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 0.6M sun. Stars up to 8M sun become carbon/oxygen white dwarfs with masses up to ~1.1M sun. Stars below 0.45M sun aren t massive enough to burn helium in their core and become helium white dwarfs. Stars with masses from 8-10M sun have an extra stage of burning in their core and make oxygen/neon/magnesium white dwarfs with masses of ~1.2M sun. White dwarfs have a mass limit 1.4M sun (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. Cataclysmic variables are bright source in the blue and ultraviolet. Accretion disk 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 1.4M sun (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. ~0.6M sun of 28 Ni 56 is produced, which is radioactive! 28Ni 56 --> 27 Co 56 --> 26 Fe 56 + lots of energy
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 (1.4M sun ), 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 M sun ) 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 1.4M sun 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 M sun 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 27 Co 56 (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 2-3M sun and 10 km radius. This is 10 11 kg/cm 3!!! 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.