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Sep 30, 2005
Tycho's Star in Theory and Practice

A supernova seen in 1572 has been understood in several ways since then. The latest x-ray observations encourage still another way to understand it.

A theory doesn’t just explain what you see. It also tells you what to look for and how to see it. This interdependency of fact and theory, of perception and conception, enables human beings to adapt their limited understandings to changing experiences. Because experiences often don’t change in a straightforward and cumulative way, theory-making extends beyond the currently accepted explanations to ask “What else could it be?”

One recent changing experience is this Chandra x-ray image of Tycho’s Star. In 1572, Tycho Brahe, the famous Danish astronomer, saw a “new star,” what we now call a supernova. When later astronomers turned their telescopes on the faint remnant of Tycho’s supernova, they saw a circular cloud of luminosity around it. The theory of stellar evolution explains supernovas as massive stars that eventually explode and throw off a shell of gas. Astronomers looked for shells of gas, and in these circular clouds of luminosity they saw shells of gas.

But as they looked more closely and looked at more supernovas, the circular clouds of luminosity began to look a little different—and sometimes a lot different—from how the theory said they should look. So the astronomers adjusted the theory and they reinterpreted how they saw the clouds until their looking and seeing were again in agreement.

Now Chandra is changing the experience of supernovas again. Conventional theory still expects to see shells of gas, but the shells are shock waves that compress and heat the gas to millions of degrees. Most of the debris from the exploded star should lag behind the shock wave.

In this image, Tycho’s supernova has two shock waves (the high-energy filaments, shown in blue), and the cloud of debris (lower-energy x-rays, shown in green and red) is not lagging behind. Measurements indicate that the blue x-rays are “non-thermal,” which means that they’re not coming from “hot gas.” (The million-degree temperature is not a direct measurement but is calculated according to how hot a gas must be to emit x-rays with the observed energy.)

Conventional astronomers think that they can adjust the theory to make the outside shock wave accelerate the nuclei of atoms to cosmic ray energies. Then if they see the blue filaments and the close-following debris as the results of this acceleration, looking and seeing may again harmonize.

But what else could it be? Plasma theory explains supernovas as stars that develop instabilities in the galactic Birkeland currents driving them. In the same way that an unstable double layer (DL) on the Sun explodes into a flare, a DL that encompasses an entire star explodes into a supernova. The energy is released in the acceleration of ions, primarily along the axis of the current, and in non-thermal radiation, especially radio and x-ray emission.

[As an aside, it’s amusing to note that conventional theory considers anything with a temperature of millions of degrees to be a plasma (although it ignores electrical properties). At that temperature, all atoms are fully ionized and their nuclei are properly called ions. But ions are accelerated by electric fields. “Nuclei” require extreme shock waves to get them to move fast. Thus the theory dictates not only what to look for and how to see it but also which words to use to describe it.]

Plasma astronomers see in this image an hourglass-shaped discharge viewed down the axis of the current. The blue ring is the outside of the tubular Birkeland current driving the discharge. They notice that it’s composed of spiraling filaments and more or less evenly spaced bright spots, behavior that’s observed in laboratory experiments and computer simulations of plasma discharges. They notice that the green and red debris is clumped into bubbles or cells, another typical behavior of plasma. This produces a “cauliflower-like” surface on the discharge. If seen from the side, Tycho’s supernova would probably look a lot like Eta Carinae.


David Talbott, Wallace Thornhill
Mel Acheson
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