Crab Nebula has long, important history


By Tom Burns - Stargazing



Around midnight this time of year, the constellation Taurus sits beautifully on the southeastern horizon. Over the centuries, thousands of amateur stargazers have trained their small telescopes and binoculars on a tiny, fuzzy patch called M1 (or more popularly, the Crab Nebula) just off the star that forms the bottom horn of the bull.

It is difficult for those not star-obsessed to understand the fascination. Many times I have pointed my big telescope at M1 so that the crowd at a public program can gaze at one of the most important astronomical objects in the sky.

Granted that it is exceedingly faint. Granted that it looks like a tiny, featureless comet — without the hint of a tail, no less. Granted that it looks like a faded, fuzzy thumbprint. But the usual responses, like “What? Where?” or “Am I supposed to see something?” or “Show me something good” seem a tad harsh, nonetheless.

The Crab Nebula is important, doggone it. When the history of the 20th century is written, it 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 finally began to bear fruit.

The Crab Nebula is decidedly 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.

Its story is a long and complex one that digresses into a discussion of comets, to which it bears more than a slight visual resemblance. Here’s the story, digression and all.

In 1054, 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 as the Chinese had observed the “guest star.”

We now know that M1 is a supernova remnant, the expanding debris of a mighty star that exploded in 1054. We revel in its beauty, but its greatest astronomical popularizer thought of it as a pain in the neck.

The “M” in M1 stands for Charles Messier, assistant to the head of the French naval observatory in 1758 when that ridiculous patch of fuzz passed into the field of his small telescope.

Messier had a job to do, and M1 wasn’t helping any.

A bit earlier, in 1695, no one was quite sure what to make of another class of fuzzies called comets. Did they pass straight through our solar system never to be seen again? Or did they orbit the sun like planets?

Edmund Halley believed that they were in orbit. If they were, they should return to view periodically as they dove from the inky depths of the solar system back toward the sun, where we live.

To prove his point, Halley examined the historical record for comets and painstakingly calculated their possible orbits around the sun. Three of them had startlingly similar patterns — the comets sighted in 1531, 1607 and 1682.

It didn’t take a genius mathematician to conclude that these apparitions might in fact be one comet that returned to our earthly environs every 75 or 76 years. If so, the comet should return in late 1758 or early 1759.

Halley would have been 102 if he had survived to see his comet, that is, if he was right and comets did return.

The rest of the astronomical community took up the challenge, and the race was on. Charles Messier’s boss handed him the task of being the first to find the comet.

For a year and a half, Messier searched night after fruitless night for Halley’s comet. In September 1758, he thought he had found it in the constellation Taurus. However, subsequent observations of it showed that his fuzzball did not move against the starry background. A comet in orbit around the sun should move.

As Messier tells it, in late 1758, he finally found what he was looking for and begged his boss for permission to proclaim his discovery to the world. Inexplicably, the head of the naval observatory made him wait a month to make his announcement. By that time, others had seen the comet, and Messier’s claim became the subject of intense ridicule.

Messier spent the rest of his life trying to live down the ignominy. He became obsessed with comets, discovering more than any other astronomer. The king of France, Louis XV, eventually bestowed on him the informal title “the ferret of comets.”

As he searched, he ran into more stray fuzzy patches that weren’t comets and therefore pains in the posterior. Comets move against the starry background as they orbit the sun. Non-comets move with the stars. It often took days of observing to determine that the cursed non-comets did not move.

To save time and effort, he decided to catalog them, draw them, and mark their locations simply to avoid them.

As time passed and telescopes got bigger, Messier’s nuisances began to explode into glittering star clusters, spinning whirlpools of light called galaxies, and beautiful clouds of glowing hydrogen gas. They are, without doubt, the most spectacular objects to see in small amateur telescopes, M1 and a few others excepted. It takes a huge telescope by amateur standards to discern any detail in M1.

The comets Messier discovered are lost in the depths of space and time. But his 110 beautiful nuisances still bear his name, each one a monument to dogged determination and accidental glory. The Crab Nebula, as featureless as it is in a small telescope, was the first of those nuisances, and therefore takes its honored position as M1 in Messier’s Catalog.

And there matters languished for two centuries.

As telescopes finally got larger, astronomers began to discern details in the fuzzball. 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 20th 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 thus 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 1950s 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.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.