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
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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