Star Death From white dwarfs…......to black holes

The Life Cycle of stars

The short story Stars begin to die as they run out of fuel Stars begin to die as they run out of fuel All stars will become red giants All stars will become red giants Medium sized stars shed their outer layers forming a planetary nebulae ring around their core. Medium sized stars shed their outer layers forming a planetary nebulae ring around their core this is a white dwarf star. High mass stars explode as supernovas, High mass stars explode as supernovas, leaving behind a neutron star or if it was originally a really big star, a black hole

Two questions – two ideas Why doesn’t the Sun collapse under its own weight? Hot gas pressure inside pushing outwards Why doesn’t the Sun explode? Gravity holds it together, pulling inwards Inside the Sun, gravity balances pressure EVERYWHERE. If a star gets out of balance, it will expand or contract……….or… explode or collapse

Red Giants As the Nuclear fuel runs out a Red Giant is formed. As the Nuclear fuel runs out a Red Giant is formed. Luminosity increases 1000 times Luminosity increases 1000 times Envelope puffs up to be as big as Mars’ orbit ….. or bigger ……. and cools to red hot. Envelope puffs up to be as big as Mars’ orbit ….. or bigger ……. and cools to red hot. Eventually the

Medium Sized Stars Eventually the outer core of Hydrogen gas is exploded outwards to form a ring around the core. Eventually the outer core of Hydrogen gas is exploded outwards to form a ring around the core. This is called a planetary nebula. This is called a planetary nebula.

More Planetary Nebulae

But what is at the middle?

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 white dwarf And is white hot.…..it’s a white dwarf One teaspoon of white dwarf weighs …. One teaspoon of white dwarf weighs ….

What happens to more massive stars ? The pile of helium ashes in the core gets so hot that ….. The pile of helium ashes in the core gets so hot that ….. It ignites !! It ignites !!

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

But all good things come to an end…. Energy-producing nuclear reactions stop at iron. Energy-producing nuclear reactions stop at 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 a which triggers a SUPERNOVA! SUPERNOVA!

The 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 collapsed? If it is less than 3 solar masses, it forms a neutron star a ball of pure neutrons with a radius of only 10 km. If it is less than 3 solar masses, it forms a neutron star a ball of pure neutrons with a radius of only 10 km. One teaspoon of neutron star weighs about a billion elephants One teaspoon of neutron star weighs about a billion elephants

Neutron stars were discovered by Jocelyn Bell in 1967, who noticed Neutron stars were discovered by Jocelyn Bell in 1967, who noticed strange pulses of radio waves that ticked like an atomic clock.

If the core is bigger than 3 solar masses ……… gravity wins Nothing can stop the collapse Nothing can stop the collapse The pull of gravity at the surface become The pull of gravity at the surface become so strong that not even light can escape…....This is a black hole.

There is a monster black hole (3 million solar masses) in the center of our galaxy. Hard to see because visible light is blocked by dust Hard to see because visible light is blocked by dust Infrared light can pass through the dust Infrared light can pass through the dust We can see the stars in orbit around a very small, but very massive object --- this is the black hole We can see the stars in orbit around a very small, but very massive object --- this is the black hole

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