Novae

Nova means "new star", but they are actually "newly visible" stars caused by brightening of an existing star by 5 to 15 magnitudes. This can be brightening by hundreds of millions of times. The brightening may occur within a few days or weeks, but usually fades again within months. There are some recurring novae, but most are one-time events. One model of novae suggests that they occur in binary systems where one is a white dwarf and the other is on its way to becoming a red giant . The red giant can lose mass which would trigger hydrogen fusion as it falls on the white dwarf. This would blow the gas off and the process could repeat itself. A notable nova example is Nova Cygni 1975. It was the second brightest observed nova in the 20th century, after CC Puppis in 1942. CC Puppis reached a magnitude of 0.35 on November 11, 1942 and is considered to be one of the four brightest novae in the past three hundred years.

The spectra of light emitted by a nova are Doppler shifted toward the blue, implying that the luminous gas is exploding off the star and traveling toward us at a high speed.

Nova Cygni 1975

The brightest Nova in recent years was Nova Cygni 1975 which became visible in Cygnus in August. It reached a magnitude of 1.8 after a rise in magnitude of about 15 in two days time. This is a million-fold increase in luminosity! (Image accessed Astrophysique gamma nucleaire, Toulouse 19-21 avril 2004. Novae- M. Hernanz)

Nova Cygni 1975 was close to the bright star Deneb, a member of the "summer triangle", and reached a brightness comparable to Deneb. The photo is a 30sec time exposure taken on August 30, 1975. (Accessed at Astronomeer.com)

The images below show the difference in brightness in two days. They were taken by Ben Mayer who was taking photographs in a search for meteors. They were 25 minute exposures on Tri-X film. The were reproduced by Jay Pasachoff, see reference. The bright star at right is Deneb.