(
apparent magnitude ~8.4) Evolution of potential novae begins with two
main sequence stars in a binary system. One of the two
evolves into a
red giant, leaving its remnant white dwarf core in orbit with the remaining star. The second star—which may be either a
main-sequence star or an aging
giant—begins to shed its envelope onto its white dwarf companion when it overflows its
Roche lobe. As a result, the white dwarf steadily captures matter from the companion's outer atmosphere in an accretion disk, and in turn, the accreted matter falls into the atmosphere. As the white dwarf consists of
degenerate matter, the
accreted hydrogen is unable to expand even though its temperature increases. Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20 million
K, initiating nuclear burning via the
CNO cycle. If the accretion rate is just right, hydrogen fusion may occur in a stable manner on the surface of the white dwarf, giving rise to a
super soft X-ray source, but for most binary system parameters, the hydrogen burning is thermally unstable and rapidly converts a large amount of the hydrogen into other,
heavier chemical elements in a
runaway reaction, liberating an enormous amount of energy. This blows the remaining gases away from the surface of the white dwarf and produces an extremely bright outburst of light. The rise to peak brightness may be very rapid, or gradual; after the peak, the brightness declines steadily. The time taken for a nova to decay by 2 or 3 magnitudes from maximum optical brightness is used for grouping novae into speed classes. Fast novae typically will take less than 25 days to decay by 2 magnitudes, while slow novae will take more than 80 days. Despite its violence, usually the amount of
material ejected in a nova is only about of a
solar mass, quite small relative to the mass of the white dwarf. Furthermore, only five percent of the accreted mass is fused during the power outburst. In 2010 scientists using NASA's
Fermi Gamma-ray Space Telescope discovered that a nova also can emit
gamma rays (>100 MeV). Potentially, a
white dwarf can generate multiple novae over time as additional
hydrogen continues to accrete onto its surface from its
companion star. Where this repeated flaring is observed, the object is called a recurrent nova. An example is
RS Ophiuchi, which is known to have flared seven times (in 1898, 1933, 1958, 1967, 1985, 2006, and 2021). Eventually, the
white dwarf can
explode as a
Type Ia supernova if it approaches the
Chandrasekhar limit. Occasionally, novae are bright enough and close enough to Earth to be conspicuous to the unaided eye. The brightest recent example was
Nova Cygni 1975. This nova appeared on 29 August 1975, in the constellation
Cygnus about 5 degrees north of
Deneb, and reached
magnitude 2.0 (nearly as bright as
Deneb). The most recent were
V1280 Scorpii, which reached magnitude 3.7 on 17 February 2007, and
Nova Delphini 2013.
Nova Centauri 2013 was discovered 2 December 2013 and so far is the brightest nova of this
millennium, reaching magnitude 3.3.
Helium novae A
helium nova (undergoing a
helium flash) is a proposed category of nova event that lacks
hydrogen lines in its
spectrum. The absence of
hydrogen lines may be caused by the
explosion of a
helium shell on a
white dwarf. The
theory was first proposed in 1989, and the first candidate
helium nova to be observed was
V445 Puppis, in 2000. Since then, four other novae have been proposed as
helium novae. == Occurrence rate and astrophysical significance ==