The Crab Nebula is a pulsar that’s only about 1,600 light years away. It is the remnant of a supernova that occurred in 1054, and was recorded by Chinese astronomers. Pulsars are rotating neutron stars that produce bursts of energy we observe as pulses. Most pulsars are observed at radio wavelengths, but the Crab Nebula pulsar can also be observed in the visible. Because of its relative proximity, it’s also one of the few neutron stars we can observe directly. You can see it as the bright central dot in the x-ray image above.
When the supernova occurred in 1054, it had a maximum magnitude of about -6, which is much brighter than the brightest stars in the sky, and even brighter than Venus at its maximum. It should have been easily visible across the globe, and yet there is limited confirmed recording of the event in historical records. There are some hints of recordings, but the Chinese observation is the only one with sufficient accuracy to confirm. It’s an interesting example of how transient events in the sky didn’t always gain attention.
One of the mysteries about the Crab Nebula is that the calculated mass is about 3 solar masses. Combined with the mass of the neutron star itself (about 2 solar masses), the estimated mass of the original star would be about 5 solar masses. However to create a supernova of this size and chemical composition, the original star should have been about 9 – 11 solar masses. We’re still not sure where the missing mass went, though a good possibility is that the outer layers of the star were pushed away by the star before the supernova occurred. We see this happen with Wolf-Rayet stars.
The Crab Nebula itself can be seen with the naked eye, and some of its structure can be observed with binoculars or a small telescope. So if you get the chance, it’s worth checking out.