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 29.3 – Stellar EvolutionEvolution

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/Homewor

k• 29.3 WKT – just the front side