Jimmy Westlake: Exploding star visible before dawn
About 10,000 years ago, in a star system far, far away, a layer of superheated hydrogen gas on the surface of a dead star called a white dwarf erupted in a thermonuclear inferno. The light flash from that explosion finally arrived at Earth last week producing the brightest “nova stella” in our skies since at least August 2013.
Australian nova hunter John Seach spotted the exploding star on March 15, just at the limit of naked eye visibility. Since then, the nova brightened steadily and seems to have peaked Sunday, well above the naked-eye brightness limit.
In ages past, when a star suddenly appeared in the sky where none had been seen before, it was called a “nova stella,” or “new star.” In spite of its name, a nova is not a new star at all, but is the sudden explosion of a very old star near the end of its life. Modern astronomers have determined that two stars are required to create a nova explosion, a tiny white dwarf star orbiting a red giant star.
The nova explosion is triggered when a bloated red giant star dumps fresh hydrogen gas onto the surface of its orbiting white dwarf companion. The hydrogen accumulates in a thin layer on the white dwarf’s surface until it reaches the critical temperature of 10-million degrees and then… KABOOM!
A runaway thermonuclear explosion erupts on the surface of the white dwarf as the hydrogen fuses rapidly into helium. It’s a hydrogen bomb the size of the Earth. The star increases in brightness by a factor of 63,000 in a matter of hours and blazes into view in earthly skies as a “new star.”
Over the following days and weeks, the erupting star fades back into obscurity. As amazing as it sounds, the red giant and white dwarf can survive this conflagration and repeat the nova process again in the future. Many recurrent novae are known to exist.
The new nova is officially designated Nova Sagittarii 2015 No. 2. It is the brightest nova to occur within the constellation of Sagittarius in well over a century.
Astronomers discover many novae every year, but most never break the naked-eye limit because of their extreme distance. It is a rare nova, indeed, that becomes visible without a telescope as this one did.
If you would like to see this newcomer to our sky for yourself, you’ll need to rise early before dawn starts to brighten the sky. Try looking around 5:30 to 6 a.m., low in the southeastern sky, for the familiar teapot-shaped outline of Sagittarius, the Archer. The nova was briefly the brightest star inside of the teapot’s outline, almost dead center.
The latest reports from Monday morning confirm that the nova has already peaked in brightness and is starting to fade. It is still bright enough to see without optical aid, but just barely so, and probably not for much longer. Binoculars will enhance the view and make spotting the nova much easier.
Not many people can claim to have seen a nova in the sky, so if you are up early, and the sky is clear, give it a shot.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. His “Celestial News” column appears weekly in the Steamboat Today newspaper and his “Cosmic Moment” radio spots can be heard on local radio station KFMU. Check out Jimmy’s astrophotography website at http://www.jwestlake.com.
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