The last two months have treated astronomers with two unusually bright arrivals in the sky, one a nova and the other a supernova. Each is interesting in its own right, but I find the comparison particularly attractive.
The first thing to note is that one was discovered by an amateur, while the other was a professional putting on a show for undergrads, rather than conducting research. Both hurried to alert the professional astronomical community. It is often said that astronomy is one of the few fields of science where amateurs still make a genuine contribution. As there become more and more professional telescopes scouring the sky one might expect this to stop being true, and in some areas it is – not long ago most comets were discovered by amateurs, while now the big search programs dominate.
It’s not clear to me whether the dramatic fall in the prices of larger home telescopes is keeping amateur contributions up, or whether these two discoveries were the late examples of a dying tradition, but I think for most people there is something romantic in the idea of the individual at home with his (or increasingly her) home scope coming up with something that expands our collective knowledge of the universe. Both events are bright enough that their eventual discovery was inevitable, but with short-lived features like this astronomers appreciate every extra hour of observing time, so early notice counts for a lot.
I can’t resist noting that the way amateur astronomers rush to report their discoveries so that professionals can verify them and start important work contrasts with the behaviour of “amateurs” in many other fields. Those who term themselves amateur climatologists or experts in vaccines do not feed in what they think they have learned to professionals for assessment, rather they announce that they know more than those who have spent their lifetimes studying the topic, often topped with accusations of fraud. An amateur astronomer whose report was found to be wrong would be more likely to express embarrassment for wasting people’s time than to go on the Internet and accuse government observatories of a cover-up.
Back to the events, Nova Centuari saw out 2013 for southern hemisphere observers. It peaked at a brightness of 3.3, easily visible to the naked eye under dark skies if you knew where to look. However, at 59º south that was pretty much just southern hemisphere observers, and it only got high in the sky quite late during the night, so the number of people who got to see it was probably small. I was told there was a nova in Centauri, but it is a big constellation. I had a look, and no doubt saw it shortly after the peak, but could not be confident of recognising the new star in such a large constellation. If I had known how close it was to Beta Centauri I would have had no trouble picking it out. It should still be visible in a small telescope or even binoculars, particularly if you live outside the city.
SN 2014J is quite a bit fainter, and while it is still brightening, it is never likely to be visible to the naked eye. Nevertheless, it is likely to be seen by quite a few more people – a product of being at 70º north, and therefore a possibility for anyone in the northern hemisphere with binoculars. The fact that it is located in a galaxy beloved of amateur astronomers will no doubt help, and being dubbed a supernova has a lot more cachet than a nova.
The fact that by the 21st of January we were up to J in the alphabet shows that supernova discoveries are not rare these days. However, two things make this special. The first is that Messier 82, the galaxy in which it is located, is a mere 11.5 million light years away. The second is that this is a Type 1a supernova, described by Dr Brad Tucker of the Siding Springs Observatory as “the golden goose egg”. I’m not sure if Tucker has his mythology right, but he was trying to get across that 1a supernova are poorly understood.
The nomenclature is confusing here. Type 1b and c share with 1a an absence of hydrogen in their spectrum. However, the processes that cause them are fairly similar to Type 2 Supernovae, that is a massive star ends its life with a bang not a whimper. Although there have been no supernovae of either sort in our galaxy since the invention of the telescope, 1987A gave us a close-up look at a Type 2 supernova. While much of the astronomical community would crawl over broken mirror glass for a front row seat with the benefit of the improved instrumentation of the last 30 years, the hunger is much greater for a nearby Type 1a.
2014J is the closest of its sort for 150 years, so while we wish it were closer still, it will have to do. As Tucker explained to me, “We’ve seen nearly 2000 type1a supernovae,” but none have been close enough to see what causes them. The theory is that this type of supernovae occurs when a white dwarf star lies close enough to a companion star to pull material off the companion (or should that be victim?) eventually exploding. However, this theory is largely a product of elimination – we haven’t been able to see the processes involved, it’s just the only thing we can think of that explains what occurs.
According to Tucker, we are particularly keen to learn about the nature of the unfortunate companion star. “It used to be thought that the companion was a red giant,” says Tucker. This is still what a lot of textbooks and popular science accounts will tell you. However, whenever we witness supernovae astronomers go hunting through previous images to find the progenitor stars. With type 2 and 1b/c we usually find something, at least if the source is close enough that a large progenitor could be detected. Not so with 1as. “No one expects to find the white dwarf,” says Tucker. However, in some cases explosions have been close enough that we would expect to have been able to see the red giant before it got stripped. We haven’t, so the thinking has turned to more ordinary sun-like stars, or possibly “subgiants”. Previous supernovae of this type have been too far away to pick up such stars, but Messier 82 is a different matter.
As well as tracking the light curve and analysing the spectrum for the elements present, astronomers are trying to place the event as accurately as possible. This will allow them to search old images for prospects. Give the star a couple of years to cool down and they will go back to look for stars that have moved. It is thought the explosion will shift the companion star noticeably, allowing us to identify it.
Why does it matter what sort of star is feasted upon to lead to such an explosion? Tucker says that we hope that by learning more about the star we can calibrate our models of the event itself. This is important, because 1a supernovae have a very important role in astronomy. They are used as what is called “standard candles” to give us a measure of the distance of the galaxies in which they occur, when these are too great to be measured in other ways. The speed with which 1a supernovae fade from their peak can be used to estimate their intrinsic brightness, which coupled with their measured brightness tells us how far away they are.
It was this measure that upended our understanding of the universe and won Brian Schmidt a Nobel Prize. By comparing the distances to galaxies in which 1a supernovae were observed, and the speed with which they are moving, Schmidt demonstrated that the expansion of the universe is accelerating, rather than slowing down as was previously thought. The finding was sufficiently robust that few now doubt the conclusion, or the existence of “dark energy” which this implies. However, the rate of expansion is still a rough estimate. A better understanding of the candles we are using should improve our precision.
Back to poor Nova Centauri. Besides being relegated to a neglected part of the sky, it lacks the same scientific significance. Nevertheless, Tucker says “anything new in the sky is of interest”. It is thought that similar processes were involved here, but instead of the white dwarf pulling off so much material the whole star explodes, mini explosions “like a string of fire crackers” are occurring as clumps of gas land on the dwarf’s surface, leading to fluctuations in its brightness despite the general downward trend. Learning more about this process will improve our understanding of white dwarves, and possibly the processes when they really go off, and being in our galaxy we get to see it about a thousand times closer than 2014J.
Forensics, Fossils and Fruitbats 