Chapter 12 The Universe Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

The night sky Celestial observation dates to ancient civilizations Stars Appear as point sources Generate their own light Twinkle from atmospheric turbulence Distance measured in light years (ly): 9.5x1012 km Planets Visible by reflected light Extended sources

Stars Origin of stars Massive, dense balls of incandescent gas Powered by fusion reactions in their core Sun An average star Reference for understanding other stars Origin of stars Gaseous nebula Mostly hydrogen Shock waves induce gravitational collapse Gravitational energy released into higher temperatures and pressures Protostar Accumulation of gases that will become a star

Stellar modeling Core Radiation zone Convection zone Very hot, most dense region Nuclear fusion releases gamma and x-ray radiation Radiation zone Radiation diffuses outward over millions of years Convection zone Structured by hot material rising from the interior, cooling, and sinking Upper reaches: visible “surface” of star Sun surface temp. ~5,800 K

Lifetime of the Sun Converts about 1.4x1017 kg of matter to energy each year About 2,700 000 SUVs! Born 5 billion years ago Enough hydrogen for another 5 billion years Lifetime depends on stellar mass Less massive stars have longer lifetimes More massive stars have shorter lifetimes

Brightness of stars Differences in stellar brightness Amount of light produced by star Size of star Distance to star Apparent magnitude Scheme to quantify observed brightness Brightness values range from 1 (brightest) to 6 (faintest) Later some stars found to be brighter than +1 Sirius -1.42

Absolute magnitude Brightness adjusted to a defined, standard distance Example: Sun Apparent magnitude = -26.7 Absolute magnitude = +4.8

Star temperature Color variations apparent: red, yellow, bluish white Color related to surface temperature Blackbody radiation curves Red: cooler stars Blue: hotter stars Yellow: in between (Sun) Classification scheme Based on temperature: hottest to coolest O, B, A, F, G, K, M

Star types Hertzsprung-Russell diagram Plot of absolute magnitude versus stellar temperature Each dot = star Characteristic grouping Main sequence stars Red giants Novas White dwarfs Cepheid variables Standard star for distance calibrations (Hubble)

Life of a star Protostar stage Main Sequence stage Gravitational collapse Density, temperature and pressure increase 10 million K: fusion ignition temperature Dynamical equilibrium Inward force of gravity Outward pressure of fusion energy Main Sequence stage Depends only on mass More massive stars: Higher core, surface temperatures Use up hydrogen more rapidly O type stars

Stellar evolution Traces path across HR diagram Red giant stage Hydrogen in core exhausted Core collapses, heats Outer shell expands, cools Lifetimes O star ~ millions of years M star ~ trillions of years Mass determines ultimate fate of star

Stellar evolution, cont. Late red giant stage Further core collapse and heating Helium fusion to carbon initiated Radius and luminosity decrease, moves back toward main sequence The end - less massive stars Helium fuel in core used up; helium and hydrogen fusion in shells exhausted Instabilities blow off outer layers into a planetary nebula Carbon core contracts to white dwarf; cools to black lump of carbon

The end - massive stars Supernova More mass: more gravitational contraction and heating Critical temperature: 600 million K Carbon fusion Heavier nuclei fuse, up to iron All fusion energy sources used up Energy expansion pressure lost Dynamic equilibrium disrupted Supernova Star collapses and rebounds from core Elements beyond iron created in explosion and distributed throughout Universe Subsequent events depend on mass of remaining core

End states for massive stars Neutron star Remaining core between 1.4 and 3.0 solar masses Gravitational pressure fuses protons and electrons into neutrons Pulsar: rotating, magnetized neutron star Black hole Remaining core greater than 3 solar masses Gravitational collapse overwhelms all known forces Even light cannot escape the dense, compact object

Stellar evolution - summary

Larger scale structures Binary systems Two gravitationally bound stars Most stars are in binary pairs, not ours Star clusters Tens to hundreds of thousands or more gravitationally bound stars Often share a common origin Galaxies Basic unit of the Universe Billions and billions of gravitationally bound stars Larger scale still Clusters of galaxies Superclusters of galaxies Billions and billions of galaxies!

The Milky Way Visible as a diffuse band on a dark night Billions of stars, some bound in galactic clusters Structure Galactic nucleus Rotating galactic disk Diameter ~ 100,000 ly Spherical galactic halo Contains ~150 globular clusters

Other galaxies Our nearest neighbors - the Local Group Andromeda Dwarf galaxies ~1,000 light years in diameter Nearest dwarf being disrupted gravitationally by the Milky Way Andromeda 2 million light years away Very similar to Milky Way Classification scheme (Hubble) Elliptical, spiral, barred and irregular

The life of a galaxy Big Bang Theory Universe evolved from an explosive beginning Supporting evidence Einstein’s Theory of General Relativity Expansion of the Universe Diffuse cosmic background radiation (COBE spacecraft) Relative abundances of elements The end: Expansion forever or the big crunch?