The Crab Nebula - An Astrophysics Laboratory in the Sky

On a July morning nearly 1000 years ago, those looking at the waning crescent moon saw an astounding sight - a new star, far brighter than any other in the heavens, mostly likely brighter than the planet Venus, shone within a short distance of the moon.

In July of the year 1054 AD, a brilliant new star appeared in the constellation of Taurus, visible in broad daylight for nearly a month. Our knowledge of this truly astounding event is based upon records from Chinese and Japanese annals, while in Europe, then suffering through the depths of the Dark Ages, scant evidence remains of an appearance that must have struck everyone on Earth with awe and fear. At least one prehistoric civilization of the period - the native Americans occupying Arizona - left behind paintings showing a crescent moon beside a bright star.

After remaining visible in the night sky for about two years, the "guest star" as the Chinese described it, gradually faded from view. Over the next several hundred years, the event remained only as a forgotten memory recorded on parchment. 

In the year 1731, the British astronomer John Bevis was the first to point a telescope toward the region in which the star of 1054 had appeared. He found a faint cloud which he realized was not a comet. During this time period, the discovery of comets was a very active pursuit of many astronomers; however many observers became easily confused in this quest by other faint, hazy objects with the appearance of comets, but which did not move through the night sky as a comet must. In 1758, the Frenchman Charles Messier decided to start a catalog of such objects after observing the same faint cloud which Bevis had earlier discovered. This cloud was given the label M1 (Messier 1) in this catalog.

The first detailed observations and sketches of the nebula were made in the later 18th century. The overall shape of the cloud was seen in the shape of a crab's shell, from which this object now derives its popular name. In the early 20th century, comparisons of photographs taken several decades apart led to the realization that the observed cloud is expanding at a dramatic rate. Assuming a roughly constant rate of expansion, the cloud was found to have started expanding about 900 years before these comparisons were made, coinciding very well with the Chinese records of the new star seen in 1054.

Recent measurements of the rate of expansion have found that the gas in the cloud moves at an average speed of 3,000,000 miles per hour. The cloud lies 6500 light years from Earth, and now covers an area over 11 light years in diameter.

The Crab Nebula is among the most studied objects in astrophysics. The event observed in 1054 was a supernova - the amazingly violent death of a star about 10 times more massive than the Sun. The resulting cloud of expanding gas is a supernova remnant, and at the relatively close distance of 6500 light years, the Crab Nebula allows detailed observation of the evolution of a supernova remnant that is not possible with any other object in the Universe.

But as amazing as this object was known to be by the mid-20th century, the Crab Nebula contained an additional surprise. It had been already recorded that the center of the nebula contained two stars. One of these was the remains of the star that supernovaed in 1054; the other just a "normal" star. In 1967, an Air Force operator in Alaska noticed a fluctuating radio source whose position coincided with the Crab Nebula. Independent observations at the massive Green Bank radio telescope in Aricebo, Pueto Rico confirmed an oscillating source of
radio waves in the Crab Nebula with a frequency of 30Hz (30 cycles a second).

Subsequent observations show that the remains of the star that produced the supernova emits intense radiation at all wavelengths - from gamma rays through visible light, to radio waves - all pulsating at the rate of 30 cycles per second. Such an object is a pulsar, one of the most bizarre objects in the Universe known to man.

A pulsar forms when a massive star exhausts its sources of energy and collapses violently under its own gravitational pull. Following the resulting supernova, a star of sufficient mass will continue to collapse, forcing the electrons and protons that form normal matter to fuse into neutrons. The star, originally 100's of thousands or miles in diameter, is crushed down into a ball of neutrons only 10 miles or so in diameter.

As the star collapses, it spins faster and faster - just as a figure skater accomplishes very fast spins by pulling her arms into her body after starting a moderately fast spin, the star's mass rushing to its center casues it to speed up tis spin from a rate of days per turn to less than a second per turn. A spinning ball of neutrons produces an incredibly powerful magnetic field. And streaming out from the star along the north and south poles of this magnet, light of all
possible wavelengths, from gamma radiation to visible light, to radio waves, will be focussed into two opposing beacons.

The appearance of such an object - with two beacons of light swinging around rapidly - is like that of a vast lighthouse search light. If Earth lies in the path of the swinging beam, we see pulses of the light each time the search light crosses us. In the case of the Crab Nebula, this happens 30 times a second.

The pulsar at the center of the Crab Nebula is the second brightest source of gamma and X-ray radiation (after the Sun) in Earth's skies. So bright and consistent is this source, that astronomers actually use this object to probe other objects in our solar system and beyond when they cross in front of the nebula. The absorption patterns seen in gamma and X-ray wavelengths as the radiation from the Crab Nebula passes through the atmospheres of the Sun and the planets allow us to determine in detail the chemical make up of those atmospheres.