Red Giant Groove
Earlier this month I attended a scientific conference on a small Spanish island called Lanzarote, just off the coast of Morocco. The meeting was for scientists who study the insides of the Sun and other stars using a technique similar to seismology. Without a doubt, the coolest result presented at the meeting came from Italian astronomer Andrea Miglio and his colleagues in Belgium. For the first time in any star other than the Sun, they measured an abrupt variation in the internal structure -- like the boundary between the crust and the core of the Earth, but in a red giant star more than 350 light years away.
A red giant star is what our Sun will become when it begins to run out of hydrogen fuel, about 5 billion years in the future. As the hydrogen begins to burn faster in a shell around the helium core, our star will slowly bloat and cool -- eventually engulfing the inner planets, including the Earth. Like the Sun, the surface of a red giant seems to boil as convection brings heat from below and radiates it into the coldness of outer space. The boiling churns more slowly in a red giant, but the turbulent motions still create sound waves that travel down through the star and then back toward the surface. Some of these sounds have just the right pitch, a million times lower than we can hear with the human ear, and they set up standing waves that cause the entire star to slowly change its brightness over hours and days.
Like the standing waves on a vibrating string, the brightness changes that we can see in a red giant have very specific frequencies. The fundamental frequency of a vibrating string is sort of like a jump rope, with the entire length moving up and down together. If the people at the ends of the rope move their hands up and down at twice the fundamental frequency, the standing wave will have two lobes with a stationary point in the middle -- one side will move up while the other moves down. At three times the fundamental frequency you get three lobes, and so on to create a whole series of evenly spaced frequencies. Now imagine that somewhere along the jump rope you attach a small lump of clay -- the extra weight at this position will change the way that waves travel along the rope, and the frequencies of the standing waves will change slightly from the evenly spaced pattern. Here's the cool part -- by looking at the deviations from uniform spacing, you can actually figure out where along the rope you have attached the clay!
This is essentially what Andrea and his friends did for the red giant. Using measurements taken over 5 months from a French space telescope called COROT, they identified a series of almost evenly spaced frequencies of brightness variation in a red giant known as "HR 7349". By looking at how the frequencies they observed differed from perfectly uniform spacing, they determined the position of a boundary layer on the inside of the star. Comparing this prediction with theoretical models of the star, they identified this layer as the depth below the surface where the temperature was just high enough to strip both of the electrons from every helium atom. From a distance of more than 350 light years, the team had pinpointed the position of a subtle change in the composition and density of the star -- just by measuring its regular changes in brightness over time.
This beautiful result promises to be just the first demonstration of the types of measurements that are possible using stellar seismology. The same basic techniques will allow us to measure the size of the boiling layer on the surface of a star like the Sun, providing new ways to test our understanding of how stars are actually built.
