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The appearance of this new star was not unique, although only about a dozen or so such events have been recorded in human history.  The ancient Chinese called it a "Guest Star."  Modern astronomers refer to it as a supernova.  A supernova is one of more cataclysmic events in the Universe and marks the extraordinary death of a very massive star.  Such massive stars live their lives on the edge, fusing their elements at an enormous rate.  In addition to causing the star to shine brightly, the outflow of fusion energy provides pressure to hold up the outer layers of the star against gravity; without this flow of energy the gaseous star would begin to collapse into an ever more dense ball.  Compared to the low mass stars which can fuse their elements for billions of years, the very massive stars live short, bright lives.  In the end they can no longer fuse enough energy to hold themselves up and the collapse begins.  In less than a second the core of such a star can collapse from the size of the earth to a mere 20km in diameter!  As suddenly as it began, the collapse is just as suddenly halted.  The remaining falling matter rebounds, exploding back outward in a flurry of nuclear reactions, producing for a brief moment in time as much light as all of the rest of it's parent galaxy!  The expanding material explodes into the interstellar medium as a strong wind, moving outward at about 22,000 km/hour.

Nearly 700 years after the supernova of 1054, John Bevis of England discovered a faint nebula at the same location in his small telescope.  Later Charles Messier independently discovered this nebula, at first mistaking  it for a comet (the expected return of comet Halley).  It is said that the discovery of this object resulted in his decision to begin his own catalog; the nebula appears as his first entry (Messier 1).  Of course, at that time the supernova of 1054 was unknown to both of these men, largely because no records of the event had survived in Europe.

An 1844 drawing of the nebula made by Lord Rosse using the 36-inch reflector at Birr Castle indicated intricate filaments.  This drawing  eventually led to the name "Crab Nebula."

In 1921 J.C. Duncan of Mt. Wilson Observatory compared photographs of the nebula taken 11 eleven years apart.  He found that the nebula was expanding and measured the rate.  He estimated that at the present rate of expansion the nebula had begun expanding about 900 years earlier.  At about the same time Knut Lundmark  made the connection between the position of the nebula and the supernova of 1054.  It was soon apparent that the two were somehow connected.  In the decades  that followed our understanding of stars and their evolution grew by leaps and bounds, eventually leading to a basic understanding of the supernova phenomenon.

In 1948 the Crab was discovered to be a bright radio source (Taurus A).  It was later discovered to be a bright source of x-rays as well.  In 1968 a quickly pulsating radio source was found near the center of the Crab.  These pulsing radio sources, or Pulsars, had only been recently discovered and the discovery of the Crab pulsar generated a great deal of interest.  This  pulsar was found to pulsate 30 times per second.  Astonishingly, it was recognized that this pulsation was due to the rotation of the ultra-compact core of the star that had exploded in 1054.  This core was now a neutron star, a star made entirely of neutrons tightly packed together.  Like an ice skater who brings in her arms, the rotation of the core had been sped up greatly as it collapsed.  The Crab pulsar is also known as the variable star CM Tau, and appears as a 17th magnitude star near the center of the nebula.
 
 

Observing the Crab

Walter Scott Houston wrote that "The Crab Nebula usually shows in small telescopes as a featureless gray ghost."  He goes on to say that his "4-inch Clark refractor has revealed hints of the nebula's ragged edge that appears so prominently in photographs" and that "these edge serations are usually apparent in a 12-inch telescope and easy in a 17-inch."

Observations made with large telescopes in the 1800's revealed the filaments for the first time, reportedly becoming visible at magnifications in excess of 500X.   I didn't find that many modern observations of this nebula in larger instruments, perhaps because it is assumed that the Crab is essentially featureless.   Most observers, including myself,  describe the Crab as "mottled" in large instruments rather than as filamentary.  I wonder if we aren't using enough magnification?  I'm looking forward to trying the Crab again in the 18-inch this month at higher magnification, perhaps with an OIII filter as well.  I'd be greatly interested to hear the results of others!