The Crab Nebula is the first entry in Charles Messier’s famous catalog of objects that look like comets but are not comets. Messier observed the nebula several times while searching for the return of Halley’s comet. He puzzled over its appearance (quite like a comet) and lack of movement among the stars (unlike a comet). In the early 20’th century repeated observations showed changes in the nebula over time: If Messier had had better instruments he might have detected these changes. The nebula I saw last night does not look the same as what Messier observed in 1758.
Messier was certainly not the first person to see this object, not by a long shot. Half the living population of this planet had probably seen the object at one point. It was a supernova, visible to the naked eye from July 1054 until April 1056, substantially brighter than Venus. By far the best documentation for the supernova comes from Chinese records of that time.
Yang Weide declared: "I humbly observe that a guest star has appeared; above the star there is a feeble yellow glimmer. If one examines the divination regarding the Emperor, the interpretation is the following: The fact that the star has not overrun Bi {Chinese Net constellation} and that its brightness must represent a person of great value. I demand that the Office of Historiography is informed of this." All officials congratulated the Emperor… (Wikipedia entry for SN-1054).
The Crab Nebula is the smoking gun, a remnant from that massive star explosion. In astronomical terms 1054 was quite recent and the object is relatively nearby (6500 light years away). Conveniently we know exactly (within a day) when the star exploded and we can measure how rapidly the shell of gas is expanding. At the centre of the nebula is an optical pulsar, flashing at approximately 30 Hz. It is so well known as an X-Ray source that a "crab" has become a unit of X-Ray flux! This nebula is an experiment in astrophysics.
The nebula is roughly 5 arc minutes by 7 arc minutes. Recall that the moon’s diameter is ~30 arc minutes. It is bright enough to be seen with binoculars under very good conditions, but relatively small to get a lot of detail in an image. Nevertheless, a lot of amateur astronomers have produced fantastic images of this object because there are so few like it in the night sky. My own previous attempt (October 2020) produced quite a good image. It is very different - probably from using a filter to bring out more colour. That image is below.
Messier 1, M1, Crab Nebula. Observed Dec 26, 2021 at Lac Teeples. 76 minutes of data (1-minute subframes), the telescope was the RASA11, Camera was the ZWO ASI6200, no filter. Red colour was enhanced slightly to match the colour observed on the screen during data collection (no other colours were visible).
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