A nova has flared up in the constellation Sagittarius, and it’s grown bright enough to see with the naked eye in the pre-dawn sky. As of this weekend, according to Sky and Telescope, the nova has reached an impressive magnitude 4.3 which is plenty bright enough to see without optical aid in clear sky. The flaring star has been officially named Nova Sagittarii 2015 No.2.
“Nova” is a pre-telescopic term which means “new star”, because a nova seems to appear in a place where no star appeared before.
We now understand a nova is not a new star, but a flare-up of an existing binary star system in which a stream of hydrogen gas from a main-sequence or red giant star falls onto the surface of a close white-dwarf companion star. When the thin hydrogen layer on the white dwarf gets sufficiently thick and hot, it detonates like a hydrogen bomb and the star brightens for several days before fading over the next weeks. Some material is blown into space, but both stars remain intact. Astronomers suspect most such novae repeat this process every 1,000 to 10,000 years.
This nova was first discovered on March 15, 2015 by John Seach of Chatsworth Island, NSW, Australia, who was searching for novae in images from a DSLR camera. The star was already 6th magnitude upon discovery. The night before, the camera recorded no bright star in this area down to magnitude 10.5.
To see the nova for yourself, look for the striking “Teapot” of Sagittarius in the pre-dawn sky low over the southeastern horizon at 5-6 a.m. as seen from the northern hemisphere. From the southern hemisphere, Sagittarius lies well overhead as dawn arrives. The nova is just under the “lid” of the “Teapot” (see image above). In binoculars or a telescope, the flaring star may appear yellowish. As it expands and dims, the color will turn to orange and then red as the expanding shell of gas expands and cools.
Nova are not rare events in the night sky. But a bright nova is still a sufficiently unusual event to see for yourself. The last naked-eye nova (to my recollection) happened in 2013 in the constellation Delphinus.