Download The Death of Stars

The Death of Stars
Today’s Lecture:
Post main-sequence (Chapter 13, pages 296-323)
• How stars explode: supernovae!
• White dwarfs
• Neutron stars
White dwarfs
• Roughly the size of the Earth with the mass of the Sun!
• If you try to pack electrons into the same place they must be
at different energy levels (like the energy levels of an atom).
Each electron must be at a higher energy than the one before it.
• All these energetic electrons in one place give rise to a
pressure: ELECTRON DEGENERACY PRESSURE
• This is weird stuff: one teaspoon of white dwarf weighs 3 tons!
If a white dwarf is more massive, it actually has a smaller
radius.
• No nuclear reactions are taking place, the white dwarf just
radiates its heat and continues to cool over time.
• White dwarfs are sometimes used as age indicators in
globular clusters.
Types of White Dwarfs
• The Sun will become a carbon/oxygen white dwarf with a
mass of 0.6Msun.
• Stars up to 8Msun become carbon/oxygen white dwarfs
with masses up to ~1.1Msun.
• Stars below 0.45Msun aren’t massive enough to burn
helium in their core and become helium white dwarfs.
• Stars with masses from 8-10Msun have an extra stage of
burning in their core and make oxygen/neon/magnesium
white dwarfs with masses of ~1.2Msun.
• White dwarfs have a mass limit 1.4Msun (the
Chandrasekhar limit), above which electron degeneracy
pressure can’t hold up the star.
Mass exchange in binaries
• Single stars evolve in a simple manner. Lifetime on the
main sequence mostly depends on mass.
• Most stars are in binary systems, allowing exciting things
to occur: mass exchange!
Once material
passes this point,
it flows onto the
white dwarf.
Cataclysmic Variables
• If one star is a white dwarf and the other star fills its
Roche lobe (like a growing red giant), material can
accrete onto the white dwarf.
Red Giant
• Angular momentum
prevents material from
directly hitting the white
dwarf, forming an accretion
disk.
Accretion
disk
• Cataclysmic variables are
bright source in the blue and
ultraviolet.
White dwarf
Novae (this is the plural)
• Cataclysmic variables undergo phases of brightening,
called novae (Latin for “new star”)
• Dwarf Novae: A rush of material flows through the disk,
falling onto the white dwarf and releasing gravitational
energy. Last a few days to weeks, and brightens by a
factor of ~100.
• Novae (or Classical Novae): Material that has built up
on the surface of the white dwarf ignites in a
thermonuclear explosion. Only happens every 1,000 to
100,000 years (need to build up enough material) and
brightens by a factor of ~1,000,000!
Supernovae: exploding stars!
• Previously “normal” star suddenly (few days to weeks)
becomes much more luminous. Up to 10 billions times
brighter than the Sun!
• Rivals the entire galaxy in brightness for a few weeks!
Fades over months to years.
Two main classes:
Type I: no hydrogen lines
Type II: hydrogen lines visible (in spectra)
Also, Type I seen in all kinds of galaxies, while Type II
seen in spiral galaxies in star forming regions. Light curve
shape and other differences as well.
Spectra are different
Light curves are different
Supernovae and remnants
• Supernovae produce remnants: expanding shells of gas
rich with heavy elements.
• Perhaps the most famous is the “Crab Nebula” from a
supernova in 1054 AD. It was so bright, Chinese, Japanese,
and Arab astronomers saw it for months during the day,
and could be seen for 2 years at night.
• The remnant merges with other gas and forms new stars.
• Supernovae occur 1 to 3 times per 100 years per galaxy.
• The last observed supernova in the Milky Way was in 1604
(Kepler’s supernova). Are we overdue? Gas and dust may
hide supernovae on the other side of the Galaxy.
Composition of our Universe
Type Ia Supernovae
• White dwarf in a binary system (white dwarf plus red giant or
2 white dwarfs -- we’re not sure!)
• White dwarf accretes matter and begins growing
• When the mass of white dwarf exceeds 1.4Msun (Chrandra
limit), there is a runaway chain of nuclear reactions.
Heating happens --> Reactions get faster --> Pressure doesn’t
increase because of electron degeneracy --> more heating -->
reactions get fast --> and so on!
• Carbon and oxygen burn into heavy elements and are
exploded out into space.
• ~0.6Msun of 28Ni56 is produced, which is radioactive!
56 -->
56 -->
56 + lots of energy
Ni
Co
Fe
28
27
26
Type Ia SNe are super important!
• Most of the iron in our Universe is from Type Ia
supernova.
• Because all the Type Ia supernovae ignite at a similar
mass (1.4Msun), they have similar luminosities: they are
standard candles!
• They are really bright 5 billion times brighter than our
Sun: so we see them at huge distances.
• By comparing the apparent brightness with the intrinsic
luminosity we can measure vast distances and measure
the shape of the space in between. This is the main
evidence that our Universe is dominated by Dark Energy.
Type II Supernova: massive star
• Massive stars (> 10 Msun)
continue to burn fuel until iron (Fe)
forms in the core. Fe is the most
bound atomic nuclei. No more
reactions in the Fe.
• The Fe core continues to grow.
• When the Fe core reaches
1.4Msun the core collapses.
p + e- --> n + ν
The core is converted into neutrons
• The outer layers bounce off the core creating an explosion!
(neutrinos also very important for driving the envelope away)
• A neutron star is formed.
Supernova 1987A (Type II)
• Nearby! Only 170,000 light years away in the Large
Magellanic Cloud (a small “satellite” galaxy of the Milky
Way).
• It initially was a 20 Msun star (but blue supergiant, NOT
red -- this is a mystery, is it because of the low metallicity?
Perhaps it was two stars merging?)
• Neutrinos detected! (25 of them) Explosion mechanism
was core collapse and rebound. A neutron star was
probably formed, but we still haven’t seen it -- this is
another mystery.
• This supernova confirmed many of our ideas about how
stars exploded, but also brought up many new questions.
Supernova 1987A - energy output
• Total energy (emitted in about 1 second) was comparable
to the energy emitted in 1 second by ALL normal stars in the
entire observable Universe!
> 99% of the energy was in neutrinos
< 1% was energy of motion of ejecta
< 0.01% was in visible light
• Supernovae are truly incredible explosions!
• Gamma-rays with certain specific energies were seen
coming from SN 1987A. This confirms that radioactive 27Co56
(Cobalt) was produced.
• Confirms that heavy elements are made in stars and
explosion and then dispersed into the Universe to form
new stars, planets, and eventually life.
Neutron Stars
• The core left over by a Type II supernova
• Held up from gravity by neutron degeneracy pressure
• First predicted in the 1930s, and confirmed with the
discovery of pulsars in 1967 by Jocelyn Bell (her advisor got
the Nobel Prize for the discovery).
• We think the maximum mass (like the Chandra limit for
WDs) is 2-3Msun and 10 km radius. This is 1011 kg/cm3!!!
• We now know about hundreds of neutron stars doing many
exciting things (pulsing, surface explosions, highly
magnetized).
• Neutron stars are important for testing Einstein’s General
Theory of Relativity and testing our understanding of
subatomic particles.