The Deaths of Stars

What Do You Think?• Will the Sun someday cease to shine brightly? If so, how will

this occur?• What is a nova? How does it differ from a supernova?• What are the origins of the carbon, silicon, oxygen, iron,

uranium, and other heavy elements on Earth?• What are cosmic rays? Where do they come from?• What is a pulsar?

Low-Mass Stars• Recall stars with a mass less than 0.4x Msun evolve to red dwarfs: all H is

fused to He, stop fusion, and cool off• Stars greater than 0.4x Msun evolve very differently• When hydrogen shell fusion begins, energy causes star to expand and

becomes a giant• As star expands, its surface temperature decreases• Mass is expelled into space in the form of stellar winds, which reduces

the mass of the star• Core helium fusion then begins, with stars less than 2.0x Msun undergoing

a core helium flash• Stars more than 2.0x Msun begin helium fusion gradually• Cores are eventually converted into C and O and He fusion stops• Final destiny of stars depends on mass• Two mass ranges to consider• 0.4 – 8.0x Msun and stars greater than 8.0x Msun

Low-Mass (0.4 – 8.0x Msun) Stars

• C and O atoms require a temp. of at least 600 million K to fuse• Core of low-mass stars only reaches about 200 million K so

fusion of C and O does not occur• As He is used up, the inner regions of the star contract,

compressing, and heating the shell of He just outside the core• Helium shell fusion begins outside of the core (H shell fusion is

also occurring) • Once helium shell fusion starts, the new outpouring of energy

pushes the star outward• Star is now called an AGB (asymptotic giant branch) star, and

becomes brighter than ever before• Star expands to a radius of about 1 AU and are now so bright

that they are low-temperature, red supergiants

AGB stars• Are destined to self destruct• All giant stars emit mass through solar winds• AGB stars reduce their mass significantly, greater than 30% of

their mass is lost• Loss of mass decreases the force of gravity available to

compress the star’s core• AGB star is compressed just enough to cause its core to

become degenerate• Growing repulsive force between electrons stops its

contraction• Core temp does not reach 600 million K to fuse C or O so no

further core fusion occurs

Thermal Runaway• Final stage through which a low mass star passes begins with a

thermal runaway – a rapid rise in temp in the helium shell• A helium shell flash occurs, expanding the star and decreasing

the shell temp, slowing the rate of fusion• Several helium flashes occur as its helium shell thickens• Eventually, outer gases are cool enough for the electrons and

ions to recombine• Mass is lost from the star and the core becomes visible• Planetary nebula: the luminous dust and gas ejected from the

star• Such nebulae are quite common in our galaxy – at least 1800• Ultimate fate of our Sun• Gas expulsion is very slow – not explosive at all

Planetary Nebulae• Outflowing gases take a variety of shapes• Helix Nebula, Hourglass Nebula, Red Spider Nebula, etc.• Are short-lived• Average 50,000 years• After that time, gases are spread over distances so far that it

fades from view• Gases mix with and become part of the interstellar medium

White Dwarfs• Burned-out cores of low-mass stars become white dwarfs• Fate of our Sun’s core• Roughly the size of Earth, covered with a thin coating of

hydrogen and helium• Very dense (109 kg/m3): a teaspoonful of white dwarf matter

on Earth would weigh 5 tons• Over time, an isolated white dwarf radiates its energy into

space• After billions of years, it cools enough so its C and O solidify

Nova• Occurs if a white dwarf is in a binary system• The other star fills its Roche lobe and slowly deposits fresh H

into the white dwarf• New mass covers the surface of the white dwarf, causing its

temp and pressure to increase• At 107 K, fusion ignites in the layer – the star suddenly

becomes brighter and gas is blown throughout the sky (nova)• After a nova, fusion stops in the white dwarf

Type Ia Supernova• Normal nova: just removes H and

He from the surface of a white dwarf

• Sometimes, the star itself is blown apart

• Type Ia Supernova: occurs with a white dwarf in a semidetached binary system

• The companion star deposits lots of mass onto the white dwarf

• The increased pressure causes carbon fusion to begin in the core

• Rate of C fusion increases dramatically and the star blows up

• Very bright