Old, Fat Stars Flicker
Observing the pattern of flickers in a star’s light offers a new way for astronomers to measure one of the basic properties of stars — and any exoplanets they might host.
Astronomers can tell the difference between dwarfs and giants by measuring the strength of the star’s gravity at its surface. High surface gravity indicates a stable, healthy star in the prime of its life. Low surface gravity means the star has swollen in old age.
Unfortunately, surface gravity is devilishly difficult to measure. One hint is the width of dark lines imprinted in a star’s spectrum by elements in the stellar atmosphere as they absorb certain wavelengths of outgoing light. (Wider lines means a higher surface gravity: the higher pressure jostles the atoms, which can then absorb a broader range of light due to one of quantum physics’ many bizarre effects.) However, this method is only accurate to about 25–50%. Another rough estimate comes from a star’s mass and radius, which in turn are estimates from measurements of the star’s brightness. But like a passing phrase in a game of telephone, this chain can result in errors as large as the measurements themselves. It’s possible to measure gravity via “starquakes” — which can give accuracy to within 2%! — but this method only works for the very brightest stars.