Download Chapter 27.2

Ch. 27.2 Stellar
Evolution
• Nebula—a cloud of dust and
gas. 70% Hydrogen, 28%
Helium, 2% heavier elements.
• Gravity pulls the nebula
together; it spins and flattens
into a disk of matter with a
central concentration called a
protostar.
• Pressure and temperature
increases in the protostar, until
the core reaches about
10,000,000 degrees C, and then
nuclear fusion starts, releasing
energy, and a star is born.
Main-Sequence Stars
• Main-sequence stage is the
longest stage in a star’s life.
• Hydrogen is converted to helium
by nuclear fusion, and energy is
released.
• The star is in balance. The
inward pull of gravity is
balanced by the energy from
fusion.
Giants and Supergiants
• 3rd stage, when hydrogen in the
star’s core is exhausted.
Hydrogen fusion continues in
the star’s outer layers.
• Core contracts due to gravity,
and its temperature increases.
• Helium in the core fuses into
carbon.
• Combined helium and hydrogen
fusion causes outer shell of star
to greatly expand.
• Star is now a red giant (10
times bigger than sun) or a red
supergiant (100 times bigger
than sun)
White Dwarf Stars
• For medium-sized stars, after
helium fusion, the giant stage is
over.
• Outer gasses are lost, and a
core is revealed, which heats
and illuminates the expanding
gasses, forming a planetary
nebula.
• The last inner matter collapses
inward due to gravity, forming a
hot, dense white dwarf star.
• It shines for billions of years,
but eventually becomes cooler
and fainter, until energy
emmission stops, and it
becomes a black dwarf.
Novas
• Explosions on the surface of
some white dwarfs, causing
brightness to increase by
thousands of times for a few
days.
• Believed to be caused by gas
(from a companion star) buildup
on the white dwarf’s surface.
Supernovas
• Occur when high-mass stars (10
to 100 times mass of the sun)
finish the supergiant stage.
• Gravitational collapse causes
fusion to restart…carbon is
converted into iron…then
gravity causes the iron core to
collapse and fuse into all
heavier elements, in a gigantic
explosion.
Neutron Stars
• What may be left after
supernovae explosions…super
dense rotating balls of pure
neutrons, with a diameter of
about 30 km.
• Pulsars are neutron stars that
emit beams of radiation.
Black Holes
• In the most massive stars,
gravity crushes the matter
down into zero volume.
• Detected by x-rays given off as
surrounding matter falls into a
black hole.