The Crab Nebula


Every astronerd, both from the most casual of amateur stargazers to the most learned of professional astronomers, has experienced some version of the following conversation.

Here’s my version of it.

Casual acquaintance: “So what do you do?”

Astronerd: “I’m a diehard stargazer.”

Casual acquaintance: “Oh, really! I love astrology! I’m a Libra. What’s your sign?”

Of course, astronomy studies the universe in a systematic and scientific way. Astrology is an ancient superstition that assumes that events in the heavens somehow foretell or influence events on Earth.

Most astronomy types detest astrology and resent the confusion of their beloved vocation (or avocation) a set of antiquated delusions.

They shouldn’t. In fact, astronomy rose out of astrology. In some cases, the observations of ancient astrologers shed some light on the findings of modern astronomy.

My recent trip to China revealed a case in point. We took a cruise down the Yangtze River and briefly escaped the horrible light pollution prevalent in the sprawling Chinese urban centers.

As is my habit, I arose early to watch the sunrise. Just before morning twilight, the bright star Aldebaran in the constellation Taurus was rising in the east.

Dimly, I could see the bull’s long horns. I had no telescope, but in my mind’s eye, I could see an astronomical object called the Crab Nebula I had observed many times halfway around the world.

Here then was the same sky I saw many times in Ohio. Instantly, I felt the presence of the ancient stargazers who floated down the old Yangtze so many centuries before, and I felt a deep kinship with them. In particular, my mind raced back to observations made by Chinese astrologers almost a millennium ago.

From a dark, rural sky, you can see the Crab Nebula in binoculars or a small telescope as an oval patch of light just above the bottom horn of Taurus. You are looking at the expanding remnants of a star that exploded over nine centuries ago.

Such supernovas, as the cataclysmic explosions of stars are called, probably happen every 50 years or so in our Milky Way galaxy. However, most are far too distant or are shrouded by interstellar dust and gas to be visible to our eyes.

Thus, naked-eye supernovas are exceedingly rare events. The last observed supernova in our Milky Way galaxy occurred in 1604. The event was observed and studied by one of the great astronomers of any age, Johannes Kepler.

He carefully recorded the rise and fall of its brightness, but he lacked both a telescope and our more modern scientific instruments to determine its manifold characteristics.

Luckily, with the invention of the telescope, we are no longer limited to naked-eye observations. Astronomers daily, both professional and amateur, scan thousands of galaxies other than our Milky Way. They are looking for bright stars in those galaxies that often briefly outshine the collected brilliance of their hundreds of billions of stars.

Still, proximity has its advantages. Thus, when astronomers want to study a supernova remnant like the Crab Nebula, which is in our own Milky Way galaxy, it is helpful to have careful observations of the explosion that led to them.

And in terms of the longevity of their observations, no culture on Earth beat the astrologers at the court of the Chinese emperor. The Chinese believed that their emperor, who was called the “Son of Heaven,” was imbued with power derived from the sky. By its careful study, astrologers believed they could help their emperor to make correct political and societal choices and even predict the future.

The most important and powerful events were unexpected. The appearance of a comet or a new star in the sky portended great events on Earth.

In 1054, those Chinese court astronomers 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 someday 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.”

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 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 had the distinction of being the first “supernova remnant” ever recognized as such.

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.

How must those long-dead Chinese astrologers have felt as they stared up at their discovery of a new star? What amazement they must have felt as it flared into view! What wonder and sadness they must have experienced as the star faded slowly into darkness.

Do we chastise them for their ignorance, or do we envy them for being among the chosen few in all of human history to see a supernova fill the night sky with otherworldly light?

Thus, when people confuse my love of stargazing with astrology, I treat it as a teachable moment. I explain the difference, of course. I try to show them that even though astrologers get the relationship between the heavens and the Earth completely wrong, we ought to respect their love of the sky and their sometimes-useful observations of it.

Of course, I reject the notion that the stars somehow rule our lives. However, I recognize in my hearts of hearts that the stars have ruled my life. They have done so since I first looked up at them — with the same curiosity and wonder as those Chinese astrologers — so long ago.

By Tom Burns


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

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