Sometimes big things come in small packages. Last April, DG CVn, a red dwarf star only one-third the size of the sun, cut loose with a flare 10,000 times more powerful than any solar flare ever recorded. The sun’s grandest was an X 45 on November 4, 2003 which happily was directed off its western limb away from Earth. Had it happened closer to the center of the solar disk, damage to satellite electronics and power grids on the ground might have been substantial.
The superflare erupted from one or the other of two closely-orbiting red dwarfs in the constellation of Canes Venatici (abbreviated CVn) located beneath the handle of the Big Dipper. While only 60 light years from Earth, the two stars orbit each other only three times Earth’s distance from the sun which is too close for the Swift satellite to know which one did the deed.
At its peak the flare shot up to 360 million degrees F (200 million C) or 12 times hotter than the center of the sun. Despite its magnitude, the star is too far away to pose any harm to Earth. As to how a smaller, cooler dwarf could unleash such an energetic blast, we have two important leads.
Astronomers estimate DG CVn was born about 30 million years ago, which makes it less than 0.7% the age of the solar system. Like children, youthful stars are blessed with energy and show it through rapid rotation – DG completes one spin in just under a day or 30 times faster than the sun. The sun also rotated faster in its youth and may well have produced a few of its own superflares. Now it spins once every 27 days, fast enough to amplify magnetic fields to X-class strength but no match for the younger set.
Magnetic energy gets concentrated around sunspots or starspots in the case of DG CVn. In the turbulent environment, opposite polarities (north and south poles) can snap together and reconnect, releasing gobs of stored energy as a flare.
Flares are classified according to their energy output. The weakest – A,B and C-class – have almost no effect on Earth. M-class or medium flares accompanied by blasts of solar particles can cause radio blackouts and fire up northern and southern lights. The strongest are the X-class, which can lead to long-lasting radiation storms and nights-long auroral displays.
At 5:07 p.m. EDT on April 23, the rising tide of X-rays from DG CVn’s superflare triggered Swift’s Burst Alert Telescope (BAT).
“For about three minutes after the BAT trigger, the superflare’s X-ray brightness was greater than the combined luminosity of both stars at all wavelengths under normal conditions,” noted Goddard’s Adam Kowalski, who is leading a detailed study on the event. “Flares this large from red dwarfs are exceedingly rare.”
Three hours later the system exploded with another weaker flare. More flares continued in a series for the next 11 days like aftershocks from an earthquake. Astronomers have observed the same phenomenon with the sun called “sympathetic flaring” where one explosion triggers another.
Stars delight the eye and make the Earth an abode for life, but don’t get too close. They’re scary.