Believe it or not, this is the Crab Nebula, one of the most famous cosmic objects. Except that this image—9.8 light years across—doesn't look like the Crab Nebula at all, but the Electric Blue Puff Puff Jellyfish.


You don't recognize it because that's the X-ray image taken by NASA's Chandra X-ray Observatory. And it's precisely its X-ray signature that has astronomers puzzled.

For decades, the Crab Nebula—the remains of a nova that reached our planet in 1054—has been considered the steadiest known high-energy source in the universe. Spinning at 30 times per second, the super dense neutron star nebula core was used to calibrate instruments.

Not anymore. A team of astrophysicists have discovered that the nebula's X-ray energy has been steadily declining "at four different "hard" X-ray energies, from 12,000 to 500,000 electron volts (eV)." Combining measurements from several X-ray observatories, they have been able to establish a seven percent energy decrease in just two years. This means that astronomers need to find a new way to calibrate their gear. [NASA]