The History and Study of the Crab Nebula

One of the most intriguing aspects of astronomy is its reliance not only on observations being made everyday, but also by observations made centuries ago. One of the best examples of this is the Crab Nebula, which was first observed almost a thousand years ago and is still captivating astronomers today. In 1054, Chinese and Arab astronomers observed an extremely bright star that seemed to have just appeared. It could be seen in the mid-day sky for three weeks and was visible to the naked eye for over a year. Interestingly, it is possible that the phenomena was also observed in Europe early in the year but was dismissed as a bad omen of the Great Schism of the church that was occurring at that time (Trimble 3). The brilliant light, astronomers would find centuries later, was the death of a massive star, a supernova that would ultimately give birth to the Crab Nebula.

In the 1700s, astronomers John Beevis and Charles Messier independently observed a fuzzy object in the constellation Taurus that they couldn't really explain. At the same time, Messier was compiling a list of the many stellar objects he discovered while hunting for comets in the night sky. The first on his list was this fuzzy object near the star Zeta Tauri, which he labeled M1 (Shubinski 71). In 1842, William Parsons observed the object and sketched a drawing of it that he concluded looked the claw of a crab. Thus, the name "Crab Nebula" was born (Trimble 3).

Out of this unique history came centuries of study that has left the astronomical community still inquisitive about the Crab Nebula. Around 1900, spectroscopy expert Vesto Slipher studied the nebula's spectral emission and found that it was expanding extremely quickly. Further research since Slipher's discovery has revealed that the Crab Nebula expands at about 3.3 million miles per hour (Shubinski 72). This rate of expansion allowed the nebula's existence to be traced back to about 900 years earlier, and its correspondence to the sighting in 1054 seemed obvious. Although there have been some objections to this deduction, it is still the most commonly held belief to this day.

The observation of the Chinese astronomers, we now know, was a supernova explosion, the violent death of a large star. The Crab, in a similar fashion to many other nebulae, was formed out of this supernova and the remnants it left behind. The expansive cloud of dust and debris left after the explosion is centered around a single, tiny mass of extremely dense material called a neutron star.

Using a series of photographs taken over a number of years, coupled with the rate of expansion deduced from Slipher's spectroscopy, astronomers have concluded that the Crab Nebula is roughly 6,000 light years from Earth. This measurement, however, is still a topic of study and has not been completely accepted (Shubinski 72). Astronomers have also concluded that the nebula's diameter is roughly 10 light years across. The Crab Nebula is a winter object, and it roughly lies centered on the celestial meridian at midnight on December 10 (Trimble 6).

So, what is at the heart of the Crab Nebula? From where does its rapid expansion originating? In 1967, Jocelyn Bell, a student at Cambridge University, detected pulsations around the center of the Crab Nebula that were beating inexplicably regularly at around 33 times a second. Bell and her colleagues joking labeled these strange pulsations LGMs, Little Green Men (Shubinski 70). This discovery, along with a few other objects being studied at the time, introduced the idea of pulsars to the astronomical community. This "Crab Pulsar" is a neutron star, the cosmic leftovers of the supernova explosion seen in 1054, and resides near the center of the nebula. The pulsar is approximately 25 kilometers in diameter. Despite its small size, the pulsar is as massive as our Sun and puts out as much energy as 100,000 Suns ("The Crab's Heart" 22). The "pulsing" of the Crab Pulsar is actually the result of its rapid rotation. Massive jets of matter spew out of the poles of the neutron star, and the pulsing we see from Earth is actually these jets swinging into our field of view.

The intense wind generated from the pulsar's spinning pushes matter outward toward the rest of the nebula at about half the speed of light. When this wind and matter collide with the surrounding nebula, they form long wisps that grow and then fade, similar to waves crashing on a beach (Talcott 24). The high amount of intense activity taking place in this central region of the nebula creates an environment that shifts and changes at almost unparalleled speed. Jeff Hester, an Arizona State University astronomer who is leading much of the research on the nebula comments on this rapid movement. "In astronomy, normally if you see something change over your career you get excited about it," Hester says. "But the Crab Nebula is so dynamic that if you wait for longer than about a week to get your next look, you'll get the wrong impression because things have changed so terribly much" ("The Crab's Heart" 22).

Although over 1,500 pulsars have now been discovered, the one at the center of the Crab Nebula is still one of the most unique. It is the only to be associated with a supernova seen from Earth. It is also much brighter than the average pulsar and emits energy in the ranges of radio waves, x-rays, and gamma rays (Irion 533).

In addition to being the object of study, the Crab Nebula has also aided astronomers in their study of other objects in the sky. For example, in 2003 Saturn's largest moon, Titan, passed across our view of the Crab Nebula. Astronomers were able to study the interaction of x-rays coming from the nebula with Titan's atmosphere to learn more about its composition in a way that telescopes and spacecrafts were unable to before. Titan, it turns out, has the thickest atmosphere of any of the solar system's moons, which is actually thicker than the Earth's at some point (Muir 15). Similar studies have also been conducted to learn more about the Sun's corona.

It is clear that the Crab Nebula is one of astronomy's most intriguing objects of study. The nebula continues to astonish and perplex astronomers and compel them to further investigate what sets it apart from other objects in the sky. Although it was first seen nearly 1000 years ago, it seems evident that the process of unraveling the Crab Nebula is just beginning.

Sources:

Irion, Robert. "The Pulsar Menagerie." Science 304.5670 (23 Apr. 2004): 532-533.
Academic Search Premier. EBSCO. Nicholson Library, Anderson, IN. 9 Nov. 2008.

Muir, Hazel. "Crab throws light on Titan's big air." New Scientist 182.2443 (17 Apr.
2004): 15-15. Academic Search Premier. EBSCO. Nicholson Library, Anderson, IN. 9 Nov. 2008.

Shubinski, Raymond. "All about the Crab Nebula." Astronomy 35.1 (Jan. 2007): 70-73.
Academic Search Premier. EBSCO. Nicholson Library, Anderson, IN. 9 Nov. 2008.

Talcott, Richard. "The Crab's Inner Workings." Astronomy 31.1 (Jan. 2003): 24.
Academic Search Premier. EBSCO. Nicholson Library, Anderson, IN. 9 Nov. 2008.

"The Crab's heart." Astronomy 24.10 (Oct. 1996): 22. Academic Search Premier. EBSCO.
Nicholson Library, Anderson, IN. 9 Nov. 2008.

Trimble, Virginia. "Pineapples and crabs: When young supernova remnants were even
younger." AIP Conference Proceedings 565.1 (02 May 2001): 3. Academic Search Premier. EBSCO. Nicholson Library, Anderson, IN. 9 Nov. 2008.