Download 29.3 – Stellar Evolution

Bell Ringer 10/13
• Why do we celebrate Columbus Day?
Bell Ringer 10/14
• Identify three stages of the life cycle of stars
from your work yesterday.
29.3 – Stellar Evolution
Evolution of Stars
•Typical star exists for billions of years
•Astronomers cannot study a star
through its entire life
–Developed theories about
evolution of stars
–Different stages of development
1st Stage of Development
•Nebula = a cloud of dust and gas
–70% hydrogen, 28% helium, 2%
other elements
•Particles in a nebula have weak
gravitational attraction
Nebula
•A force causes the particles to connect
–Particles continue to increase in size
•Region of dense matter builds up in
nebula cloud and begins to spin
•Spinning causes matter to shrink into a
disk = protostar
–May produce more than one star
Protostar
•Protostar = shrinking, spinning
region of a nebula
•Pressure builds up in protostar and it
gets hotter
–Heats up for several million years
–Fusion begins
Main-Sequence Stars
•2nd & Longest stage
•Energy is generated in the core of star
–Hydrogen atoms fuse into helium
atoms
–Releases enormous amount of energy
•Star does not change in size because of
balance of energy and force of gravity
Giants & Supergiants
• Third stage
• Almost all hydrogen has converted to helium
– Without hydrogen, core of star contracts
• Temperature increases in core
– Higher temperature causes helium atoms
fuse into carbon atoms
– Causes outer shell to expand greatly
Giants & Supergiants
•As star’s shell of gases expands,
it cools and star is no longer
main-sequence star
•Becomes red giant or red
supergiant
Bell Ringer 10/15
• What happens after almost all of the
hydrogen has been converted into helium in
a star?
– Hint – we talked about it yesterday in our notes.
White Dwarf Stars
•No energy left for fusion
•Loses its outer shell, revealing core
•Core heats and illuminates expanding
gases = planetary nebula
•Gravity causes star to collapse inward
•Hot, dense core of matter = white dwarf
White Dwarf
•As white dwarf cools, it becomes fainter
and fainter
•When no longer emitting energy, becomes
a dead star or a black dwarf
Nova
•Some white dwarfs do not cool and die
•A large explosion may occur
–Releases energy, gas, and dust
•A nova may appear 1 million times
brighter than the sun
– Quickly fades back into a white dwarf
Supernova
•Stars with very large masses may
cause more intense explosions
•Star contracts causing high
temperatures and high pressure
Neutron Stars
•Neutron stars = after an explosion, the
core of a supernova contracts into a
small dense ball of neutrons
•Neutron star has more mass than the
sun and rotates very rapidly
•Some neutron stars emit 2 beams of
radiation = pulsar
Black Holes
•Some massive stars are too big to
become neutron stars
•Stars contract with great force and crush
the core of the star = black hole
•Gravity is so great that light cannot
escape
•Astronomers locate black stars by
observing effect on neighboring stars
In-Class
Assignment/Homework
• 29.3 WKT – just the front side