A nova is a cataclysmic nuclear explosion on a white dwarf, which causes
a sudden brightening of the star. Novae are not to be confused with other
brightening phenomena such as supernovae or luminous red novae. Novae are thought
to occur on the surface of a white dwarf in a binary system when they are
sufficiently near to one another, allowing material to be pulled from the
companion star's surface onto the white dwarf. The nova is the result of the
rapid fusion of the accreted hydrogen on the surface of the star, commencing a
runaway fusion reaction. Development
The development begins with two main sequence stars in a binary relation. One
of the two evolves into a red giant leaving its remnant white dwarf core in
orbit with the remaining star. The second star then evolves and begins to
shed its envelope onto its white dwarf companion when it overflows its Roche
lobe. As a result, the white dwarf will steadily accrete matter from the
companion's outer atmosphere. The companion may be a main sequence star,
or one that is aging and expanding into a red giant. The captured gases build up
on the white dwarf's surface and begin burning via the CNO cycle.
While hydrogen fusion can occur in a stable manner on the surface of the
white dwarf for a narrow range of accretion rates, for most binary system
parameters the hydrogen burning is thermally unstable and rapidly converts
a large amount of the hydrogen into other heavier elements in a runaway
reaction, liberating an enormous amount of energy, blowing the remaining gases
away from the white dwarf's surface and producing an extremely bright outburst
of light. The rise to peak brightness can be very rapid or gradual and is
related to the speed class of the nova; 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 to classify a nova via its speed class. A fast nova
will typically take less than 25 days to decay by 2 magnitudes and a slow nova
will take over 80 days. In spite of their violence, the amount
of material ejected in novae is usually only about 1⁄10,000 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. Nonetheless,
this is enough energy to accelerate nova ejecta to velocities as high as several
thousand kilometers per second—higher for fast novae than slow ones—with a
concurrent rise in luminosity from a few times solar to 50,000–100,000 times
solar. In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope were
surprised to discover, for the first time, that a nova can also emit
gamma-rays. A white dwarf can potentially generate
multiple novae over time as additional hydrogen continues to accrete onto its
surface from its companion star. An example is RS Ophiuchi, which is known
to have flared six times. Eventually, the white dwarf could explode as a type
Ia supernova if it approaches the Chandrasekhar limit.
Occasionally a nova is bright enough and close enough 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 five degrees
north of Deneb and reached magnitude 2.0. 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 is so far the
brightest nova of this millennium reaching magnitude 3.3.
= Helium novae= A helium nova is a proposed category of
nova explosion that lacks hydrogen lines in the spectrum. This may be caused by
the explosion of a helium shell on a white dwarf. It was proposed by Kato,
Saio and Hachisu in 1989. The first candidate helium nova to be observed was
V445 Puppis in 2000. Since then, four other novae explosions have been
proposed as helium novae. Occurrence rate and astrophysical
significance Astronomers estimate that the Milky Way
experiences roughly 30 to 60 novae per year, with a likely rate of about 40.
The number of novae discovered in the Milky Way each year is much lower, about
10. Roughly 25 novae brighter than about magnitude 20 are discovered in the
Andromeda Galaxy each year and smaller numbers are seen in other nearby
galaxies. Spectroscopic observation of nova ejecta
nebulae has shown that they are enriched in elements such as helium, carbon,
nitrogen, oxygen, neon, and magnesium. The contribution of novae to the
interstellar medium is not great; novae supply only 1⁄50 as much material to the
Galaxy as do supernovae, and only 1⁄200 as much as red giant and supergiant
stars. Recurrent novae like RS Ophiuchi are
rare. Astronomers theorize however that most, if not all, novae are recurrent,
albeit on time scales ranging from 1,000 to 100,000 years. The recurrence
interval for a nova is less dependent on the white dwarf's accretion rate than on
its mass; with their powerful gravity, massive white dwarfs require less
accretion to fuel an outburst than lower-mass ones. Consequently, the
interval is shorter for high-mass white dwarfs.
= Subtypes= Novae are classified according to the
light curve development speed, so in NA: Fast novae, with a rapid brightness
increase, followed by a brightness decline of 3 magnitudes — to about 1⁄16
brightness — within 100 days. NB: Slow novae, with a 3 magnitudes
decline in 150 days or more. NC: Very slow novae, staying at maximum
light for a decade or more, fading very slowly. It is possible that NC type
novae are objects differing physically very much from normal novae, for example
planetary nebulae in formation, exhibiting Wolf-Rayet star like
features. NR/RN: Recurrent novae, novae with two
or more outbursts separated by 10–80 years have been observed.
Etymology During the 16th century, astronomer
Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia. He
described it in his book De stella nova, giving rise to the name nova. In this
work he argued that a nearby object should be seen to move relative to the
fixed stars, and that the nova had to be very far away. Though this was a
supernova and not a classical nova, the terms were considered interchangeable
until the 1930s. Novae as distance indicators
Novae have some promise for use as standard candle measurements of
distances. For instance, the distribution of their absolute magnitude
is bimodal, with a main peak at magnitude −8.8, and a lesser one at
−7.5. Novae also have roughly the same absolute magnitude 15 days after their
peak. Comparisons of nova-based distance estimates to various nearby galaxies and
galaxy clusters with those done with Cepheid variable stars have shown them
to be of comparable accuracy. = Bright novae since 1890=
Over 53 novae have been registered since 1890.
= Recurrent novae= There are ten known galactic recurrent
novae. The recurrent nova typically brightens by about 8.6 magnitude,
whereas a classic nova brightens by more than 12 magnitude. Some of the better
known and more easily observed recurrent novae are listed below.
Extragalactic novae Novae in M31 are relatively common.
There are roughly a couple dozen novae discovered in M31 each year. The Central
Bureau for Astronomical Telegrams tracks novae in M31, M33, and M81.
See also References
= Further reading= Payne-Gaposchkin, C.. The Galactic
Novae. North Holland Publishing Co. Hernanz, M.; Josè, J.. Classical Nova
Explosions. American Institute of Physics.
Bode, M.F.; Evans, E.. Classical Novae. Cambridge University Press.
External links Schaefer. "Comprehensive Photometric
Histories of All Known Galactic Recurrent Novae". arXiv:0912.4426
[astro-ph.SR]. Shafter et al.. "A Spectroscopic and
Photometric Survey of Novae in M31". arXiv:1104.0222 [astro-ph.SR].
General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow
AAVSO Variable Star of the Month. Novae: May 2001
Extragalactic Novae