Saturday, July 18, 2015

A Little Light Music

When astronomers began turning their telescopes towards the sun to examine it as they had the moon and the planets, they found out that the universe has a sense of irony. The very same source of light which reflects off those other bodies and lets us see them can't be studied by just light.

While the sun's surface features can be made out in great detail if you're using the right filters, there's no way to see past the light generated by that surface to view what lies underneath. So for much of the history of observational astronomy, there were only guesses about what was beneath the corona we could see. Some readings showed different interior spots and areas were cooler than the surface, which made little sense given what was thought to be the source of the sun's energy. Other factors had to be playing a role. Also troublesome was the difference between the amount of neutrinos we do detect from the sun and the number we figure we should detect. Again, something else had to be going on than just what the visible surface showed.

Enter, some 50 years ago, the scientific discipline of "helioseismology," or study of how pressure waves move through the burning gas of the sun and how those movements tell us what's underneath the light.

William Chaplin's 2006 book Music of the Sun sketches the history of this branch of science, which started to take hold when astronomer Robert Leighton recorded regular oscillations of the sun's surface -- you'd expect a ball of hot gas to be a little quavery, but Leighton's data showed regular periodic vibrations in addition to the regular random kind of skips. Scientists then began to realize that if the sun produced those kinds of waves, there was a ready discipline at hand to provide the conceptual framework: Acoustics.

See, sound is just the movement of pressure waves through some medium. The kind of medium affects how the waves travel, which is why things sound different underwater than above, but it's all acoustics. And since the sun is a medium like an atmosphere, then regular pressure waves from different activities beneath the surface would affect that surface and could be compared to known properties of acoustics. On earth, study of the same kinds of waves helps geologists identify kinds of rocks miles below the surface.

Chaplin outlines the development of the discipline as more sophisticated instruments find new and more precise variations and movements. Essentially, helioseismology bootstraps itself along. When a new discovery tells scientists something about the sun's interior, they use that information to help refine their measurements and then learn something from those new measurements.

His book is carefully written and he takes the time to explain a lot of the acoustical science needed to understand how the gas and temperature of the sun's interior produce the waves helioseismologists observe. It's a lot of detail to keep straight and can make the book as a whole slow going when you have to flip back some pages to remind yourself of what some terms mean. But the structure, which moves from explanation to the history of the science and then some of the questions it's answered, offers a pretty thorough introduction to the field.

After all, once you can get your head around the fact that the best way to study the greatest source of light in our sky is not to look at it but listen to it, you can handle a few quirky terms.

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