Starquake

Starquake

by Adam Frank for McGraw-Hill

It's a beautiful day in Los Angeles. The eternal sunshine bathes the city in a warmth so perfectly balanced people can hardly feel the air on their skin. On the beaches, surfers are searching for the perfect wave. In the streets of Beverly Hills shoppers are searching for the perfect pair of black pumps. Suddenly, and without warning, the ground begins to tremble. The seconds pass and the shaking intensifies to a stomach churning pitch. All around the helpless city solid steel buckles and bends. Freeways twist like ribbons of taffy as the now roaring earthquake turns solid ground into a quivering liquid.

Earthquakes make for great drama (especially if you have never had the misfortune of being in one). They also make for great science. Every quake represents a profound opportunity giving geophysicists a chance to peer deep into our planet. Such "vision" is only possible because earthquakes create powerful waves that travel across and into the planet, emerging as tremors (ground oscillations or "ringing") at distant locations. A quake in L.A. will cause tiny tremors in Japan. By first recording these tremors with seismographs and then analyzing the data with the equations of mathematical physics, geophysicists can actually reconstruct the path the waves took through the Earth. More importantly, they can reconstruct the properties of the Earth's inner layers through which the waves traveled. This science of Seismology has been incredibly successful, revealing much about the internal structure of our planet. About twenty years ago, astronomers picked up on this success and asked if a similar technique might be applied to the stars.

Astronomers are always in search of new ways to see into their distant and untouchable objects. In the 1960s and 1970s, intense observations of the Sun showed that the surface of the giant ball of gas was rising and falling with a distinctive five-minute period. After scratching their heads for a while astrophysicists realized that these oscillations of the solar surface were a kind of permanent sun-quake. Powerful waves were running around inside the Sun, bouncing from one location to the other and together they created the periodic heaving of the star's vast surface. The science of "Helioseismology" had been born. By doing a careful and mathematically exhaustive analysis of the ways these waves propagate inside of the Sun, astrophysicists were able to reconstruct the properties of much of the Sun's interior.

Even though the oscillations of the Sun's surface may be short (five minutes or less), doing Heliosiemology properly requires staring at the surface and measuring its ringing continuously. This is not easy to do on a rotating planet. To overcome this difficulty, astronomers learned to cooperate in new ways. Using the power of computers they created the Global Oscillation Network Group or GONG (astronomers, like everyone else, seem addicted to clever, catchy acronyms). GONG consists of telescopes located at different locations around the world. Using the precession timing that is possible with networked computers, GONG team members in Iowa could observe oscillations of the Sun during the day and then "hand" the observing off to a group in Australia as North America swung into night. With the continious observations provided by GONG, the surface oscillations were mapped with such precision that the Suns insides were mapped out down to just above the core where fusion reactions occur.

The success of GONG led astronomers to ask if they could turn Heliosiemology into Astroseismology - that means using oscillations on the surface of stars to recover their internal structure. In 1994 researchers at Århus University in Denmark first attempted to find oscillations on the surface of the star eta Bootis in the constellation of (you guessed it - Bootes) near Arcturus. eta Bootis is a yellow star 3 times the size of the Sun and about 38 lightyears away. After only six nights the team discovered signs of starquakes very similar to what has been seen on the Sun. In the never-ending desire to push at the boundaries of their questions, astronomers have now gone beyond GONG to create the Whole Earth Telescope (WET), a world-wide collaboration of astronomers with telescopes continuously-pointed at the distant stars rather than the Sun.

With the advent of WET and other tools Asteroseismology continues to prove itself. In the last few years its made important gains in showing others how useful this new tool can be. Perhaps the most amazing aspect of the star oscillation story however is the depth of ingenuity humans can call on when they want to know something.

Questions to Ponder

Compare and contrast the difficulties of doing seismology for the Earth, Sun and distant stars. Is there any way that working with the Sun and distant stars might be easier?
What would be the biggest problem with carrying out GONG or WET type observations? Why do computers matter so much?

Check Out This Website

Helioseismology

http://soi.stanford.edu/results/heliowhat.html