Astronomers world-wide are thrilled by news of a nova, just visible to the unaided eye in the constellation Delphinus (the Dolphin), just beside the familiar Summer Triangle. Why are they so excited and what will you see if you look toward this interloper in the sky?
To stargazers in olden times a nova was a “new” star which just erupted into the night sky. Today we have a different view. What really happens is even more dramatic. A white dwarf is the inert but still white hot core of a dead star. A white dwarf is very small, about the same size as our planet, but very dense, having perhaps as much matter as the Sun squeezed into its volume. We know of many binary star systems comprised of a healthy main sequence star with a white dwarf companion, the pairing of Sirius with its unseen companion Sirius B are a good example.
In a binary system where the pair are very close, the white dwarf’s gravity can tear material, including hydrogen, from the surface of its partner star. This stellar parasitism cannot last. Eventually the hot and compressed hydrogen squashed over the white dwarf surface’s reaches a critical threshold of temperature and pressure and detonates in a runaway nuclear fusion reaction across the surface of the white dwarf. A staggering amount of energy is released in this titanic nuclear explosion; the star system brightens ten thousandfold in mere hours, and within a few days could be a million times brighter than it was. This by the way, is not the same thing as a supernova, an even more cataclysmic explosion (brightness increases about 100 million times) which actually rips stars apart. Perhaps surprisingly both stars always survive the nova explosion (any nearby planets are probably left a little worse for wear) and in fact may repeat the experience many times.
Often the star system where the explosion occurs has been overlooked by astronomers, only the sudden flare of light reveals its existence, as though a new star has leapt into being (hence the term ‘nova’, meaning ‘new’). After attaining peak brightness the nova dwindles back over weeks to its original brightness. Astronomers discover about 10 novae every year in the Milky Way, but there are probably many more occurring block from our view by molecular clouds, the Galactic Core and other obstructions.
Nova Delphini 2013 (also known as PNV J20233073+2046041) was discovered on 14 August 2013, by Japanese amateur astronomer Koichi Itagak when it was at about magnitude 6. Since then it has brightened even more into the region of magnitude 4.4 to 4.5 (UPDATE: it ultimately reached 4.3 on 16 August 2013 before beginning to fade). This means if you have a dark enough sky the nova is visible to the naked eye but to pick it out you will need to be very familiar with the dim stars between Delphinus and Sagitta (the Arrow). Helpfully Sagitta is pointing right at the nova! Binoculars too will make seeing it much easier (to be honest they are essential)- this is not a spectacular object dominating the sky. The bright moonlight makes spotting it hard still but do persevere, there is a useful chart on the Sky and Telescope magazine website. I saw the nova myself about midnight on Saturday 17 August. In the eyepieces of my 15×70 binoculars it was one star among others- special only because the week before it would not been there to see.
(UPDATE: I have been asked how far away is the site of the nova. I did a quick calculation. Peak magnitude was 4.3, novae peak at absolute mag -8.8 (this constant makes them splendid “standard candles” for distance determinations) . Using D = 10 (m-M+5)/5 ,where m=4.3, M=-8.8, that means a high estimate for the nova’s distance from Earth (D) is about 4170 parsec or
1280 13590 light years (see comment by Charles Paluska).
A spectrum has been taken of Nova Delphini and it is typical for a binary star system undergoing a nova event.)
(Article by Colin Johnston, Science Communicator)