Flare star
Variable stars with unpredictable, dramatic brightness increases.
A flare star is a type of variable star that unpredictably and dramatically brightens for a few minutes. These outbursts are thought to be similar to solar flares, driven by magnetic energy stored in the star's atmosphere. The increase in brightness spans the entire electromagnetic spectrum, from X-rays to radio waves. Most flare stars are faint red dwarfs, though newer findings suggest that even less massive brown dwarfs may also produce flares.
The first reports of flare activity on late-type stars came from A. van Maanen in 1945, for WX Ursae Majoris and YZ Canis Minoris. The most famous flare star, however, is UV Ceti, first seen flaring in 1948. Today, similar stars are classified as UV Ceti type variables in catalogs like the General Catalogue of Variable Stars. More massive RS Canum Venaticorum variables (RS CVn) also flare, but these are understood to be triggered by a companion star in a binary system, which tangles the magnetic field. Before the Kepler observatory provided a wealth of superflare data, nine Sun-like stars had also been observed to flare. It has been suggested that these flares, like those in RS CVn systems, might be induced by an unseen Jupiter-like planet in a close orbit.
The solar flare model serves as the framework for understanding stellar flares. The general idea is that flares result from the reconnection of magnetic field lines in the corona. The process has several phases: preflare, impulsive, flash, and decay. In the preflare phase, lasting a few minutes, coronal plasma slowly heats to tens of millions of Kelvin, visible mostly in soft X-rays and EUV. During the impulsive phase, lasting three to ten minutes, electrons and sometimes ions are accelerated to high energies (keV to MeV), producing gyrosynchrotron radiation in radio and bremsstrahlung in hard X-rays; this is when most energy is released. The flash phase is marked by a rapid increase in Hα emissions, as particles stream along magnetic lines, heating the chromosphere and causing it to expand into the corona. Emission here is primarily thermal. In the decay phase, lasting one to several hours, the corona cools and returns to its original state.
This model applies to isolated stars, but flares can also arise from interactions with a companion or environment. In binary systems like RS CVn stars, magnetic field interactions between the two bodies produce flares. Fo
- first reported
- 1945 by A. van Maanen for WX Ursae Majoris and YZ Canis Minoris
- best-known example
- UV Ceti, first observed to flare in 1948
- classification
- UV Ceti type variable stars (abbreviation UV)
- typical host
- dim red dwarfs
- also known to flare
- RS Canum Venaticorum variables (RS CVn) and some Sun-like stars
Lore & Background
Flare activity among late-type stars was first reported by A. van Maanen in 1945, for WX Ursae Majoris and YZ Canis Minoris. However, the best-known flare star is UV Ceti, first observed to flare in 1948. Today similar flare stars are classified as UV Ceti type variable stars in variable star catalogs such as the General Catalogue of Variable Stars. Most flare stars are dim red dwarfs, though less massive brown dwarfs might also be capable of flaring. The more massive RS Canum Venaticorum variables (RS CVn) are also known to flare, but these flares are induced by a companion star in a binary system which causes the magnetic field to become tangled. Nine stars similar to the Sun had also been seen to undergo flare events prior to the flood of superflare data from the Kepler observatory. 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.
Reader's Guide
Flare stars are significant because they demonstrate that stellar magnetic activity can produce dramatic, unpredictable energy releases far exceeding solar flares. The solar flare model has been used as the framework for understanding other stellar flares, with phases including preflare, impulsive, flash, and decay. The most powerful stellar flare detected as of December 2005 may have come from the active binary II Peg. Nearby examples include Proxima Centauri, Wolf 359, Barnard's Star, EV Lacertae, and TVLM 513-46546. On April 23, 2014, NASA's Swift satellite detected the strongest, hottest, and longest-lasting sequence of stellar flares ever seen from a nearby red dwarf, DG Canum Venaticorum, with an initial blast up to 10,000 times more powerful than the largest solar flare ever recorded. Flare stars are intrinsically faint but have been found to distances of 1,000 light years from Earth. Their study helps understand magnetic reconnection processes, stellar evolution, and the potential habitability of exoplanets around active stars.
Did You Know?
- The best-known flare star is UV Ceti, first observed to flare in 1948.
- On April 23, 2014, NASA's Swift satellite detected a flare sequence from DG Canum Venaticorum up to 10,000 times more powerful than the largest solar flare ever recorded.
- Barnard's Star, long assumed quiescent, was observed to flare in 1998.
- TVLM 513-46546, a very low mass M9 flare star, flares every 7054 seconds with precision of one one-hundredth of a second.
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