Supernovae and their Host Galaxies

this coming week, sydney is hosting a professional astronomical conference called "Supernovae and their Host Galaxies," sponsored by the Australian Astronomical Observatory and CSIRO Astronomy and Space Science (i can never remember what CSIRO stands for...).

while supernovae (exploding stars) are not my research area of expertise, i signed up to attend the conference because, well, who doesnt like a good extragalactic stellar explosion?!? no seriously, i study the galaxies that these evolved stars live in and there are so many international experts in the field attending that i thought it would be an excellent way to catch up with the best current ideas as to how and why these stars explode.

Tycho's Supernova Remnant.  Credit: NASA/CXC/Rutgers/ J.Warren & J.Hughes et al.

i'll explain the basics of my knowledge of supernovae here (supernovae or supernovas are the plural forms of supernova) and throughout the week, i'll try to expand and update the current understanding based on what i find interesting at the conference. i'll also be using twitter to mention things in real time so follow there as well if you wish.

first, the lovely conference poster:

there are two fundamental ways stars explode as supernovae: from the central collapse of a single massive star, or from a thermonuclear detonation caused by too much extra material falling onto an old star. astronomers descriptively call these various explosions Type Ia, Type Ib, Type Ic, Type II, etc... based on the chemical signatures seen in their spectra when they were first being discovered and observed with telescopes.

1) Core-Collapse Supernovae are the inevitable explosions of big stars that are between 9 and ~40 times the mass of our sun (note that our sun will not go supernova because it is not massive enough). these massive stars burn big and bright during their lifetimes because deep in their central cores, they are fusing smaller elements into heavier ones through nuclear fusion. just as a campfire will burn out if you stop adding wooden logs for fuel, the centers of stars eventually run out of fuel for nuclear fusion. once this happens the core collapses in less than a second due to gravity and forms an extremely dense beast called a neutron star!

the heat and energy produced by the formation of the neutron star reverses the gravitational implosion and causes all the other material to shoot out in all directions at 50 million kilometers per hour! a powerful shock wave propagates outwards creating a visual outburst that can be as intense as the light of several billion suns. after reaching a maximum brightness, the supernovae fade slowly over a few months. these are classified as Type II, Type Ib and Type Ic supernovae and shown schematically below.

these supernovae are found mostly where there are lots of bright, young stars - in the arms of spiral galaxies. they are not found in elliptical galaxies which typically have old stars and very few young, newly-formed ones.

2) Thermonuclear Supernovae are called Type Ia and result from a completely different series of events. the process starts with an elderly star, called a white dwarf, which is what remains when a smallish ordinary star finishes its life cycle after running out of fuel for nuclear fusion in its core (our sun will eventually become a white dwarf star - in another 5 billion years or so).

white dwarf stars are typically very stable, but if a white dwarf star happens to have a nearby neighboring star that happens to shed enough gas onto white dwarf star that it reaches the so-called Chandrasekhar limit of 1.4 solar masses, the temperature in the core of the white dwarf will rise to the point of triggering explosive nuclear fusion reactions and releasing a huge amount of energy. the violent stellar explosion obliterates the star in about 10 seconds and releases an expanding cloud that glows brightly for a few weeks.

Type Ia supernovae are found in all types of galaxies.

so what mysteries remain? i'll leave you with the declared motivation for this conference from the organizers:

"The current generation of wide field transient surveys will revolutionize our understanding of why stars become supernovae. Designed to revisit large areas of sky at multiple wavelengths, these surveys are now discovering hundreds of supernovae each year. During the coming year, the number of supernova discoveries will increase even further as new transient surveys come online. As well as finding rare and possibly new types of supernovae, these surveys will generate new insights into both core-collapse and thermonuclear supernovae. It is therefore timely to have a conference that explores the current (observational and theoretical) supernova landscape and the connection between supernovae and their host galaxies."

UPDATE:
intro to the conference: here
summary of day 1: here
summary of day 2: here
summary of day 3: here