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: Binary systems where a white dwarf reaches the Chandrasekhar mass limit (1.4 solar masses) so they are all approximately equally bright; "carbon fusion bombs" thought to leave nothing behind, although companion stars may survive.
• Type II, Core collapse supernovas: Implosions of massive stars, leaving compact remnants of their cores (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.

 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.

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 SNs 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.

  Neutron Stars

• What are they? Extremely compact and dense star remnants, made mostly of neutrons, held up by neutron degeneracy pressure which balances gravity.
• How do they form? From the collapsed cores of massive stars after supernova explosions; It appears that during the formation process they receive a kick that gives them a speed of up to 1000 km/s, for reasons that are not understood.
• Mass and size: 1.4 to almost 3 solar masses, 20-30 km diameter; The more massive, the smaller they are! Their density is billions of tons per cubic cm, their gravity tens of millions of times stronger than Earth's.
• Important facts: Fast rotation (spin-up from collapse, and possible later accretion); Magnetic fields up to a trillion times the Earth's (or at least that's what our models tell us).
• How do we find them? They are so small that are difficult to see directly (although they are very hot), but with some luck we can see a pulsar...

Pulsars

• What are they? Objects that emit regular pulses of radio waves, first detected in 1967.
• Examples: The center of the Crab nebula M1 or the Vela nebula (both are SN remnants); about 1800 are known as of 2009, some in globular clusters.
• Periods: Most range from 0.03 to 0.3 sec (although a few are much longer), and are very stable: They are the most precise clocks known!
• Explanation: They are spinning neutron stars with hot spots; In addition to their thermal radiation, they have rotating radio beams, like lighthouses.