Tycho's Star in Theory and Practice
supernova seen in 1572 has been understood in several ways since then.
The latest x-ray observations encourage still another way to understand
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
driving them. In the same way that an
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
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