Artist's conception of a white dwarf star
accreting hydrogen from a larger companion
A
nova (pl.
novae) is a
cataclysmic
nuclear explosion caused by the
accretion of hydrogen onto the surface of a
white dwarf star. Novae are not to be confused with
supernovae or
luminous red 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.
[Prialnik, Dina. "Novae", pp. 1846-56, in Paul Murdin, ed. Encyclopedia of Astronomy and Astrophysics. London: Institute of Physics Publishing Ltd and Nature Publishing Group, 2001. ISBN 1-56159-268-4] The number of novae discovered in the Milky Way each year is much lower, about 10.
[http://www.cfa.harvard.edu/iau/nova_list.html] 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 1/50th as much material to the Galaxy as supernovae, and only 1/200th as much as
red giant and
supergiant stars.
Recurrent novae like
RS Ophiuchi (those with periods on the order of decades) 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.
Historical significance
The astronomer
Tycho Brahe observed the
supernova SN 1572 in the constellation
Cassiopeia, and described it in his book
de stella nova (
Latin for "concerning the new star"), 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 candles. 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 (−5.5). 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
A more complete list of novae in the Milky Way since 1612 is maintained by the
Central Bureau for Astronomical Telegrams, CBAT
Recurrent novae
See also
