Some Stars Wobble

Claire Pendrigh, Some Stars Wobble, 2017, rocks and wire

This mobile is part of an exhibition of the same name, currently on show at Sawtooth ARI in Launceston. You can see all the artworks in the show here.

Some stars really do wobble. It’s one of the ways we can tell if a star has a planet orbiting it. Anything that is locked in an orbit with anything else, wobbles.

It’s easier to see If you look at two objects of similar mass that are orbiting each other, for instance, Pluto and its inner most moon Charon. With half the diameter and one-eighth the mass of Pluto, Charon is a very large moon. Its gravitational influence is big enough that both bodies spin around a point in space between them.

All orbital systems have this balance point; it’s called the “barycentre”. When a tiny planet orbits a massive star, the barycentre exists within the body of the star, so instead of making a discernable orbit, the star just appears to wobble.

The barycentre is nicely modelled in a mobile, as each side of each arm needs to be equally weighted. In Some Stars Wobble each mobile starts with two rocks on either end of a wire. I find the balance point, and add a loop and a swivel. Then I add another wire with a rock on the end. This time, the balance point has one rock on one side, and two on the other. To make the mobile hang flat, the balance point needs to be in the centre of mass between the two starting rocks, and the one new rock. The next level balances one rock against three rocks and the pattern continues.

In this mobile, I wanted to see just how complicated a balancing act between rocks in space might be. Imagine how many paths each rock could travel on its journey around this cluster – how many variations are possible.

Claire Pendrigh, Some Stars Wobble (detail), 2017, rocks and wire

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