Nova Delphini was discovered on the 14th of August, 2013 by Japanese amateur Koichi Itagak. Eventually designated V339 Delphihi by the International Astronomical Union, Nova Delphini became an easy naked-eye object throughout the end of August and one of the 30 apparently brightest in recorded history. It’s location in the northern mid-summer skies made it a favorite target for stargazers throughout the late summer and early fall of 2013. It also appeared just as amateur astronomers have begun to engage in regular spectrographic monitoring as numerous relatively low-cost but very capable spectrometers have become available from commercial vendors, most notably from Shelyak Instruments in France. It has become, by far, the single most spectrogaphically-observed object in history as amateurs from around the world contributed over a thousand high quality spectra to the ARAS (or Astronomical Ring for Access to Spectroscopy) Nova Delphini database, available here.
I had previously signed on to another campaign (the WR 134, 135, and 137 campaign coordinated by Noel Richardson and Tony Moffat at U Montreal) so I did not start regularly monitoring Nova Delphini until the WR stars disappeared behind the large maple tree in my back yard. Once I could no longer acquire high signal-to-noise data from the WR stars I switched to monitoring this object, and ultimately contributed 20 or so spectra to the ARAS effort.
Novae, like other members of the cataclysmic variable star class, derive from binary star systems comprised of a collapsed white dwarf star (think something the size of earth but the mass of the sun) and a red dwarf star (something a third the mass and half the diameter of the sun). The two stars are very close to one-another, so close that the much stronger gravitational pull of the white dwarf primary star is sucking material from the outer atmosphere of the red dwarf secondary. That mass, comprised of almost pure hydrogen, must obey the laws of energy and angular momentum conservation and so settles into a disk of material, called an accretion disk, which surrounds the white dwarf primary. Eventually the disk material manages to radiate away enough energy that it finally settles onto the surface of the white dwarf (a fairly violent process in itself) and ultimately forms a very thin (some inches thick) atmosphere of pure hydrogen. Once enough Hydrogen has accumulated the temperature and pressure at the bottom of this very thin shell become sufficient to trigger the fusion of Hydrogen into Helium. When that happens the shell experiences a thermonuclear runaway – and essentially becomes a massive Hydrogen bomb, blasting itself into space at thousands of kilometers per second and taking with it anything else that might have been near the surface of the white dwarf. The spent fuel remnants from the white dwarf’s previous life as a normal star, including Helium, Carbon, Nitrogen, Oxygen, Silicon and Iron, anything that might have been near the surface, is also ejected at very high velocities into interstellar space. When all of this occurs the total luminosity of the system can increase by a factor of more than 10,000 times. And thus we see it as a nova.
In the days and weeks immediately following the outburst rapid changes in the temperature and density of the expanding shell of gas lead to dynamic and complex changes in the appearance of the nova’s spectrum. Additionally, unlike a supernova, the primary star that was the source of the explosion is still there, strongly radiating in the ultraviolet and x-ray region and thus ionizing material from the inside of the rapidly expanding shell. Within hours following the announcement of the nova’s discovery members of the ARAS forum began around-the-clock monitoring of Nova Delphini 2013, acquiring high-quality spectra from over three dozen locations around the world.
Immediately recognizing the value these spectra would represent towards a better understanding of novae in general Dr Steven Shore, a world-renown expert on the topic, began a series of communications helping to guide the ARAS effort. The website has a series of pages of Steven’s explanations of what the spectra were indicating and what observers might expect to see next based on observations of past novae. Nicely illustrated with dozens of the ARAS spectra these pages are an invaluable and unique resource for amateur spectroscopists wanting to better understand the data they’ve gathered. Check it out HERE.
Anyway, shown in Figure 1 is a combined plot of all of the Nova Del spectra acquired at the Beverly Hills Observatory.