StarDate: March 24, 2009

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Astronomers have a lot of ways to study the stars and galaxies that fill the universe. They can take pictures. They can split light apart to learn what an object is made of and how it's moving through space. And they can measure how its brightness changes over time.

Another important measurement is an object's position on the sky. Tiny changes can reveal the object's distance from Earth, or tell us whether it has companions.

Measuring positions in the sky is called astrometry.

Astronomers began using this technique in the 1800s to measure the distances to the closest stars. As Earth orbits the Sun, nearby stars appear to shift back and forth a little against the background of more-distant stars.

Later, they used the technique to measure stars' true motions through the galaxy.

Today, astrometry provides far more information.

A team led by Texas astronomer Fritz Benedict, for example, is using Hubble Space Telescope to measure tiny changes in the positions of stars caused by the tug of orbiting planets. These observations reveal the masses of the planets, as well as how planetary systems are laid out.

The researchers use Hubble's Fine Guidance Sensors -- instruments that keep the telescope pointed in the right direction in space. The sensors are also good instruments for astrometry, because they can detect incredibly tiny motions in the sky -- like those caused by extrasolar planets. More about that tomorrow.

Script by Damond Benningfield, Copyright 2009

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