Stellar Evolution Continued…

White Dwarfs Most of the fuel for fusion is used up Giant collapses because core can’t support weight of outer layers any more Nuclei of atoms squeeze together White dwarf gives off leftover heat & glows for billions of years Very small (size of Earth) Cools until becomes cold & dark

Nova Sometimes a white dwarf flares up & is 100s of times brighter Novas fade in a few days or few years This explosion can occur as a white dwarf pulls gases from a nearby main sequence star (gravitational attraction) & EXPLODES!

Supernovas Happens to stars x more massive than sun Supergiant contracts with such force that it produces high temps & pressure… So nuclear fusion begins again! Extremely bright…outshines an entire galaxy! Observed by Chinese astronomers in a.d.1054

Supernovas cont. New Carbon atoms – Mg – Fe Fusion stops with Fe Iron absorbs energy & collapses Outer part of star explodes Outer shell of gas is changed into an expanding cloud of gas (Crab Nebula) 1987 Large Magellanic Cloud modern observations made & still being analyzed.

Neutron Stars Core of supernova after explosion 1 spoonful = 100 million tons Rotate rapidly Only 10 km in diameter Composed of neutrons (heaviest part of atomic nucleus)

Pulsars Neutron stars that emit 2 beams of radiowaves across universe Detect as “blinking” signal First one discovered was in Crab Nebula Remaining core of a neutron star & supernova explosion that created the Crab Nebula

At Last… Black Holes Very massive stars collapse into a very small volume Gravitational forces are so powerful…even light cannot escape Matter gives off X-rays prior to entering black holes First one detected in Cygnus 1970 Evidence suggests central cores of galaxies contain black holes

Milky Way Galaxy 200 billion stars 100,000 light years in diameter 2,000 light years thick Sun is about 30,000 light years from galaxy’s center Local Group: small cluster of 17 galaxies (our neighbors) Andromeda is 2,000,000 lights years away

Types of Galaxies Spiral Elliptical Irregular Galaxies are millions of light years apart!

Spiral galaxies Central lens-shaped bulge with millions of stars Bright nucleus with millions of stars 2 Spiral arms come out from opposite sides of nucleus Arms trail behind as galaxy rotates Milky Way is a spiral; ¾ of galaxies are spirals Contain stars of various ages

Elliptical galaxies Range from spherical to lens-shaped No arms Most of the stars are close to center Little gas & dust clouds No young stars or ongoing star formation Contain old stars

Irregular galaxies Small, faint, less common Stars spread unevenly Contain young, blue stars & old stars Abundant gas & dust; vigorous ongoing star formation Often found close to larger galaxies

Quasars Discovered 1961 Emit radio waves, IR, visible, X rays Most luminous objects (like 20 trillion suns) Larger & more massive than any known star Radiate light and radio waves at high rates May be whole galaxies in early stage of development, but so distant that we can’t see the galaxy itself.

Quasars continued Most distant objects in the universe 30,000 known, but the number is increasing as we have better probes

Big Bang…explanation of formation of galaxies 15 billion years ago mass of hydrogen began expanding 1929 Edwin Hubble found red shifts in spectra of galaxies Red shifts showed distant galaxies were receding from Earth faster than nearby galaxies Expansion explains various speeds of recession

CBR 1964 physicists discovered cosmic background radiation in microwave wavelength Coming from all directions in space Echo from Big Bang