A
flare star is a
variable starA star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to
solar flareA solar flare is a sudden brightening observed over the Sun surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy . The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day...
s in that they are due to
magnetic reconnectionMagnetic reconnection is a physical process in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration...
in the atmospheres of the
starA star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s. The brightness increase is across the
spectrumThe electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
, from X rays to
radio waveRadio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
s. The first known flare stars (V1396 Cygni and AT Microscopii) were discovered in 1924. However, the best-known flare star is
UV Ceti, discovered in 1948. Today similar flare stars are classified as
UV Ceti type
variable starA star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
s (using the abbreviation
UV) in variable star catalogs such as the
General Catalogue of Variable StarsThe General Catalogue of Variable Stars is a list of variable stars. Its first edition, containing 10,820 stars, was published in 1948 by the Academy of Sciences of the USSR and edited by B. V. Kukarkin and P. P. Parenago. Second and third editions were published in 1958 and 1968; the fourth...
. Flares can happen once every few days or in the case of
Barnard's StarBarnard's Star, also known occasionally as Barnard's "Runaway" Star, is a very low-mass red dwarf star approximately six light-years away from Earth in the constellation of Ophiuchus . In 1916, the American astronomer E.E...
much less frequently.
Most flare stars are dim
red dwarfAccording to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
s, although recent research indicates that less massive
brown dwarfBrown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
s might also be capable of flaring. The more massive
RS Canum Venaticorum variableRS Canum Venaticorum variables are a type of variable star. They are close binary stars having active chromospheres which can cause large stellar spots. These spots are believed to cause variations in their observed luminosity...
s (RS CVn) are also known to flare, but it is understood that these flares are induced by a companion star in a binary system which causes the
magnetic fieldA magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
to become tangled. Additionally, nine stars similar to the
SunThe Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
have also been seen to undergo flare events. It has been proposed that the mechanism for this is similar to that of the RS CVn variables in that the flares are being induced by a companion, namely an unseen Jupiter-like planet in a close orbit.
Nearby flare stars
Flare stars are intrinsically faint, but have been found to distances of 1,000 light years from Earth.
Proxima Centauri
The Sun's nearest stellar neighbor
Proxima CentauriProxima Centauri is a red dwarf star about 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun, although it is too faint to be seen with the naked eye...
is a flare star that undergoes random increases in brightness because of magnetic activity. The star's
magnetic fieldA magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
is created by
convectionConvection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
throughout the stellar body, and the resulting flare activity generates a total
X-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
emission similar to that produced by the Sun.
Wolf 359
The flare star
Wolf 359 is another near neighbor (2.39 ± 0.01 parsecs).
Wolf 359, also known as Gliese 406 and CN Leo, is a
red dwarfAccording to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
of spectral class M6.5 that emits X-rays. It is a
UV CetiLuyten 726-8 is a binary star system that is one of Earth's nearest neighbors, at about 8.7 light years from Earth in the constellation Cetus...
flare star, and has a relatively high flare rate.
The mean magnetic field has a strength of about , but this varies significantly on time scales as short as six hours. By comparison, the magnetic field of the
SunThe Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
averages , although it can rise as high as in active
sunspotSunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....
regions.
Barnard's Star
Barnard's StarBarnard's Star, also known occasionally as Barnard's "Runaway" Star, is a very low-mass red dwarf star approximately six light-years away from Earth in the constellation of Ophiuchus . In 1916, the American astronomer E.E...
, the second nearest star system, is also suspected of being a flare star.
TVLM513-46546
A very low mass flare star is
TVLM513-46546TVLM513-46546 is an ultracool red dwarf in the constellation Boötes, exhibiting a flare star activity and all the same varying semi-regularly due to dissimilarly bright hemispheres and a rotation period of about two days...
, slightly heavier than the lower limit for red dwarfs.
External links