A tiny star, barely above a brown dwarf, has been spotted emitting a solar flare ten times more powerful than anything seen on our own Sun.
The L dwarf, which just barely qualifies as a star because it is a low-mass, ultracool orb, is the coolest and smallest to be seen giving off a rare white-light superflare.
“The activity of low mass stars decreases as you go to lower and lower masses and we expect the chromosphere (a region of the star which support flares) to get cooler or weaker. The fact that we’ve observed this incredibly low mass star, where the chromosphere should be almost at its weakest, but we have a white-light flare occurring shows that strong magnetic activity can still persist down to this level,” said James Jackman, a PhD student in the University of Warwick’s Department of Physics, in a statement.
“It’s right on the boundary between being a star and a brown dwarf, a very low mass, substellar object. Any lower in mass and it would definitely be a brown dwarf. By pushing this boundary, we can see whether these types of flares are limited to stars and if so, when does this activity stop? This result takes us a long way to answering these questions.”
The flare gave off energy equivalent to 80 billion megatons of TNT, making the event ten times larger than the Carrington event in 1859, the highest energy ever observed from our Sun.
Usually we wouldn’t even see this L dwarf star, as it is 250 light years away and only a tenth the radius of our Sun, far too small to shine brighter than the Universe’s background radiation. But the flare was so bright that the Next Generation Transit Survey (NGTS) facility at the European Southern Observatory’s Paranal Observatory picked it up during an optical survey of the surrounding stars.
“Our 12 NGTS telescopes are normally used to search for planets around bright stars, so it is exciting to find that we can also use them to find giant explosions on tiny, faint stars. It is particularly pleasing that detecting these flares may help us to understand the origin of life on planets,” said Professor Peter Wheatley, also at University of Warwick.
You need UV radiation to get chemical reactions going on planets and this is the kind of light that hotter stars emit. But L dwarfs emit most of their light towards the infrared end of the spectrum and wouldn’t normally be able to set off any chain reactions. A flare like this, however, would have the potential to do just that.
“It is amazing that such a puny star can produce such a powerful explosion. This discovery is going to force us to think again about how small stars can store energy in magnetic fields. We are now searching giant flares from other tiny stars to push the limits on our understanding of stellar activity,” said Wheatley.