The bright nova that flared up in the constellation Sagittarius has made a comeback of sorts over the last few days. The star was already 6th magnitude when first discovered on March 15, 2015 by John Seach of Chatsworth Island, NSW, Australia. The nova brightened magnitude 4.3 by March 22, making it easy to see with the naked eye, then faded to magnitude 6 by March 25. It surprised astronomers by brightening again to magnitude 4.8 on March 27. The flaring star has been officially named Nova Sagittarii 2015 No.2.
The term “nova” 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 some novae repeat this process every 1,000 to 10,000 years.
To see the nova for yourself before it fades again, 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, then turn to orange and red as the expanding shell of gas expands, cools, and dims.