Star DeathStar DeathFrom white dwarfs…......to black holesFrom white dwarfs…......to black holes

The Life Cycle of starsThe Life Cycle of stars

The short storyThe short story

Stars begin to die as they run out of fuelStars begin to die as they run out of fuel

All stars will become All stars will become red giantsred giants Medium sized stars shed their outer layers Medium sized stars shed their outer layers

forming a planetary nebulae ring around their forming a planetary nebulae ring around their core. core.

------ this is a ------ this is a whitewhite dwarfdwarf star. star. High mass stars explode as supernovas,High mass stars explode as supernovas,

------ leaving behind a ------ leaving behind a neutron starneutron star or if it or if it was originally a really big star, a was originally a really big star, a black holeblack hole

Two questions – two ideasTwo questions – two ideas

Why doesn’t the Sun collapse under its own weight? Why doesn’t the Sun collapse under its own weight?

Hot gas Hot gas pressurepressure inside pushing outwards inside pushing outwards

Why doesn’t the Sun explode?Why doesn’t the Sun explode?

GravityGravity holds it together, pulling inwards holds it together, pulling inwards

Inside the Sun, Inside the Sun, gravitygravity balances balances

pressurepressure EVERYWHERE. If a EVERYWHERE. If a

star gets out of balance, it will star gets out of balance, it will expand or contract……….or… expand or contract……….or…

explodeexplode or or collapsecollapse

Red GiantsRed Giants As the Nuclear fuel As the Nuclear fuel

runs out a Red runs out a Red Giant is formed.Giant is formed.

Luminosity Luminosity increases 1000 increases 1000 timestimes

Envelope puffs up Envelope puffs up to be as big as to be as big as Mars’ Mars’ orbitorbit ….. or ….. or bigger ……. and bigger ……. and cools to red hot.cools to red hot.

Eventually the

Medium Sized StarsMedium Sized Stars

Eventually the outer Eventually the outer core of Hydrogen gas core of Hydrogen gas is exploded outwards is exploded outwards to form a ring around to form a ring around the core.the core.

This is called a This is called a planetary nebula.planetary nebula.

More Planetary Nebulae

But what is at the middle?But what is at the middle?

The star at the center ……The star at the center ……

is not much bigger than the Earth …is not much bigger than the Earth …

And is white hot.…..it’s a And is white hot.…..it’s a white dwarfwhite dwarf

One teaspoon of white dwarf weighs ….One teaspoon of white dwarf weighs ….

What happens to more massive stars ?What happens to more massive stars ?The pile of helium ashes in the core gets so hot The pile of helium ashes in the core gets so hot

that …..that ….. It ignites !!It ignites !!

A helium nucleus can crash into a carbon A helium nucleus can crash into a carbon nucleus and make an nucleus and make an oxygen nucleusoxygen nucleus …………and still more energy!!and still more energy!!

But all good things come to an end…. But all good things come to an end….

Energy-producing nuclear reactions stop at Energy-producing nuclear reactions stop at iron.iron.

The core of the star collapses, The core of the star collapses,

which sends shock waves through the star,which sends shock waves through the star,

which triggers awhich triggers a

SSUUPPEERRNNOOVVAA!!

The atoms made inside the coreThe atoms made inside the core

mix with the interstellar gas,mix with the interstellar gas,

become part of new stars,become part of new stars,

and planets, and you and me.and planets, and you and me.

We are all made from the stars!We are all made from the stars!

And the part of the core that And the part of the core that collapsed?collapsed?

If it is less than 3 solar masses, it forms a If it is less than 3 solar masses, it forms a neutron star ------ a ball of pure neutrons neutron star ------ a ball of pure neutrons with a radius of only 10 km.with a radius of only 10 km.

One teaspoon of neutron star weighs One teaspoon of neutron star weighs about a billion elephantsabout a billion elephants

Neutron stars were discovered by Jocelyn Neutron stars were discovered by Jocelyn Bell in 1967, who noticedBell in 1967, who noticed

strange pulses of radiostrange pulses of radio

waves that ticked like an waves that ticked like an

atomic clock.atomic clock.

If the core is bigger than 3 solar If the core is bigger than 3 solar masses ……… gravity winsmasses ……… gravity wins

Nothing can stop the collapseNothing can stop the collapseThe pull of gravity at the surface becomeThe pull of gravity at the surface become

so strong that not even light can so strong that not even light can escape…....escape…....This is a black holeThis is a black hole. .

There is a There is a monstermonster black holeblack hole (3 million solar masses) in the (3 million solar masses) in the

center of our galaxy.center of our galaxy.Hard to see because visible light is blocked Hard to see because visible light is blocked

by dustby dust Infrared light can pass through the dustInfrared light can pass through the dustWe can see the stars in orbit around a very We can see the stars in orbit around a very

small, but very massive object --- this is the small, but very massive object --- this is the black holeblack hole

In distant galaxies, there are much bigger In distant galaxies, there are much bigger black holes -- a billion solar masses or black holes -- a billion solar masses or more -- but that’s another story.more -- but that’s another story.

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