THE CELESTIAL CRUSTACEAN
One of the official duties of the Chinese emperor around the end of the first millennium was to explain to his people any unusual occurrences in the heavens. To this end, he employed astronomers to observe the sky. In 1054 A.D., these astronomers reported to Emperor Renzong (1010 – 1063 A.D.) the appearance of a “guest star” in the constellation we call Taurus the Bull. “Guest star” was their term for any bright object that suddenly appeared in the sky and then disappeared after a relatively short time. This “guest star” was a supernova, which is a star that has used up its nuclear fuel, collapses inward, and then explodes. The object first appeared in April and grew brighter for many weeks, with a magnitude as great as –7.0 in July (please see my previous article for an explanation of “magnitude”). This was brighter than Venus and anything else in the sky except the full moon. Other historical records record observations of this supernova in Korea, Japan and the Middle East.
In 1054 A.D., Europe had recently emerged from the period known as the dark ages into what is now called the Middle Ages. What would become known as the Americas was sparsely populated. No reports exist of a bright object in the sky seen in those geographical regions. Thus, several hundred years passed before astronomical observation in the West caught up with the “guest star” of 1054 A.D. The remains of the supernova were discovered by John Bevis in 1731 and independently by Charles Messier in 1758.
Between 1840 and 1844, William Parsons, Third Earl of Rosse, observed this object through the 30-inch reflecting telescope at Birr Castle in Ireland, located about halfway between Dublin and Galway. This was a rather large telescope for its time and provided the opportunity for good-quality images. The sketches that Parsons drew of the object showed an oval central body with filaments extending outward, somewhat like the anatomy of a crab, and he dubbed it the “Crab Nebula”. The name stuck, even though later observations by Parsons and others, using superior telescopes, failed to produce the same crab-like image.
There are different mechanisms for the explosion of a supernova. These are beyond the scope of this article and the result depends on the size of the original star. In general, as stars age, they “burn up” (using nuclear fusion rather than combustion) their hydrogen fuel, converting it to helium. The star then shrinks and heats to the point that the helium fuses into carbon and then to higher elements up to iron. When sufficient iron accumulates in the core of the star, no further fusion is possible and gravity causes the star to collapse and explode, usually producing either a neutron star or a black hole. In the case of the Crab Nebula, the central object is a neutron star rotating about 30 times per second.
The Crab Nebula is a visually striking object showing a predominantly blue central area. About 70 years ago, it was proposed that this blue color was produced by electrons traveling in a curved path through an intense magnetic field. This was confirmed by the observation of the central neutron star in the 1960s. The star’s magnetic field is so strong that it focuses the radiation from the star into narrow beams, pulsing about 30 times per second. This object is thus known as a pulsar.
One of the interesting features of the Crab Nebula is that changes in the filamentous structure take place over a short period of days and weeks, rather than years for other objects. For this reason, the Crab Nebula remains the subject of significant research interest. It emits radiation ranging from visible light through gamma rays and in 2019 it was discovered that the Crab Nebula is the first object found to emit gamma rays with an energy in excess of 100 TeV (tera electron volts, or trillions of electron volts).