages from the Main-Sequence Turn-off Open Clusters Young clusters of few 100s - 1000s of stars Blue Main-Sequence stars & few giants Shapes of their H-R diagrams Range of ages from Few million to few billion years Globular Clusters Old clusters of a few 100,000 stars No blue Main-Sequence stars & many giants Shapes of their H-R diagrams 10-13 Billion years old