Thread by @realscientists

Supernova progenitors! Wooooo!

First question, what is a supernova? Second question, what is a progenitor?

Well a supernova is a very bright source we see in the sky, a single star almost outshines all the rest of the stars in a galaxy, when galaxies have billions of stars.

Here is a good picture of a supernova and it's host galaxy (taken from apod.nasa.gov/apod/ap150531.…). As you can see that source in the lower right is as bright as the central nucleus of the galaxy. And it can stay that way for months or weeks.

Now we classify supernova by their observational characteristics. Most probably type I have no hydrogen in them while type II have hydrogen observed. As hydrogen is the most abundant element in the Universe its odd that something don't have it! The other important lines are...

...of silicon, if we see lots of silicon then the explosion is a "thermonuclear" explosion or a type Ia. If not then the supernova is a "core-collapse" supernova and type Ib, Ic or II. Then Ib have helium lines, Ic don't and type II are split up into other classes...

like IIP, IIL, IIb and IIn by either how quickly they fade of other specific features of their spectra and evolution but we'll come back to those.

Core-collapse supernovae are the deaths of a massive star, more massive than 8 times that of the Sun, when it's formed a core of...

...iron in it's center which is the most stable element in the Universe so it can't get any more energy from fusion. It collapses down to a neutron star or black hole and releases a large amount of gravitational potential energy that causes the star to explode.

The thermonuclear supernovae occur when another type of stellar remnant, a white dwarf, reaches 1.4 times the mass of the Sun, when it can't support itself anymore so it collapses which then triggers an explosion, explosively fusing carbon and oxygen to iron...

...which is the major source of iron in the Universe and much of the iron in your blood that make it red (it's called hemoglobin after all) comes from this time of supernova.

So supernova = exploding stars.

A supernova progenitor is the star that exploded in the supernova. :o)

So we really want to know what different types of star produce the different types of supernovae. But how do we do this? Well one thing to do is to thing and make models of stars and explode them but this has always been difficult with needing lots of computer power that...

...we've only just recently started to get. The other method is to see a supernova and then look back in archival images to see if they had the star that exploded in them. This has only really become possible this century. However the first one was...

found for the supernova 1987A which was the closest supernova observed since 1604! This image (Credit: David Malin / Australian Astronomical Observatory.) shows the supernova on the right with the star that exploded on the right. Oddly it wasn't a "normal" supernova nor expected

type of progenitor as it was a blue supergiant rather than a red supergiant and it's lightcurve was "peculiar" and fainter than expected. But we did also detect neutrinos from the core-collapse as theory predicted so everyone got very excited as it confirmed that a neutron star

...was formed. One other interesting fact is that we really understood the star as rather than just a image Nolan Walborn had taken a spectrum of the star before it exploded so we understood it's temperature very exactly too. So while it was "unusual" it really was an exciting...

...event. There are lots of other unusual things about it but today we are pretty sure that the supernova was odd as the star was a binary that merged a few thousand years before it exploded. This mixed up the evolution of the star so it exploded much smaller and hotter than...

...theory predicted for single stars. A running theme of this week will be how stars interacting in a binary system really produce very different outcomes to those we get taught in textbooks about how stars evolve.

The next detected progenitor was for supernova 1993J which was a type IIb, which stars out looking like it's hydrogen rich but then the hydrogen lines disappear so it looks like a Ib hydrogen-free supernova. This time the progenitor was a red supergiant but it was odd...

...(I won't show the image as it's really bad as it was in a much more distant galaxy) as while there was a lot of red light there was a lot of blue light so people thought that again we might be looking at a binary system and it was! As years later scientists looked for the...

...surviving companion and found it (taken from here - apod.nasa.gov/apod/ap040212.…). But this image also shows that with the Hubble Space Telescope we can zoom right in and pick out individual stars in quite distant galaxies. So maybe using HST we can find more progenitors?

So the first people to try to do this were Stephen Smartt and Schyler Van Dyk and it was highly competitive. The first detected progenitor was for supernova 2003gd. (Image from ing.iac.es/PR/SH/SH2004/s…) the star that exploded isn't that bright red smudge it's the small red dot.

So now we're good? We've seen a red supergiant explode! And the star remained at a constant brightness for about 3 months which makes it a IIP - lightcurve plateau supernova which are about 60% of all supernova. So yay!

To make really sure this was the case we went back to some of the progenitor sites and observed to make sure the star disappeared which is what Seppo Mattila did here just as Justyn Maund had done for 1993J except this time the star was gone with only a faint glow from the...

...explosion left. So all good. But what about the other 40% of supernovae, what stars explode as them? Right time for a new thread!