The Crab Nebula

When historians write about the 20th century, they often emphasize the worst elements of the human spirit: two world wars, the Cold War, this war and that war. When we finally get some distance from it, I hope that the century will be best remembered as the time when we began to understand the workings of the universe. Humans have wanted that knowledge for a long time. At long last, the patient measurements of scientists over many centuries had just begun to bear fruit.

The Crab Nebula is a case in point. You can see it in binoculars or a small telescope as an oval patch of light just above the bottom horn of the constellation Taurus, the Bull. You are looking at the expanding remnants of a star that exploded almost a millennium ago.

In 1050, Chinese astronomers in the court of the Emperor saw a star where none had been seen before. They had no idea what stars really were, so the new star was little more than a fearsome curiosity. But they made careful observations anyway, perhaps in the hope that their observations would some day mean something. The “guest star” blazed so brightly that it was visible during the day for three weeks. They watched it slowly fade to black over the following three months.

Seven centuries later in 1731, John Bevis discovered another curiosity, a misty patch of light, called a “nebula,” in the same location that the “guest star” had been observed by the Chinese.

There matters languished for two centuries. As telescopes finally got larger, astronomers began to discern details in the fuzz ball. The 19th-century astronomer Lord Rosse described it as having “resolvable filaments” with a gap at its south end, which led to its nickname, the “Crab.”

In the first decades of the twentieth century, astronomers began to make some sense of the Crab. By comparing photographs taken several years apart, they discovered that the nebula was rapidly expanding.

By studying its outward motion, they concluded that the expansion must have begun about 900 years earlier, and the connection with the Chinese guest star was finally established. It had all the earmarks of a stellar explosion of intense magnitude. The Crab had the distinction of being the first “supernova remnant” ever discovered.

Improved photographs of the object showed that it was blue at its center with filaments of red gas at its edges. Why was the gas producing different colors? The answer came in the 1950’s from Soviet experiments with a particle accelerator called a synchrotron. The Soviets discovered that electrons rotating in a powerful magnetic field produced a bluish glow of exactly the same kind as seen inside the Crab.

What then was causing the spinning magnetic field that in turn caused the electrons to whirl around? As astronomers pondered that question, they also began to use new-technology telescopes to study stars in parts of the energy spectrum that visual telescopes cannot see. The Crab was emitting X-ray and radio energy, as they had come to expect from supernova remnants. However, unlike most supernova clouds, the Crab was rich in those energy bands deep inside the heart of the Crab.

In the end, astronomers discovered a tiny and extremely dense star rotating at the center of the Crab. By studying its radio waves, they determined that it was, like a celestial lighthouse beacon spinning out of control, rotating at an unbelievable 30 times every second.

Other, older supernova stars rotated much more slowly. This, then, was what happened when stars explode. Some of their material is ejected explosively into space. The rest collapses into a rapidly spinning ball. As the energy of its spin is converted over time into a massive magnetic field with spinning electrons, its rotation slows down.

The great Chinese mystery was at last solved, and it only took 900 years — a long, long time in human history but the smallest trifle in the life and death of a star.

Tom Burns is the director of Perkins Observatory. He can be reached at tlburns@owu.edu.