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Resonances

Start with a metal surface attached to an acoustic oscillator (a speaker). Add salt to the surface. Increase the frequency until the plate reaches its natural frequency, at which point standing waves are set up. The salt settles down along the nodal lines on the surface where the plate is relatively still. Similarly, salt grains are bounced out of the regions where the surface oscillates.



(I found this video on ThisIsColossal. h/t Bri for pointing me to the site.)

This is a 2-dimensional analogy of oscillations in the Sun. Hot material just below the Sun's surface rises in convective cells, which then "ping" the outer layers of the Sun. This pinging causes the Sun to oscillate and set up standing waves throughout it's 3-dimensional interior. Some of these vibration modes can be observed by making repeated measurements of the Sun's brightness as its surface oscillates.

An illustration of spherical harmonics in the Sun's interior. These harmonics can be measured by watching the Sun's (or another star's) brightness oscillate in time.

In principle, one could estimate the density of the plate in the video above by reading off the nature of its natural frequencies. Plates of different densities (made of different materials) will vibrate differently and give rise to patterns at different characteristic frequencies. The same goes for stars. Astronomers can read off the density of stars based on the fluctuations in their brightness, which allows them to measure their mass and radius.

This is one of the major side benefits of the Kepler mission. By making repeated brightness measurements (photometry) of hundreds of thousands of stars, the mission has discovered thousands of planet candidates. For some stars, their natural frequencies show up in the photometry, which enables the detection of planets and a characterization of the host star's physical properties. Science!


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