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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 10-100x 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.
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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
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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
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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
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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
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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
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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
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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