StarsStars::

The beginning of The beginning of the endthe end

HELIUM BURNINGHELIUM BURNING::

For stars that live most of their lives in the For stars that live most of their lives in the main sequence, helium burning is the main sequence, helium burning is the beginning of the end. The overall beginning of the end. The overall thermonuclear reaction for helium burning thermonuclear reaction for helium burning is as follows: is as follows:

3 He -> 1 C + energy released 3 He -> 1 C + energy released

HELIUM FUSES TO FORM HELIUM FUSES TO FORM CARBONCARBON

Increase of core tempIncrease of core temp

hydrogen in the core is gone.hydrogen in the core is gone. to maintain equilibrium between gravity to maintain equilibrium between gravity

and gas pressure, it needs increased and gas pressure, it needs increased temperatures in the core to re-ignite temperatures in the core to re-ignite fusion. fusion.

temperature of 10×10temperature of 10×1077 °K to initiate helium °K to initiate helium burning, burning,

whereas temperature of 2×10whereas temperature of 2×1077 °K to °K to initiate hydrogen burning.initiate hydrogen burning.

Star increases in size…Star increases in size…

Helium burns inside the core, Helium burns inside the core, but a rapid hydrogen reaction occurs but a rapid hydrogen reaction occurs

faster in the shell of the star.faster in the shell of the star. As the temperature in the shell of the star As the temperature in the shell of the star

increases, the outer layers of the star increases, the outer layers of the star expand. expand.

At this point, the star is often termedAt this point, the star is often termed

a a red giant.red giant.

What next ?What next ?

Eventually, the core will run out of helium Eventually, the core will run out of helium fuel, and in order to maintain equilibrium, fuel, and in order to maintain equilibrium, the core will contract again to initiate the the core will contract again to initiate the last type of fusion last type of fusion

– – carbon burning!carbon burning!

CARBON BURNING: DeathCARBON BURNING: Death

star after the red-giant phase – evolution is star after the red-giant phase – evolution is not known that certainly.not known that certainly.

a star, regardless of its size, must a star, regardless of its size, must eventually run out of fuel and collapse.eventually run out of fuel and collapse.

In theory, GRAVITY WINS. In theory, GRAVITY WINS.

THE THREE END STATES…THE THREE END STATES…

we will consider the death of stars and we will consider the death of stars and group them into three categories group them into three categories according to mass: according to mass:

Low-Mass Stars (0.5 solar mass or less) Low-Mass Stars (0.5 solar mass or less) Medium-Mass Stars (0.5 solar mass to 3.0 Medium-Mass Stars (0.5 solar mass to 3.0

solar mass) solar mass) Massive Stars (3.0 solar masses or larger) Massive Stars (3.0 solar masses or larger)

Evolution of low mass starEvolution of low mass star

A lower-mass star like the sun can survive for A lower-mass star like the sun can survive for billions of years, but after the hydrogen and billions of years, but after the hydrogen and helium fuel is gone it cannot get hot enough to helium fuel is gone it cannot get hot enough to fuse carbon. fuse carbon.

This type of star dies by puffing off its outer This type of star dies by puffing off its outer layers to produce expanding planetary nebulae. layers to produce expanding planetary nebulae. These nebulae, which are the remains of dying These nebulae, which are the remains of dying stars, serve as the birthplace for future stars, serve as the birthplace for future protostars.protostars.

H-R DIAGRAM POST MAIN H-R DIAGRAM POST MAIN SEQUENCESEQUENCE

A low mass star becomes A low mass star becomes a white dwarf a white dwarf

Low mass stars (0.08-5 SM during main Low mass stars (0.08-5 SM during main sequence) will go the planetary nebula route. sequence) will go the planetary nebula route.

The core cannot contract and heat up to a The core cannot contract and heat up to a temperature needed to initiate carbon fusion.temperature needed to initiate carbon fusion.

Electrons degeneracy pressure prevents further Electrons degeneracy pressure prevents further collapse. collapse.

A low mass core (<1.4 SM) shrinks to white A low mass core (<1.4 SM) shrinks to white dwarf, composed mostly of carbon. dwarf, composed mostly of carbon.

The size of the white dwarf is close to that of The size of the white dwarf is close to that of earth, and the outer layers are planetary nebula. earth, and the outer layers are planetary nebula.

A Planetary nebula with a A Planetary nebula with a White dwarf at the centreWhite dwarf at the centre

What happens to the white dwarf?What happens to the white dwarf?

As a star contracts into a white dwarf, its As a star contracts into a white dwarf, its surface becomes very hot. But without surface becomes very hot. But without energy, it cannot sustain itself. Because energy, it cannot sustain itself. Because the core is out of fuel, the white dwarf will the core is out of fuel, the white dwarf will eventually cool to a black dwarf. This will eventually cool to a black dwarf. This will take many billions of years to cool. take many billions of years to cool. Degenerate gas pressure inside the core Degenerate gas pressure inside the core continues to support the star so it doesn’t continues to support the star so it doesn’t collapse. collapse.

Size and MassSize and Mass

The smallest white dwarfs are the most The smallest white dwarfs are the most massive, and the largest white dwarfs are massive, and the largest white dwarfs are the least massive. the least massive.

Why is this? Why is this? The more massive star, because of its The more massive star, because of its

gravitational force, is able to squeeze itself gravitational force, is able to squeeze itself into a smaller, more densely packed object into a smaller, more densely packed object than can a less massive star.than can a less massive star.

Medium-mass stars become Medium-mass stars become neutron starsneutron stars

A higher mass core (between 1.4-3 SM) A higher mass core (between 1.4-3 SM) shrinks to neutron star. shrinks to neutron star.

Supernova happens when a neutron star Supernova happens when a neutron star is created.is created.

Neutron degeneracy pressure prevents Neutron degeneracy pressure prevents further collapse. further collapse.

The size of a neutron star is about that of The size of a neutron star is about that of a large city. a large city.

This supernova was first observed This supernova was first observed in 1987 by the Hubble Telescope in 1987 by the Hubble Telescope

(NASA)(NASA)

The Supernova Explosion….The Supernova Explosion….

The overall reactions that occur for carbon The overall reactions that occur for carbon burning occur so rapidly and with so much burning occur so rapidly and with so much energy that the star blows apart in an explosion energy that the star blows apart in an explosion called a called a supernovasupernova. .

The outer layers of the star blast into space, and The outer layers of the star blast into space, and the core is crushed to immense densities. the core is crushed to immense densities.

Carbon burning occurs when the helium in the Carbon burning occurs when the helium in the core is gone. The core needs to maintain core is gone. The core needs to maintain temperature to keep the gas pressure up; temperature to keep the gas pressure up; otherwise the star cannot resist gravity. otherwise the star cannot resist gravity.

When carbon burning does occur, iron is formed.When carbon burning does occur, iron is formed. Iron is the most stable of all nuclei, and ends the Iron is the most stable of all nuclei, and ends the

nuclear fusion process within a star. nuclear fusion process within a star. When these heavier elements form in the core, they When these heavier elements form in the core, they

take away energy rather than release it. take away energy rather than release it. With the decrease in fuel for fusion, the temperature With the decrease in fuel for fusion, the temperature

decreases and the rate of collapse increases. decreases and the rate of collapse increases. Just before the star totally collapses, there is a Just before the star totally collapses, there is a

sudden increase in temperature, density, and sudden increase in temperature, density, and pressure.pressure.

The pressure and energy compact the core further. The pressure and energy compact the core further. The compact core becomes a rapidly whirling ball of The compact core becomes a rapidly whirling ball of

neutrons, and that’s why now this star is termed a neutrons, and that’s why now this star is termed a neutron star. neutron star.

Onion Ring Structure at the last Onion Ring Structure at the last stage before Supernova explosionstage before Supernova explosion

A model ….A model ….

Supernova explosions happen because Supernova explosions happen because the core has formed a very stiff neutron the core has formed a very stiff neutron star and the infalling outer layers rebound star and the infalling outer layers rebound off of itoff of it

Analogy:Analogy: drop a basketball with a tennis drop a basketball with a tennis ball on top of it; see the tennis ball really ball on top of it; see the tennis ball really bounce off the basketball when the bounce off the basketball when the basketball hits the floor.basketball hits the floor.

THE MASS OF THE STAR THE MASS OF THE STAR DECIDES….DECIDES….

Remember it was mentioned that the Remember it was mentioned that the amount of mass a star has at birth amount of mass a star has at birth determines how long it lives? determines how long it lives?

Well, here’s the stage in star life where the Well, here’s the stage in star life where the differences show up dramatically. differences show up dramatically.

The largest mass stars may The largest mass stars may become black holesbecome black holes

The highest mass star has a core that The highest mass star has a core that shrinks to a point. shrinks to a point.

On the way to total collapse it may On the way to total collapse it may momentarily create a neutron star and the momentarily create a neutron star and the resulting supernova rebound explosion.resulting supernova rebound explosion.

Gravity finally wins. Nothing holds it up. Gravity finally wins. Nothing holds it up. Space so warped around the object that it Space so warped around the object that it effectively leaves our space – black hole! effectively leaves our space – black hole!

The life cycle of a low mass star The life cycle of a low mass star and a high mass starand a high mass star . .