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The Crawl of the Crab

Week of February 15, 1999

On occasion, I will take a few weeks to explore a theme in astronomy on these pages. In 1997 that theme was the solar system. This time, for the next few weeks, I invite you to take a look at the Universe In Motion.

We think of the skies as static, unchanging. The only motions we see easily are the rising and setting of the Sun, Moon and stars, but that is a reflection of our own Earth's rotation. The stars themselves don't seem to move at all among themselves, and it takes a keen eye to discern the motion of the planets, which takes days or weeks to become obvious.

Yet objects in the sky are in constant motion, and some move with an incredible intrinsic velocity. Usually, these objects are so far away that the distance itself shrinks the apparent motion, the way distant mountains hardly seem to move at all even though you may be driving past them at 100 kilometers an hour. Sometimes it does take many years to perceive the movement of heavenly bodies, and sometimes it happens in the blink of an eye. Every Monday, we'll take a look at some of these celestial travelers.

A couple of snacks ago, I talked about how the pulsar in the center of the Crab Nebula is helping the inner nebula expand. The expanding arcs and filaments are being driven by the tremendous magnetic field of the collapsed star.

But long ago, before the star collapsed, it was a star something like the Sun, though much more massive and hotter. On July 4, 1054 (as we reckon the time), the star ran out of fuel. Its core was filled with iron, which in stellar terms is the equivalent of eating celery: it takes more energy to process than you get out in the end. Fusing iron rings the death knell for a massive star. It takes away energy vitally needed to support the star, and when that support is gone, the core collapses. In a process still not well understood, a huge number of ghostly neutrinos are produced, which deposit energy into the outer envelope of the star. That may not sound like a big deal, but it might help to understand just how much energy we're dealing with here: it's roughly the amount of energy the Sun will put out in its entire lifetime, and that energy is released in a matter of milliseconds! Like a heavy hammer hitting a small rock, the outer parts of the star absorb the energy and literally explode. Something like several solar masses of material are flung outwards at velocities a good fraction of the speed of light.

The ejecta glows from the initial heat of the blast, and would quickly cool, except that the violence of the explosion actually changes the nature of the material. The elements themselves transmute, from mostly hydrogen and helium to more exotic species as nickel and cobalt. A lot of the new material is highly radioactive, and actually continues to heat the expanding gas. Eventually though those materials also burn out, and the tortured matter, once part of the upper layers of a star, now coast into space. Even the tiny amount of matter between stars can rob enough momentum from the ejecta, and eventually the supernova remnant slows to a stop as it runs into the not-quite-empty regions of space.

image of Crab Nebula expansion

But while it is still hot, we can see it. The Crab Nebula is one such supernova remnant, and the nebula we see is the what's left of the tremendous explosion that spelled doom for the star. Even though the material we see is expanding at hundreds of kilometers per second, at a distance of 2000 parsecs that expansion is difficult to see. But not impossible; it just takes time. Walter Baade, one of the world's all time greatest observational astronomers, captured that expansion on film. He took an image of the Crab, then took another 14 years later. He made a positive print from the first image, so the gas looks white, and a negative of the second, so the gas looks black. He then printed the two on top of each other, and a reproduction of that image is shown here. If you look carefully, you'll see that in the outer parts of the nebula, where the black and white parts are adjacent, the black parts are always outside the white parts. That's because those parts of the nebula have expanded measurably during the time between the two images. Again, like the Leonid explosion or the inner regions of the Crab, you can actually see an astronomical object change in this image. It represents one of the earliest examples of the actual detection of motion of such a distant astronomical object (one of the most recent is a series of images of Supernova 1987A taken by my friend Jason Pun, but that's a later Snack).

Incidentally, an interesting note. It was long thought that the bright ``new star'' seen by Chinese and Native American astronomers was actually the explosion event that was the birth of the Crab Nebula, but it wasn't until the image above was taken that it was known for sure. The speed of expansion of the gas can be measured directly using the Doppler effect, and the motion on the sky can also be measured. That gives an actual size of the object, which in turn means the time of the explosion could be found by the measurements of the object itself. That date: July, 1054, just when the terrestrial astronomers saw the bright star! So the Universe is indeed in Motion, and if we are clever enough we can sometimes even trace it back to its starting gate.

©2008 Phil Plait. All Rights Reserved.

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