A nova is a sudden, short-lived brightening of an otherwise inconspicuous star caused by a thermonuclear explosion on the surface of a white dwarf, a compact star not much larger than Earth. Novae occur because a stream of gas flowing from the star continually piles up into a layer on the white dwarf's surface. This layer eventually reaches a flash point and detonates in a runaway thermonuclear explosion.
Each nova releases up to 100,000 times the annual energy output of our sun. Prior to Fermi, no one suspected these outbursts were capable of producing high-energy gamma rays. Such emission, with energies millions of times greater than visible light, usually is associated with far more powerful cosmic blasts.
Fermi's Large Area Telescope (LAT) scored its first nova detection in March 2010 with an outburst of V407 Cygni. In this rare type of system, a white dwarf interacts with a red giant star more than a hundred times the size of our sun. Other members of this unusual stellar class have been observed to "go nova" every few decades.
In 2012 and 2013, the LAT found three much more typical, or "classical," novae: V339 Delphini in 2013 and V1324 Scorpii and V959 Monocerotis in 2012. The outbursts occurred in comparatively common systems where a white dwarf and a sun-like star orbit each other every few hours.
Astronomers estimate that between 20 and 50 novae occur each year in our galaxy. Most go undetected, their visible light obscured by intervening dust and their gamma rays dimmed by distance. All of the gamma-ray novae found so far lie between 9,000 and 15,000 light-years away, which is relatively nearby compared to our galaxy's size.
One explanation for the gamma-ray emission is that the blast creates multiple shock waves, which expand into space at slightly different speeds. Faster shocks could interact with slower ones, accelerating particles to near the speed of light. These particles ultimately could produce gamma rays.