How A Dwarf Nova Hit The Big Time

Although it’s starting to drop off in the western sky, you’ve still got time to find the nova V392 Persei. It’s visible at nightfall about 25° high (two and a half fists) in the northwestern sky. This map will get you oriented. More detailed maps below will guide you directly to the star. Stellarium

Yuji Nakamura of Japan was taking pictures of the sky last Sunday night (April 30), when he recorded a remarkable stellar explosion 12,685 light years away in the constellation Perseus. It turned out to be a brand new nova. That night and the following night, the star was easily visible in binoculars. By last night it had faded a bit but remains easy to see in a small 3-inch telescope. Turns out the star, V392 Persei, was already known to astronomers as a dwarf nova. The star rarely gets brighter than magnitude 15, but that night it shot up to 6, the naked eye limit!

Novae occur in close binary systems where one star is a tiny but extremely compact white dwarf star. The dwarf pulls material into a disk around itself, some of which is funneled to the surface and ignites in a nova explosion. NASA

Despite the name, a dwarf nova isn’t a smaller star but instead a smaller explosion. All novae, whether dwarf or normal, involve two stars, the reason the sun can never “go nova.” One of the stars is a super-dense, Earth-sized white dwarf that closely orbits a normal star like our sun. The strong gravity of the dwarf siphons gas from its companion into a hot, spinning disk called an accretion disk. Changes in the flow and temperature of the gas into the disk cause it to overheat and flare. Back on Earth, we see these flares as a rapid brightening of the star system. What normally looks like a faint star can swell overnight to more than 100 times its original brightness.

Click the map to get a larger version. The star at lower left is Epsilon Aurigae, the same as marked on the wide map at the top of the blog. Stars are numbered with their brightness (magnitude) with the decimals omitted, so 57 is magnitude 5.7. AAVSO

That’s how dwarf novae work. One of the easiest for small scopes is SS Cygni in Cygnus the Swan, a.k.a. Northern Cross.  In a nova, we once again have two stars, a white dwarf and a normal sun, but this time, gas in the disk gets funneled down to the surface of the dwarf. Crunched by the intense pressure and cooked at over 180,000°F, it detonates like a gargantuan thermonuclear bomb, boosting the brilliance of the original star by at least several thousand times. Instead of a faint point of light at the limit of vision, we see a bright star.

That’s the nova. It’s still appears exactly like a star. Months or years later, larger telescopes might be able to spy a faint shell of debris illuminated by the host suns. In most novae, the stars remain intact and can experience repeat blasts in the distant future. What makes V392 Persei incredibly rare is that it’s just one of two dwarf novae ever seen to transition to a nova. (The other, V1213 Centauri, never became prominent.)

This is a closer view of the nova and stars near it. Click to enlarge, save and print out. AAVSO

Whenever surprises like this happen, I can’t wait to get a look. The sky cleared last night, and I saw the nova at magnitude 8.3, still bright enough to be faintly visible in binoculars. No one know if V392 Per will fade back, hit a standstill or climb back up again. That’s why you may want to have a look yourself the next clear night. The maps included with this blog will help get you there.

Start at Epsilon (ε) Aurigae, the star directly below Capella in the map at the top of the blog. Then use the AAVSO chart (left) to star-hop northwest past a small pair of stars to the nova. There’s a magnitude 9.3 star close to and almost due east of the nova that makes a handy comparison star to gauge the nova’s changing brightness in the coming nights.

To make more maps and keep up the nova’s ups and down, check out the AAVSO website.