INTERLUDE 21-3 The Crab Nebula in Motion
The Crab Nebula is one of the most fascinating —and instructive—cosmic objects anywhere. Within this remarkable region of space we find a wide variety of physical processes at work, all the result of a supernova explosion that occurred almost a thousand years ago. A famous physics professor at MIT once taught a course exclusively on this object, claiming that if you understand all that transpires within the Crab, then you have mastered much of modern physics. Within this remarkable gas cloud, extending for a couple of parsecs and residing about 1800 pc away, we find applications of particle physics, nuclear physics, electromagnetism, thermodynamics, condensed-matter physics, plasma physics, and gravitational physics, to name but a few.

Now we are treated to new observations suggesting that the Crab Nebula is even more dynamic than had previously been thought. Apparently, things are changing rapidly among the debris, right before our eyes, giving astronomers a rare chance to study evolutionary changes that actually occur during a single human lifetime.

The first figure consists of two frames. On the left is a true-color, visible-light photo taken with the 5-m Hale telescope on Mount Palomar. (Sec. 5.1) This image is similar to the center frame of Figure 21.9. The visible light arises in two distinct ways. The outermost filaments seen in red, yellow, and green are literally glowing from the heat and violence of the explosion long ago. However, the faint bluish light toward the center of the nebula arises from a nonthermal process that releases energy as rapidly moving electrons spiral around lines of magnetic field. (This process is not studied in the text until Chapter 25, but you can skip ahead and look at

Figure 25.15 to get the gist of it.) The filamentary structure of the wispy debris leaves little doubt that this is the remnant of an exploded star.

The right frame is a recent image acquired by the Hubble telescope, covering the portion of the first image contained within the white box. The image was taken at a wavelength of around 550 nm; the red color was added artificially during computer enhancement.

The central "engine" at the heart of the Crab Nebula is a pulsar—a rapidly spinning, compact starlike object that managed to survive the supernova explosion that created the nebula itself. (We will discuss these strange objects in more detail in Chapter 22.) (Sec. 22.2) The pulsar can be seen in this image as the left member of the pair of stars near the center of the frame. The pulsar powers the Crab today by accelerating elementary particles into the nebular debris, causing knots and wisps of energetic matter to stream away from the core.

In the second set of images here we can almost see in action the tiny pulsar powering the Crab Nebula (again, the left star of the pair at upper center). These three images were taken sequentially over the course of a few months. The field of view is even smaller field than in the right frame we are now virtually inside the Crab, witnessing the minute changes among the nebular gases near the site of the ancient explosion. Astronomers can now watch the changes in the shock-driven features that brighten and fade, over and over, like ripples in a pond. The light-year-long ripples in the Crab, seen throughout the bottom half of these frames, are moving outward from the pulsar at about half the speed of light.