Life Cycle of Stars

Birth of a Star

• Born from interstellar matter (dust & gases)

– Denser portions of the nebula

• Nebula begins to contract

– Due to gravity

– Shockwave from nearby explosion

• Temperature at the core rises

– Gravitational energy is converted into heat energy

There are Nebulas…..

And then there are FAMOUS Nebulas….!

Childhood

• A star is born once fusion begins.

• If fusion does not occur, it is not a star

– Brown Dwarfs (ex. Jupiter)

• Core temperature must reach 10 *106 K

– Repulsive forces between nuclei overcome

– Higher temperatures = higher velocities

– Hydrogen nuclei combine in fusion

• 4H →1He + energy

• E=mc2

Childhood continued

• Once fusion begins, the star now enters the

protostar stage

• Gravitational contraction continues

(nothing to counteract it)

• As fusion continues, pressure from the

gases builds at the core, creating an

OUTWARD force

Adulthood

• Eventually, the outward force will balance

the inward force, and the star will become

stable; its size will remain fixed (not

contracting or expanding)

• The star is now a main sequence star

– Spends approximately 90% of its life in this stage

• Hydrogen fusion continues

A Senior Citizen

• Eventually the hydrogen core is turned into helium

through fusion

• H fusion can continue in outer shells

• Core begins to contract (why?)

– Gravity is no longer balanced by the gas pressure from

fusion

• Temperature rises in core, heat is transferred to outer

shells, which expands

– Creates a giant star

Senior Citizen continued

• Red Giant Stage

– Temperature at core becomes hot enough for helium

fusion to form carbon, and possibly oxygen

• Temperature needs to be 100*106 K

• Temperature at surface cools

– Why are these stars called RED GIANTS?

• Red Giant stage is shorter for more massive stars

– Similar to cars: SUV vs. Sedan

Red Giant

Death of a star

• A star begins to die once it runs out

of all useable fusion fuel

• Next phase is determined based on

the star’s mass

A Low Mass Star’s Demise

• Mass < .5 Sun’s mass

– 2*1030 kg or 330,000 * Earth’s mass (5.98*1024 kg)

• These stars are like the Prius

– Burn through hydrogen fuel very slowly

– Main sequence star for 100 billion years!

• Are low mass, 1st generation stars still alive?

• Core temperature never reaches 100*106 K for helium

fusion to occur

– Does not enter Red Giant phase

– Goes straight to the White Dwarf phase

Medium-Mass Star’s Demise

• Masses similar to the sun

• Red Giant phase goes rather quickly

• Red Giants blow off their outer layers

– Like a mini-explosion

– Planetary nebula

• Forms a cloud of gas and interstellar matter

• Then goes into the white dwarf phase

• Fun Fact: When the sun expands into the Red Giant

phase, it will go past Earth!

Planetary Nebula

A Massive Star’s Destruction

• Mass > 4 suns

• Short life spans (like a Humvee!)

• Becomes a SUPER Red Giant

• Temperature becomes so hot that fusion of

heavier elements can occur (nucleosynthesis)

– Occurs rapidly over a short amount of time

– Formation of elements in periodic table

– The mass determines what heavy elements can form

Going out with a BANG!

• When it runs out of fuel, gravitational contraction

occurs rapidly because it is SO MASSIVE!

• Star collapses and IMPLODES as a supernova

• Creates a shockwave that destroys the star, and

throws matter into space (IMPORTANT!)

– Distribution of elements in universe

– Responsible for forming everything, stellar bodies

– “We are all made out of star dust”

Supernova

White Dwarfs• Only low and medium mass stars

• Once fusion ends, the star contracts

– Extremely high density

– Temperature rises, including at the surface

• Why are these stars called WHITE DWARFS?

• Similar to the wick of a candle

– No heat source

– White Dwarfs will cool and dim over time

– Once all light / heat is gone, it becomes a black dwarf

• Why is it named a BLACK DWARF?

Neutron Stars

• Formed after a supernova

• Stronger gravitational pull

– More contraction

• Massive, denser, and smaller than white dwarfs

• Atoms take up less space (just like with a white

dwarf)

– Instead of electrons closer to the nucleus, they combine

with protons to form NEUTRONS (hence the name)

Black Holes

• Most massive stars collapse to become

black holes

–Mass > 20 Suns

– Denser and more massive than neutron star

• Gravitational field is so strong not even

light can escape!

H-R Diagram