Notes #2 Stellar

Evolution

• The star called the sun is changing and will burn out over a lifespan of approximately 10 billion years.

• Other than the hydrogen and helium formed at the time of the Big Bang, nuclear fusion within stars produces all atomic nuclei lighter than and including iron, and the process releases electromagnetic energy. Heavier elements are produced when certain massive stars achieve a supernova state and explode.

1. Stars are classified

based on their temperature

and luminosity using the H-

R diagram.

Main Sequence Stars have a relationship between temperature and luminosity

White dwarfs have a high temperature and a low luminosity

Giants and Supergiants have a high luminosity and a low temperature

2. A star begins as a nebula, concentrates

to form a protostar, and then finally

becomes a main sequence star. Nebula: large cloud of mostly H gas

Rotates and contracts due to gravity

Protostar: a concentrated region within a nebula

Main sequence star a star like our sun

Creates energy from the fusion of H to form He (hydrogen fusion)

Maintains a stable size and luminosity

Main Sequence Star

The force of gravity

would cause the star

to contract, but it is

balanced by the

expansion of the hot

gas

3. A main sequence star can

eventually become a red giant or

supergiant when fusion changes. Red giant: becomes larger and red (low

temperature) Was a small main sequence star

Change occurs when hydrogen fusion ends and helium fusion (combination of helium to form carbon and oxygen) begins

Supergiant Was a large main sequence star

Helium fusion occurs followed by more phases of fusion create even heavier elements (up to iron on the periodic table)

Why does a red giant become

so big?

4. A red giant will eventually become

a nova, white dwarf, and planetary

nebula when fusion ends.

Planetary Nebula: After fusion ends in a

red giant the gases will drift away

White dwarf: Left over core of a red giant

Small, high temperature star

All of the heat is left over from when fusion

happened

Nova: Explosion of a white dwarf that is small

the star will return to being a white dwarf

Planetary Nebula

5. A supergiant will eventually explode in a

supernova and become a neutron star or

a black hole when fusion ends. Supernova: Explosion of a supergiant that completely

destroys the supergiant During the explosion fusion creates heavier elements

Neutron star: Left over core of a supergiant after a supernova explode; only give off radio waves

Black hole A massive star explodes inward

The increase in gravity will not allow visible light nor matter to escape

X-rays are created as matter falls into the black hole

Supernova Explosion

Black Hole