Novae and Supernovae
Novae: White Dwarfs
in Binary Systems
- What do we see? Stars whose
brightness increases quickly by 10,000 times or more, but
may not appear exceptionally bright,
and returns to normal over a few weeks.
- How often do they occur? A few
are seen every year; Some may repeat after many years [for example, RS Ophiuchi repeats every 20 years or so].
- What's going on? Gas buildup
in the accretion disk around a white dwarf in a semidetached binary.
- Explosion: The gas heats up and emits
energetic radiation; At 10 MK we see a thermonuclear flare.
- Afterwards: The star returns
to normal after a few years, but sometimes with more mass...
Supernovae: The Explosions
- What do we see? Exploding stars
whose intensity reaches more than a billion suns, and may be visible
even in daytime.
- Type I, White dwarf supernovas:
Development like novas, almost no H lines in the spectrum, all approximately
equally bright; They are binary
system where the white dwarf reaches the white dwarf or Chandrasekhar
limit (1.4 solar masses), "carbon fusion bombs" thought
to leave nothing behind, although companion stars may survive; Brighness
differences depend on nickel content.
- Type II, Core collapse supernovas:
Fading with additional bump, H lines in the spectrum; They are
the implosion of a massive star, leaving a compact remnant of
the core (they may have a companion too, which may or may not
- Use: They are so bright we can
see them in distant galaxies, and since we know their luminosity,
we can use them as standard candles.
- Hypernovae: The most violent
ones, which produce 10-100 times more energy than supernovae;
Seen as remnants in distant galaxies, may be related to gamma
ray bursts and production of black holes.
The brightest explosion seen was SN 2006gy in NGC 1260, possibly from pair-instability
in a single hypermassive star.
- How often do we see one? We
expect to see about 1 per century in our galaxy, but the actual
rate varies; Active (starburst) galaxies can have 1 in 2-3 years!
- Historical ones: The only recorded
ones were in AD 185, 386 & 393 (Chinese records), 1006, 1054
(the "guest star" in Taurus, several times as bright
as Venus, visible in the daytime for at least 23 days, produced
the Crab Nebula), 1181, 1572 (Tycho's SN in Cassiopeia), and
1604 (Kepler's SN in Ophiuchus); A less noticeable one occurred in 1667
or 1680, and its remnant is Cassiopeia A; No naked eye ones, except SN
1987A in the LMC, in more than 400 years!
- Previous ones: Nearby stars (e.g., in
Scorpius-Centaurus, 130 pc away) have had supernovas within the past
few millions of years,
which have affected chemistry
on Earth and and damage
to the ozone layer; One in particular has left evidence in 2.8 Myr old
soil layers, and may have affected human evolution.
- The next ones? Sometimes we can tell
that an explosion is "about" to happen; Should we monitor
massive supergiants past the main sequence? Are they dangerous?
One less than 25 ly away would extinguish most life on Earth;
One that is waiting to happen is eta Carinae, but it is 7500
- Star monitoring: Many galaxies
are now being monitored, to learn more about supernova explosions,
and be able to identify the pre-explosion stars.
Supernovae: The Aftermath
- Supernova remnants: Shells of
expanding gas that we see as nebulae; Examples: Crab nebula M1
(from the 1054 SN, still expanding), Veil Nebula (a.k.a. Cygnus
loop) and Vela remnant (9000 BC), or SN1987A in the LMC.
- Stellar remnants: The cores
of massive star SN's become compact objects; Astronomers believe
that, depending on the mass, they can become neutron stars or
black holes [or possibly strange quark stars]; Extreme examples
are magnetars with magnetic fields 1.6 quadrillion times as strong
as the Earth's.
- Consequence: Production of heavier
elements than during the star's lifetime.
- Evidence: Many neutron stars
seen as pulsars in supernova remnants; 10 stellar black holes
in binaries known as of 2002, although only one [GRO J1655-40]
is clearly the result of a supernova explosion; [An example of
quark star could be 3C58 (the remnant of a SN recorded by the
Chinese in AD 1181)?]
Gamma Ray Bursts
- What do we see? Flashes of g rays (and light, and neutrinos), first
discovered in the early 1970's by satellites designed to verify
the conditions of the Nuclear Test Ban Treaty, since the 1990's
seen approximately once a day; They last for only a few seconds
or minutes, but have longer afterglows.
- Where are they? Distributed
randomly across the sky; Most are seen to occur several billions
of light years from Earth, when the universe was quite young.
- Where are they? Highly beamed
components of supernova/hypernova explosions; It seems that some
(maybe 2/3) are stars at least 25 times as massive as the Sun
collapsing into black holes, with a beam of energy shooting out
along the axis of the star's rotation, while others may be neutron
stars merging into black holes.
page by luca bombelli <bombelli at olemiss.edu>,
modified 1 sep 2012