Flare star

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 (using the abbreviation UV) in variable star catalogs such as the General Catalogue of Variable Stars. Most flare stars are dim red dwarfs, although recent research indicates that 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 it is understood that these flares are induced by a companion star in a binary system which causes the magnetic field to become tangled. 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

The general idea is that flares are generated through the reconnection of the magnetic field lines in the corona. There are several phases for the flare: preflare phase, impulsive phase, flash phase and decay phase. During the preflare phase, which usually lasts for a few minutes, the coronal plasmas slowly heats up to temperatures of tens of millions Kelvin. During the impulsive phase, which lasts for three to ten minutes, a large number of electrons and sometimes also ions are accelerated to extremely high energies ranging from keV to MeV. The later flash phase is defined by the rapid increase in Hα emissions. During the decay phase which lasts for one to several hours, the corona returns back to its original state. This is the model for how an isolated star generates flares but this is not the only way. Interactions between a star and the companion or sometimes the environment can also produce flares. In binary systems such as RS Canum Venaticorum variable stars (RS CVn), flares can be produced through the interactions between the magnetic fields of the two bodies in the systems. For stars that have an accretion disk, which most of the time are protostars or pre-main sequence stars, the interactions of magnetic fields between the stars and the disk can also cause flares.

Did You Know?

• Flare stars are intrinsically faint, but have been found to distances of 1,000 light years from Earth.
• 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.
• Proxima Centauri is a flare star that undergoes occasional increases in brightness because of magnetic activity.
• Barnard's Star was long assumed to be quiescent in terms of stellar activity, but in 1998 astronomers observed an intense stellar flare, showing that it is a flare star.
• EV Lacertae produced a record-setting flare in 2008 that was thousands of times more powerful than the largest observed solar flare.