Hubble composite image of the Crab Nebula in optical wavelengths. Blue filaments in the outer part of the nebula represent neutral oxygen, green is singly-ionized sulfur, and red indicates doubly-ionized oxygen. NASA, ESA, J. Hester and A. Loll (Arizona State University)
Hubble composite image of the Crab Nebula in optical wavelengths. Blue filaments in the outer part of the nebula represent neutral oxygen, green is singly-ionized sulfur, and red indicates doubly-ionized oxygen. NASA, ESA, J. Hester and A. Loll (Arizona State University)

How the Crab Nebula’s Pulsing Heart Stayed Hidden for Centuries

Ancient astronomers chronicled the shifting heavens, diligently charting the movements of our star-studded canopy. The moon’s face morphed nightly, our planetary neighbors came and went, and occasionally a brilliant, icy vagrant would sweep by.

But the stars? Those stayed pretty much in the same place, relative to one another. So when new, starry points of light briefly appeared and then faded away, Earth’s sky gazers noticed.

Nearly 1,000 years ago, one of these new stars began shining brightly in the northern sky. It was July 4, 1054, and the people of Earth – from North America to China – turned their attention skyward. Glimmering near the star Zeta Tauri, the new star was much more than a distant, pale point of light: For almost a month, it even shone during the day. Chinese astronomers, who politely referred to the newcomer as a “guest star,” kept detailed records of the stellar visitor. Those records show that the star stuck around for more than two years before slowly fading from the sky like a guest saying good night for the evening.

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Color-inverted reproduction of William Parsons’ 1844 drawing of the Crab Nebula. (Wikimedia)

Seven centuries years later, French astronomer Charles Messier was peering through his telescope in Paris, looking for comets. One night in 1758, Messier saw a strange, fuzzy object in the constellation Taurus. He briefly thought it might be the comet Edmund Halley had predicted would return that year. But the object wasn’t moving; it was fixed in the sky, near where Chinese stargazers had marked the appearance of their guest star almost exactly 700 years earlier. That fuzzy dot, which Messier came to realize was a gassy nebula, became known as M1 – it was the first entry in his new catalogue of astronomical objects.

By the mid-1800s, the nebula had another name: The Crab Nebula, a result of Irish astronomer William Parsons sketching the object and thinking it looked vaguely crab-like.

It wasn’t until the early 20th century that a series of observations finally revealed what the Chinese guest star was: In 1054, a massive star had exploded and died. At 6,500 light-years away, the supernova, as these stellar explosions are called, was so close by that its light pierced the heavens and arrived on Earth with no difficulty at all. The explosion produced a bright, expanding shell of gas — the nebula Messier, Parsons, and others had seen. When astronomers in the 1920s measured how fast the nebula was growing, they realized they were looking at an object that began ballooning outward nearly 900 years earlier.

By 1942, there no doubt the nebula was linked to the observations from 1054. But the story isn’t quite over yet.

For most of its lifetime in Earth’s skies, the Crab Nebula has only been observed in optical wavelengths – that small sliver of the electromagnetic spectrum that humans have evolved to perceive as colors. In addition to visible light, there are also such things as X-rays, gamma rays, infrared, ultraviolet and radio waves. They’re all part of the same spectrum, but vary in wavelength and energy. It’s only been in the last 100 years that astronomers have finished working out how to view the skies through all these different lenses.

“These are not just different ways of seeing the same thing,” says Neil DeGrasse Tyson in this week’s episode of Cosmos: A Spacetime Odyssey. “These other kinds of light reveal different objects and phenomena in the cosmos.”

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Composite of the Crab, in X-rays, optical, and infrared. X-ray: NASA/CXC/SAO/F.Seward; Optical: NASA/ESA/ASU/J.Hester & A.Loll; Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz A composite of the Crab, in X-rays (blue), optical (red and yellow) and infrared (purple).

Light does indeed reveal much that is hidden. Gamma-ray bursts, some of the most energetic events in the universe, blaze brightly in gamma rays (as the name suggests). Black holes snacking on stars can glow strongly in X-rays. Supernova remnants can appear placid in visible light yet look spectacular in the infrared or X-ray. Some galaxies shine brightly in radio, but are too far away or hidden behind dust to be seen in optical.

And the Crab Nebula – it’s got a racing pulse that astronomers wouldn’t be able to take until the late 1960’s.

The big image above shows what the Crab looks like to an eye like the Hubble Space Telescope. Here, in visible wavelengths, it shines brightly blue in the center, surrounded on its fringes by tendrils of red. Because of the spectral signatures carried by visible light – those lines that Tyson describes in Cosmos – astronomers know what kinds of chemical elements lived in the Crab Nebula. That doesn’t mean there aren’t still surprises: Late last year, observations in the infrared revealed the presence of argon hydride, a molecule scientists didn’t expect to see.

As the Cosmos script says, we perceive infrared wavelengths as thermal radiation. We can’t sense them with our eyes, but we can feel their heat with our skin – unless of course, their source is far away. Infrared light, unlike optical wavelengths, is very good at traveling through cosmic dust and clouds – so viewing a skyscape in these wavelengths can reveal objects and structures that are too cold or obscured to see in other ways.

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The Crab Nebula in infrared. The blue region traces the cloud of energetic electrons trapped within the star’s magnetic field. The yellow-red features follow the well-known filamentary structures that permeate this nebula. (

Now, how about looking at the Crab in X-rays? There, in the center, amidst all that blue is something curious: It looks like a jet, spewing from a disk encircling a central object. X-rays are produced by the most energetic events in the cosmos – things like black holes tearing things apart and neutron stars spinning like crazed street performers. To observe an object in X-rays is to know that something enormously energetic is happening.

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The Crab Nebula, in X-rays. NASA/CXC/SAO/F.Seward et al.

Here, that feature in the X-ray image is the product of a neutron star in the Crab’s center – the remnant of a star that was much more massive than our sun. When that star collapsed and died, it flung its guts outward, creating the gassy part of the Crab Nebula. Its core, though, contracted into a dense, spinning neutron star – an object with 1.4 solar masses squeezed into a diameter of only 10 miles.

When neutron stars spin, they’re called pulsars. In 1967, the first pulsars were discovered by Jocelyn Bell Burnell, who’d been looking at the sky through a radio telescope. Radio waves, like infrared, travel really well through dust and clouds; among other things, they’re produced when high-energy electrons go spiraling around magnetic field lines, like those surrounding a neutron star. When you look at the Crab in radio, the pulsar is in the middle of the image is so bright that it’s just colored white.

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The Crab in radio, where the pulsar at the center is clearly visible as a bright white spot. (Image courtesy of NRAO/AUI)

Pulsars, it turns out, are among the brightest astronomical objects in the radio. And the Crab Pulsar? That thing is among the brightest of all, as seen from Earth. Nearly 1,000 years after it formed, the pulsar is still spinning wickedly fast, sending a beam of detectable radiation toward Earth 30 times a second.

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Stroboscopic view of the Crab Nebula pulsar. When the X-ray observations were timed to match the frequency of the pulsar, the pulsar seemed to turn on and off. (Einstein Observatory image, Smithsonian Institution Photo No. 80-16234)

For more great views of the cosmos through different lenses, spend some time with the photos here.