Supernova explosions

The lives of stars Type I supernovae –Destruction of white dwarfs Type II supernovae –Core collapse of massive stars What’s left behind? –Pulsars –Black Holes

Hertzsprung-Russell diagram

Nova Explosions Nova Cygni 1975 Hydrogen accreted through the accretion disk accumulates on the surface of the white dwarf  Very hot, dense layer of non-fusing hydrogen on the white dwarf surface  Explosive onset of H fusion  Nova explosion

Artist’s impression of a nova

The Chandrasekhar Limit The more massive a white dwarf, the smaller it is.  Pressure becomes larger, until electron degeneracy pressure can no longer hold off gravity > Type I supernova WDs with more than ~ 1.4 solar masses can not exist!

Evolution of high-mass stars

No need to memorize these numbers. Main thing is to appreciate the differences in the numbers for each evolutionary stage. Time-scales and conditions

Core collapse - a star in gravitational free-fall (Q1)

Artist’s impression of a supernova Supernova facts ~1/100 yr in our Galaxy ~1/second in Universe Only 0.01% of energy released in visible light! p + e -> n + neutrino Most of energy is in the form of neutrinos!

Neutrinos Elementary particle postulated in 1930 First detected in 1942 Miniscule mass and no electric charge Come in three “flavors” electron- muon- and tau- Interact only weakly with other matter

Detecting neutrinos Sudbury neutrino detector

Historical Supernovae (Q 2) The youngest supernova explosion occurred in ~1868 and astronomers have just discovered the remnant in the X-ray image shown on the right. The explosion occurred close to the center of our Galaxy and the optical light was obscured from view.

The Famous Supernova of 1987: Supernova 1987A BeforeAt maximum Unusual type II supernova in the Large Magellanic Cloud in Feb. 1987

How much energy is liberated? An application of Einstein’s equation –Matter and energy are related as follows E = M c 2 In question 3 we will use this –Take mass of original star –Subtract mass of end product –Subtract mass of expanding shell –Apply the equation to what is left over

Extragalactic Supernovae Why wait? Supernovae are so bright that they can be seen in other Galaxies! Zwicky started such searches and found a total of 120. Currently 984 known. One of their uses is to study the expansion of the Universe (see ASTR 106)

Neutron Stars Typical size: R ~ 10 km Mass: M ~ 1.4 – 3 M sun Density:  ~ g/cm 3  Piece of neutron star matter of the size of a sugar cube has a mass of ~ 100 million tons!!! A supernova explosion of an M > 8 M sun star blows away its outer layers. The central core will collapse into a compact object supported by neutron degeneracy pressure. Pressure becomes so high that electrons and protons combine to form stable neutrons throughout the object.

Black Holes Just like white dwarfs (Chandrasekhar limit: 1.4 M sun ), there is a mass limit for neutron stars: Neutron stars can not exist with masses > 3 M sun We know of no mechanism to halt the collapse of a compact object with > 3 M sun. It will collapse forming a singularity: A BLACK HOLE!

Summary Supernova are violent stellar explosions –Type I destruction of greedy white dwarfs –Type II core collapse of massive stars Supernovae produce exotic objects –Neutron stars –Black holes Supernovae probe the size of the Universe –Type I supernovae act as “standard candles”