This spectacular image from the orbiting Solar Dynamics Observatory satellite shows a solar flare erupting from the surface of our own star, the Sun. Gaia can spot flares and outbursts like this around other stars. Image credit: NASA/GSFC/SDO.
Our own Sun is variable – using telescopes above the Earth’s atmosphere we have seen outbursts and eruptions called solar flares. The strongest solar flares can damage satellites in orbit, affect telecommunications networks, and even damage power grids. But the strongest solar flares pale in comparison to the extreme energetics of the flares seen in other stars.
The most violent flares seem to occur on small, dim red dwarf stars. These stars, known as "M-dwarfs" are cooler than our Sun. The source of energy for these flares lies in the magnetic field of the star. Magnetic fields trace out arcs on the surface of the star, similar to the pattern you can see in iron filings when you hold the pole of a magnet nearby, or the magnetic field lines around the Earth. If the magnetic fields on the surface of a star become tangled and twisted, they can build up a store of energy that is released suddenly in a flare.
Energy can be stored in the magnetic field of a star, only to be suddenly released as a bright flare when the magnetic field lines adjust. This process is called "magnetic reconnection". Image credit: Grace Treanor.
Gaia sees such flares regularly while observing the Milky Way. It is possible to distinguish them from other classes of transients, as there will be a star at the position of the alert, the star will be very red in colour, and the Alert will brighten and fade very rapidly (see http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia16aan/ for an example).