Astronomers are puzzled by a series of "superflares" from the Crab Nebula that NASA's Fermi Gamma-ray Telescope has detected.

NASA said today that a flare detected April 12 was five times more powerful than any eruption previously seen from the nebula, the remnants of a supernova 6,500 light-years away in the constellation Taurus. At the heart of the dusty, expanding, high-energy gas cloud is a super-dense neutron star, the remains of the original star that exploded.

Last month's outburst lasted six days.

"These superflares are the most intense outbursts we've seen to date, and they are all extremely puzzling events," said Alice Harding at NASA's Goddard Space Flight Center. "We think they are caused by sudden rearrangements of the magnetic field not far from the neutron star, but exactly where that's happening remains a mystery."

This NASA video explains the nebula and the flares, and this one shows what the Chandra X-ray Observatory saw.

Space.com has more about the strange goings-on.

For decades, the Crab's X-ray emissions had been so stable that astronomers used the nebula to calibrate space-borne instruments. But early this year they were surprised to discover steady declines of four X-ray energies.