The Heartbeat of an Exploded Star

By Phil Plait

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Posted Thursday, Aug. 1, 2013, at 8:00 AM

The Crab Nebula, the violently out-rushing debris from a star that exploded a millennium ago. Click to hugely chandrasekharenate.
Photo by Adam Block/Mount Lemmon SkyCenter/University of Arizona

A thousand years ago—in July of 1054, to be somewhat more precise—the light from a cosmic catastrophe reached Earth. A massive star, probably 20 or more times the heft of the Sun, exploded. This titanic event was vast almost beyond human grasp: It released as much energy in a few weeks as the Sun will over its entire ten billion year lifetime.

The devastation was nearly total: Most of the star was torn apart, its octillion tons of matter blasted outward at a good fraction of the speed of light, while the very central core of the star collapsed to form a rapidly spinning white-hot neutron star. Now, ten centuries later, the expanding debris is 100 trillion kilometers across, glowing from both the influence of the neutron star’s fierce magnetic field, and the violent collision of the filaments of the gas itself, creating epic shock waves in the material.

We call this cloud the Crab Nebula, and you can see it in the picture above, taken by my friend Adam Block using the 0.81 meter Schulman Telescope in Arizona. The total exposure time on this image was a whopping 17.5 hours, using several different filters to produce those glorious colors.

Amazing as the image is, there’s another, subtler aspect of it that will cook your brain. That debris you see is still expanding, and quite rapidly. Because the Crab is tremendously far away—6500 light years or so—any motion is shrunk down to near-invisibility. But we’ve been observing it for decades, which is a pretty long baseline. That means that if you compare an earlier image to a later one, you can actually see the physical expansion of the supernova explosion.

Adam did this: He created the video below, which shows his image taken in 2012 compared to one taken in 1999 using the ESO’s Very Large Telescope.

Holy. Wow. That’s not a trick using exposures or magnification or anything like that. Keep your eyes on the stars and you’ll see they are in the same positions in both frames; then pick a knot or filament in the nebula and you can see the material moving. To me it looks like a heart beating, especially given the gas cloud’s overall shape.

I’ve written about this visible expansion before; in fact a few years back when I was developing educational activities based on NASA satellites, I reworked an old classroom exercise where you could compare two images of the Crab and determine not only how fast it’s expanding, but then trace it back to determine how old it is. Astonishingly, you get the correct date to within a small margin of error!

It’s easy to think of the sky as static, unmoving, and unchanging. Because most objects are so terribly far away, we don’t notice the motion they undergo. But sometimes they move rapidly enough, and our technology is sensitive enough, that their velocity betrays them. And seeing that motion, as in the video, gives you a real sense of it. Remember, what you’re seeing is a superheated cloud of gas with five times the mass of the Sun screaming outward into space at speeds up to 1500 kilometers per second—well over three million miles per hour!

The Universe is an amazing place. I love that we have such a wonderful chance to study it.