Distant Flare Star Briefly Erupts to 15 Times its Normal Brightness

A star in the Ursa Major constellation, about 15.6 light years away from Earth, gave astronomers a surprise when it unleashed a flare so intense the star to become 15 times brighter in a matter of minutes.

"We recorded a strong flare of the star WX UMa, which became almost 15 times brighter in a matter of 160 seconds," astrophysicist Vakhtang Tamazian, professor at the University of Santiago de Compostela, said in a June 13 statement.

The star in question is part of a binary system; its companion normally shines about 100 times brighter, but when WX Uma flares, which happens several times a year, it is capable of outshining its companion. The recent super-bright flare was noteworthy for scientists because of its exceptional intensity.

Tamazian said during this period of less than three minutes "the star underwent an abrupt change from spectral type M to B; in other words, it went from a temperature of 2,800 kelvin to six or seven times more than that."

WX Uma belongs to a class of variable stars called flare stars. Flare stars are intrinsically weak, only observable at relatively short distances in astronomic terms, specifically in the vicinity of the Sun, up to a distance of a few tens of light years. The intensity of the WX Uma's flare-up is on par with event's randomness - the sudden flares are irregular and unpredictable, and the star will return to its normal state within 10 minutes. It is unknown how the flaring arises, but scientists have an idea of how it develops.

"For some reason a small focus of instability arises within the plasma of the star, which causes turbulence in its magnetic field," Tamazian said. "A magnetic reconnection then occurs, a conversion of energy from the magnetic field into kinetic energy, in order to recover the stability of the flow, much like what happens in an electric discharge."

Astronomers are alerted to the star by detecting significant rises in the temperature and brightness of the star initiated by changes in the radiation spectrum, namely by kinetic energy in the star's plasma transforming into thermal energy in the upper layers of the atmosphere and the star's corona.

"Photometric and spectroscopic monitoring of this kind of flare stars is very relevant because it provides us with information about the changing states and physical processes, which are in turn key to studying the formation and evolution of stars," Tamazian explains.

In a binary star system like WX Uma and its companion, "observation of flares acquires a special importance, because we can investigate whether there is any relation between the frequency of flares and the position of the pair of stars on their orbit, a question which remains open," Tamazian said.

The research was published in the journal Astrophysics.