Supernovae

An extremely rare endpoint Most stars either have M < 1.4 Mʘ, or can lose enough mass during post-main sequence evolution to get there, and end as white dwarf However if core is more massive, nothing can stop gravitational collapse The final stages take just seconds, and result in a massive fusion explosion

How rare is “rare?” There has not been one in our Galaxy since the invention of the telescope 400 years ago! They are so bright that we can easily see them in other galaxies, and estimate how often they go off A typical galaxy of 200 x 109 stars has only one SN every 100 years! If they are so rare, how do we know this rate? Do we need to watch for 100 years?

How “super” is a supernova: the “light curve” At maximum brightness, for about a week or two the SN can reach the same luminosity as the sum of all the 1011 stars in the entire host galaxy!

Our Milky Way Galaxy is “overdue” for a supernova Last local SN was more than 400 years ago (1604) [so, maybe soon!] Johannes Kepler

Our Milky Way Galaxy is “overdue” for a supernova Last local SN was 400 years ago However, in 1987 one occurred in our nearest neighbor galaxy, the Large Magellanic Cloud, 180,000 lt-yr away

Supernova “1987a” in LMC

Supernova “1987a” in LMC

Want to bet on the next Milky Way supernova? I’d put my money on η Carinae

There are actually two different paths that can lead to a supernova The collapse of a massive star – “Type II” Mass exchange on to a white dwarf in a binary system --- “Type I” Most stars are actually in binary systems – our Sun happens to be an exception

If the two stars in a binary orbit are sufficiently close, “mass exchange” can occur Process is slow, perhaps 10-8 Mʘ /yr But in 108 yr, that means 1 Mʘ of stuff piles up on the white dwarf, which often had an initial mass of 0.4 Mʘ Recall, electron pressure can only support 1.4 Mʘ Then what??

Why are supernova important? All the elements more complex than iron are formed!

SN as “standard candles” A certain subclass of Type I supernova turn out to all have the same luminosity when they are at their brightest

SN and Gamma Ray Bursts

A secret during the Cold War: the “Vela” Satellites

A Cold War Secret: Gamma Ray Burst (“GRB”)

Thousands of GRBs are now known But, are they nearby (relatively little energy released), or very far away (large energy release)?

Thirty years later GRBs prove to lie inside of very distant galaxies)

Now that we know the distance to each GRB event, we can calculate the energy (luminosity) generated in each burst There are roughly 1011 stars in a typical galaxy There are roughly 1010 galaxies within range of the largest telescopes For the few second that a GRB is in progress, that single event generates more energy than the sum of every star in every galaxy in the entire observable Universe, all 1021 stars!

Do all supernovae make Gamma Ray bursts? No Why do some make GRBs, but others not?

Discovery of new supernovae

Discovery of a new Gamma Ray Burst Swift Satellite

Stuff Supernovae leave behind

Stuff Supernovae leave behind

Stuff Supernovae leave behind

Doppler Effect: motion between source of a wave and the observer slightly changes the wavelength