Transcript Section 3-3(rev04) 2

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LIVES OF STARS
A large cloud of dust and gases is called a
nubula. It is the source of all stars.
Gravity pulls the gases closer together and they
heat up. This is called a protostar.
A star is born when the gases and dust become
hot enough for nuclear fusion to take place.
Nuclear fusion is the process in which hydrogen
is changed to helium. Other elements are formed
but hydrogen to helium is by far the most
common.
• During fusion tremendous amounts of energy are
released.
• The life of a star depends on its mass. The smaller
the star, the longer it lasts.
• Small stars can live for about 200 billion years.
• Medium stars live for about 10 billion years.
• Large stars live for about 10 million years.
For most of its life, the star glows
brightly as nuclear fusion uses up
the hydrogen fuel, but it does not
change size.
DEATHS OF STARS
• All stars eventually start to run out of hydrogen
fuel and become red giants or supergiants,
depending on how large they are.
• When fuel runs out, it becomes a white dwarf,
neutron star, or black hole.
• Small and medium stars become red giants and
then white dwarfs. These are the size of the
Earth but have the mass of the sun, so it is very
dense. A spoonful of white dwarf is equal to a
truckload of earth. It has no fuel so it just glows
faintly with left-over heat but eventually dies. It
is then called a black dwarf.
• A dying giant or super giant star can suddenly
explode. The star becomes millions of times
brighter. This is called a supernova. Some of the
exploded material may enter a nebula which may
form a new star. Our sun probably started this way.
• The material that is left behind forms a neutron star.
Neutron stars are very small and dense. They may
have three times the mass of our sun with a volume
smaller than Pluto.
• The most massive stars become black holes.
• After the supernova a mass of five times our sun
may remain. The gases pull in and a mass of five
times our sun is in a diameter the size of NYC.
• The gravity is so strong that even light cannot
escape. This is called a black hole.
• We cannot detect a black hole directly. Any gas
near it will be pulled so strongly that it will spin
rapidly around the black hole. This will cause the
gas to give off x-rays that we can detect.
• We can also detect black holes from the effect
they have on the gravity of nearby stars.
The supernova 1987A. The left hand
photo was taken before the explosion; the
arrow shows the star that blew up.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
This black
hole is
pulling
matter from
a nearby star
into its
center.
Quasars
• Discovered in the 1960s, quasars are very bright
objects that are about 12 billion light years away.
• They are on the edge of the universe.
• Each one is a galaxy with a black hole in the
center.
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Sorenson Video decompressor
are needed to see this picture.
The eagle nebula
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Orion Nebula