One of the biggest explosions astronomers have ever seen appeared recently in space, but scientists are struggling to decide what caused it. The best guess so far is that it was something called a "super-luminous supernova", which is the cataclysmic explosion produced when a large star reaches the end of its life. But the explosion this time was so powerful that it doesn't seem to fit with any of our existing theories, as David Bersier, from Liverpool John Moores University, explained to Felicity Bedford...
David - This one is just the brightest that has ever been found; something like 500 billion times the brightness of the sun. In terms of space, it's at a fairly large distance because what makes this event unique is that it's all so very rare. So, if you want to find rare objects, you also have to look at a very large chunk of universe and this object, we're talking about 3 billion light years away.
Felicity - Wow! What exactly is it that makes this star, this supernova so bright?
David - Well that's part of the problem. In a sense it's much easier to discover a supernova than to understand what makes it shine and it's so bright, so much brighter than normal supernovae that we think we understand, that for this particular event we call "Assassin 15LH," these models are really stretched because the total amount of energy that's been emitted in the first few months are really pushing the limits of what models can explain.
Felicity - So it's a challenge, but you must have some ideas?
David - We can try and speculate. Yes, there are several models around, several theories; these are superluminous supernovae; some of them are powered, in a sense, by the interaction of the matter that is ejected in the explosion with what is just lying around the star at the time of the explosion. Another possibility is, what triggers the explosion is that the core of the star, where the nuclear reactions are happening, this core collapses and forms what we call a "neutron star" and, in some cases this "neutron star" may have an enormously large magnetic field and it may be spinning very, very fast. And that is one possibility to explain the brightness of these superluminous supernovae.
Felicity - What is it that makes supernovae so interesting; why are you studying them?
David - Different types of supernovae are, essentially, responsible for a lot of what we call "the periodic table" of the elements. Very early in the universe, after a few minutes, there was just a lot of hydrogen, some helium, and very little of a few other elements, whereas supernovae make a lot of the elements; the carbon and oxygen: most of it has been made in supernovae.
Felicity - So it's something that's fairly difficult to miss. How did you detect this star?
David - This Assassin survey scans the whole sky, about every other night; using small telescopes, actually, about 14 cm telescopes. There is software to look at the images when they come in and to try and find objects that were not there before.
Felicity - Is the international nature of this project central to its success? You say you were looking at the whole night sky - that simply wouldn't be possible without the telescopes that are scattered all over the world.
David - "The sky never sleeps" - one of the principal instigators of the project puts it. He said "there's always an assassin awake"...
Felicity - There's always an assassin awake - fantastic.
David - There's always one of us awake somewhere in the world, able to do something.
Felicity - Could you draw all of this together and summarise what you found?
David - So we found this object, this extraordinary luminous supernova that beats the record at least by a factor of two in terms of brightness compared to the second brightest. Traditionally scientists are very conservative and we say - okay, this is weird, what else can it be? A bright supernova is a standard explanation but we have to make sure it's not something else and nature is not playing a trick. We think we have reasonably good arguments to rule out other explanations such as an extraordinarily massive black hole that is regularly accreting matter; there is no sign of anything like that happening. We think it's not a tidal disruption event, it's not a star that passed very close to a supermassive black hole and got ripped apart and, essentially, we are left with this explanation. It is a single star that exploded but we don't really know why it is so bright; it is really stretching current models almost to the breaking point.