EVOLUTION AND LIFE CYCLE OF STARS

EXAMPLES

LOOK ON P. 764 in TEXT

Book

STARS Life Cycles: • A Star goes through a life cycle just like

humans would.

Birth

Life Death

Birth of a star

A shockwave causes nebula gas and dust to collapse due to

gravity.

As the cloud collapses

and shrinks due to

gravity, it compresses the gas and

starts to heat up inside

(compression creates

heat).

It gets so hot that nuclear fusion starts

inside the center of the cloud

(a protostar forms).

Birth of a Star

The high heat and

temperature from the fusion

reaction increases the

forces of outward

pressure and the protostar

grows and starts to expand.

Eventually, a balance between gravity wanting to collapse the

protostar and the heat and

pressure wanting to expand the

protostar is met.

A star is born and

burns happily.

Shockwave

Gravity compress

es star

Heats up fusion begins,

pressure balances

with gravity.

Hydrogen is

burning and

balancing out the

compression from gravity.

REVIEW…

This balance between gravity and heat and pressure from fusion must be kept in order for a star to live peacefully.

If a star does not burn enough hydrogen, then gravity will overcome it and it will start to collapse in on itself again.If a star makes too much energy, it will expand too much, thus shutting down the fusion reactions, ultimately causing the star to collapse again until fusion starts back up

A star will continue to live happily as long as it balances internal pressure and gravity.

Fusion for a star is like FOOD for us, With it we are sustained and can live. Without it, we die.

HAPPY LIFE – Main Sequence

Implication

More Mass

• The more massive a star, the harder it has to try to overcome the force of gravity working to collapse it.

More Fusion

• The only way a star can overcome the collapsing force of gravity, is through the expanding heat and pressure of fusion.

More Hydrogen

• The more massive stars, need to burn more hydrogen than less massive stars.

Star live happy until it uses up its food: Hydrogen

Stars live happy until they run out

of food. It can no longer

overcome gravity and

starts to compress

again.

Bigger stars need more

food to overcome

gravity and thus use up there food faster (die earlier).

A dying star changes from

a main sequence start to

another type of star.

Depending on the mass of the star.

What happens when a star eats all its food? When it’s hydrogen inside its core runs out?

THE STAR EXPANDSAnd is now

burning Helium

BECOMES A RED GIANT or SUPER GIANT

DEATH• At this time, gravity and heat and pressure are

unbalanced. • The star begins to collapse as it cannot

overcome gravity without fusion.• What happens after this point depends on the

MASS of the star.

Medium – Low Mass Stars (like the sun).

The stars core

continues to

collapse

The star collapses so much that

the enormous amount of

heat produced

causes leftover

hydrogen in a cloud

outside the core and He in the core

to burn.

This burning causes

the outer atmosphere of the star to be

wisped off into space

leaving just the

stars core behind.

This core is called a

white dwarf. White dwarfs are the

same size as earth but way

more dense.

High Mass Stars

Hydrogen runs out in

the core much more

quickly because

of its high

mass, collapsin

g the core.

It collapses so much

that the inside heats

up much more than

inside a mediu

m mass star.

High mass stars are

more massive, fusion of elements heavier

than H and He take place,

causing the star to

expand rapidly into a super

giant star.

Eventually, the

star runs

out of fuel in

the core

causing fusion

to stop.

The star collapses

in on itself and

then explodes outward forming

a supernov

a

After the explosion however,

the core of the star remain

and becomes either a

black hole or a

neutron star

depending on just

how massive it

is.

Supernova

From a Supernova… now what?1.4 to 3 times the mass of the sun

Core will collapse to about the size of a

small city.

Pressure from neutrons in the core stop the core from collapsing further.

Neutron Star

Greater than 3.0 the mass of the sun.

Core will collapse to about the size of a

small city.

Pressure from neutrons in the core

CANNOT stop the core from collapsing.

Core shrinks down to a point called a

singularity.

Black Hole is formed.

Neutron Stars• Small• Extremely Dense - teaspoon weighs a mountain• Stupendous surface gravity• Rotate 1000 times per second (pulsar)

neutron star

marshmallow

3-megaton explosion

Black Holes• Massively dense core of a dead star

compressed to a speck.• There is an imaginary sphere around the black

hole called an event horizon.• Anything that goes into the event horizon can not escape its stupendous gravity.

Black HolesIf Sun became black hole (not possible)…orbits of planets would be engulfed.event horizon

BH

Total Distortion (“Spaghettification”)

Gas, stars falling towards BH are tidally distorted even outside the event horizon.

Distortion heats up stuff to millions of degrees and emit X-Rays strongly.

Feeding BH’s should be very bright X-Rays.

You can find a BH in space in looking for normal star and a binary companion emitting X-Rays (The star will be at least 3 times the mass of the SUN).