The flare star WX UMa was recently observed releasing a solar flare (stellar flare) that was so powerful that the star itself became 15 times brighter in a matter of just a couple of minutes. The flare was observed by researchers at the University of Santiago de Compostela in Spain, and the Byurakan Observatory in Armenia.
“We recorded a strong flare of the star WX UMa, which became almost 15 times brighter in a matter of 160 seconds,” states the astrophysicist Vakhtang Tamazian, a professor at the University of Santiago de Compostela.
The flare star — WX UMa — is located within the Ursa Major constellation, just about 15.6 light years from the Earth. The star is part of a binary system where the companion star is nearly 100 times brighter — though not always, as this new finding has shown. The star was already known to regularly release powerful flares — hence the “flare star” label — but none nearly as powerful as this recent one.
Dr Tamazian, along with his fellow researchers, observed the exceptional brightness of the “recent” flare from the Byurakan Observatory in Armenia. “Furthermore, 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 (K) to six or seven times more than that.”
The press release provides some background:
Based on their spectral absorption lines, stars are classified using letters. Type M stars have a surface temperature of between 2,000 and 3,700 K; Type B between 10,000 and 33,000 K.
WX UMa belongs to the limited group of ‘flare stars,’ a class of variable stars which exhibit increases in brightness of up to 100 factors or more within a matter of seconds or minutes. These increases are sudden and irregular, practically random, in fact. They then return to their normal state within tens of minutes.
While the “causes” are still not clearly understood, the development process is: “For some reason a small focus of instability arises within the plasma of the star, which causes turbulence in its magnetic field,” explains Tamazian. “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.”
Next, kinetic energy in the plasma transforms into thermal energy in the upper layers of the atmosphere and the star’s corona. This significant rise in the temperature and brightness of the star enables astronomers to detect changes in the radiation spectrum.
“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 continues.
“Additionally, in cases of binary systems such as that which unites WX UMa with 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.”
Something to note — flare stars are inherently weak, they are only observable (with current technology) at relatively close astronomic distances. In general, only those flare stars which are rather close to us — up to a distance of a few tens of light years — are also visible to us.
The new findings were recently published in the journal Astrophysics.