Tuesday, June 5, 2007

Images of Distant Stars

Last week, astronomers from the University of Michigan announced that they have successfully obtained the first image of a distant Sun-like star. Using a technique known as optical interferometry, the scientists combined the light from a group of four small telescopes scattered across the top of a mountain in California -- effectively creating one huge telescope more than 200 meters across. The resulting image is about 100 times sharper than the view from the Hubble Space Telescope.

Astronomers have been making images of the nearest star (our Sun) since the time of Galileo, but other stars are so far away that even the most powerful telescopes see them as single points of light. In 1995, the Hubble Space Telescope obtained the first direct image of a distant star -- the supergiant Betelgeuse in the constellation Orion, which is so enormous that it would span the orbit of Jupiter in our solar system. The technique of interferometry -- combining the signals from many individual telescopes to produce a sharper image -- has been used in radio astronomy for a long time, most famously in New Mexico at the Very Large Array that was featured in the movie Contact, with Jodie Foster. Recent advances in technology have now made it possible to do a similar thing with optical telescopes.

The target of the University of Michigan study was Altair, the brightest star in the constellation Aquila, which is about 80% more massive than the Sun and spins about 60 times faster. It's rotation is so fast -- more than 600,000 miles per hour at the equator -- that Altair bulges out around the middle. The new image not only reveals this flattened shape, it also shows that the star is much cooler around the equator than near the pole. Although this feature was expected, the predictions do not match the observations exactly -- suggesting that astronomers may need to improve the theory.

There are several arrays of telescopes around the world that are designed for this type of imaging. As the technology continues to improve, imaging of distant Sun-like stars will become routine. This is just the beginning of an exciting era for interferometry.