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 survive...).
• 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.

 Supernovae: Rate of Occurrence

• 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 ly away.
• 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.