Everything about Nova totally explained
A
nova (pl. ) 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
Type Ia supernovae, or another form of stellar explosion first announced by
Caltech in
May 2007,
Luminous Red Novae.
Development
If a white dwarf has a close companion star that overflows its
Roche lobe, the white dwarf will steadily accrete gas from the star's outer atmosphere. The companion may be a
main sequence star, or one that's aging and expanding into a
red giant. The captured gases consist primarily of
hydrogen and
helium, the two principal constituents of
ordinary matter in the
universe. The gases are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists of
degenerate matter, and so doesn't inflate at increased heat, while the accreted hydrogen is compressed upon the surface. The dependence of the hydrogen fusion rate on temperature and pressure means that it's only when it's compressed and heated at the surface of the white dwarf to a temperature of some 20 million
K that a
nuclear fusion reaction occurs; at these temperatures, hydrogen burns via the
CNO cycle. 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. (Hydrogen fusion can occur in a stable manner on the surface, but only for a narrow range of accretion rates.) The enormous amount of energy liberated by this process blows the remaining gases away from the white dwarf's surface and produces an extremely bright outburst of light. The rise to peak brightness can be very rapid or gradual which 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 1/10,000th of a
solar mass, quite small relative to the mass of the white dwarf. Furthermore, only five percent of the accreted mass is fused to power the outburst.
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 (in
1898,
1933,
1958,
1967,
1985, and again in
2006). Eventually, the white dwarf could explode as a type Ia
supernova if it exceeds 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
August 29,
1975 in the constellation
Cygnus about five degrees north of
Deneb and reached
magnitude 2.0 (nearly as bright as Deneb). The most recent was
V1280 Scorpii which reached magnitude 3.7 on
February 17,
2007.
Occurrence rate, and astrophysical significance
Astronomers estimate that the
Milky Way experiences roughly 20 to 60 novae per year, with a likely rate of about 40. By comparison, the number of novae discovered each year in the nearby
Andromeda Galaxy is much lower; roughly ½ to ⅓ that of the Milky Way.
Spectroscopic observation of nova ejecta
nebulae has shown that they're enriched in elements such as helium,
carbon,
nitrogen,
oxygen,
neon, and
magnesium. 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.
Further Information
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