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Winds from a pulsar

Week of February 1, 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.

[Note: I originally intended to have the Universe In Motion series of Snacks start at the Earth and move outwards, like the Solar System series did. However, I have decided instead to skip around the cosmos a bit. I Have My Reasons; you'll see why next week.]

For almost all of the history of the scientific study of astronomy, it was thought that the Universe was a relatively quiet place. Most events, we thought, happen slowly, and when things did happen it was a stately, orderly process. This turns out not to be the case. The Universe is a shockingly violent place, fraught with explosions, supersonic travel, radiation and just general nastiness. No place exemplifies this better than a supernova.

When a massive star reaches the end of its life, it literally explodes. The outer regions- more massive than our entire Sun!) are sent screaming outwards at a respectable fraction of the speed of light. A thousand trillion trillion tons of hydrogen, helium and other elements are accelerated outwards by the force of the explosion, and after time they cool and form a nebula, or gas cloud. We see many supernova nebulae in the sky, but perhaps none is more famous than the Crab Nebula. It was the first object in Charles Messier's famous list of astronomical objects (ironically, this list of the 100 or so most interesting and bright objects in the sky was compiled because Messier kept confusing them with comets; the list was for things to avoid!), and it has been said that you can nearly evenly divide astronomers into those who study the Crab and those who don't.

In the center of the Crab shines the leftover remnant of the supernova explosion. The core of the star does not itself explode, but instead can collapse to fantastic densities. The so-called neutron star in the center of the Crab is such an object, with a density a trillion times that of water. Imagine compressing the mass of the Sun into a sphere only 15 kilometers across! In the same way a skater spins faster when she brings her arms in, the collapsing star spins faster as well. The magnetic field of the original star gets compressed too, which strengthens it by an incredible amount. The rapid spinning of the magnetic field can pump huge energies into the surrounding gas, which heats and accelerates the gas even more. In the Crab, the gas very close to the neutron star was never seen directly until the Hubble Space Telescope was pointed that way.

image of Crab Nebula The image to the left is one frame of a movie you can download from the Space Telescope Science Institute. The bright arc shape just below and to the left of the star in the center is a blob of gas being heated by the neutron star. In the movie, you can watch as the gas is pushed away from the center of the nebula. The images were taken over the course of a few weeks, and the vast energies involved are difficult to imagine: that blob is accelerated to nearly half the speed of light! You can find out more about the images and the people that took them on the text of the press release by the astronomers.

Incidentally, don't think this is some remote object that no one can see. The Crab Nebula is only about 6000 light years away, which is relatively closeby in astronomical terms. It's up in the sky right now, just north and west of Orion. You only need a small telescope to see it, although without a big one all you'll see is a faint smudge. But don't be fooled: some of Nature's most terrible fury is locked in the heart of that dim blob.

©2008 Phil Plait. All Rights Reserved.

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