THE CRAB NEBULA

Saturday 2 February 2019 — ASTROPHOTOGRAPHY 

In July 1054(AD), a bright new star appeared in the constellation of Taurus. We have records from China, Korea and Japan and from some Islamic scholars. The star was visible even in the daytime (if you knew where to look) for about 23 days before gradually fading from view. It was one of the brightest things in the night sky, but does not seem to have been recorded in Europe. Civilizations in China and the East set great store by astrology at this time and observation of the night sky was systematic and careful. The appearance of a new star was highly significant.

Much later, when telescopes began to be used, astronomers soon noticed a small, fuzzy patch in the same area of the sky. Charles Messier noted it as the first of his catalogue (the Messier or M objects) of things that could easily be confused with a comet, which is a small fuzzy patch trough a simple telescope. Messier was looking for the return of Halley’s comet predicted in 1758 but kept seeing this object and went on to find others himself and list those noted by other astronomers. This object is thus M1, or the Crab Nebula, after William Parsons published a drawing from 1840 which showed the object looking rather like a crab.

The bright star was a supernova, in this case a type II. This is a gigantic explosion from a large star nearing the end of its main life-cycle. More massive stars (unlike our Sun) can fuse elements heavier than hydrogen and helium. They burn hot and burn though their fuel more quickly. As the hydrogen is used up, increasingly heavier elements are fused. Up to this point, the collosal energies produced by nuclear fusion counteract the gravity of the star, but once nickel and iron are produced a very heavy central core made of these elements is created in the star, as fusing nickel and iron releases no net energy and so fusion processes cannot continue. Once the core grows beyond a certain size (about 1.5 times the mass of our sun) the energy from fusion suddenly cannot prevent the star from collapsing under gravity. The star implodes, in a matter of seconds. The extremely rapid implosion (about 23% of the speed of light or roughly 42,000 miles per second) heats the gas up to about 100 billion degrees and the gases rebound from the heavy core fast enough to escape from the star. It’s an explosion on a simply astonishing scale. From a safe (many trillions of miles) distance, we see a star that is bright enough to outshne all the other stars in its galaxy for a time, which then fades away.

The Crab Nebula is the debris from the supernova explosion that we saw in 1054. It is still expanding rapidly, and changes in the size and shape of the debris field can be detected over a few years using modern telescopes. It is currently about 11 light years (6,460,000,000,000,000 miles) across. At the heart of the nebula, we now know there is a pulsar: a rapidly spinning neutron star that pulses light and radio waves many times a second (30 times a second for this pulsar which means it is rotating at 1,800 rpm, about the speed of a car engine). This tiny star (about 20km diameter) is the left-behind core of the original star. Pulsars were detected by radio waves initially, but this pulsar has been imaged optically too. The aftermath of the supernova explosion means that the nebula is still emitting lots of gamma and x-rays, as well as radio waves and visible, infra-red and ultra-violet light.


This image

I made this image with telescope 5 of iTelescope. This is a 250mm f4 reflector sited in New Mexico, USA fitted with a specialist camera. The colour image was made from 4 two-minute exposures through Ha, red, green and blue filters. A Ha filter allows the deep red light from glowing hydrogen through to the camera. This was used to bring out more detail in the nebula itself. I processed it in PixInsight software integrating the separate images with a “drizzle” algorithm which helps to bring out fine detail and also makes a much larger image.