The March 2005 issue mentioned a newly discovered stellar neighbor, SIPS 1259-4336. How do astronomers use parallax to determine if a star is a close neighbor of the Sun?

The two images (above) that appeared in the news story “New star found near Sol” show the star’s proper motion, not its parallax. Proper motion is the apparent movement of a star across the sky year after year. It results from both the star’s movement through space and our own. In the same way, as you drive down the highway, nearby objects — such as road signs — appear to pass by fast, whereas distant objects — such as mountains — seem to move slowly. The two images of the new star were taken 10 years apart.

A large proper motion is a great way to discover a nearby star, but it’s not a good way to measure distance. The size of the proper motion depends on both the star’s distance from us and its motion through space. For example, the stars Alpha (α) Centauri and Mu (μ) Cassiopeiae have almost equally large proper motions — but Mu Cas is nearly 6 times farther away than Alpha Cen. The proper motions appear equally large because Mu Cas moves through space nearly 6 times faster. Alpha Cen has a large proper motion primarily because it’s close to us; Mu Cas has a large proper motion primarily because it moves through space so fast.

To determine a star’s distance, astronomers measure its parallax — the small shift that occurs in a star’s apparent position as Earth orbits the Sun. The larger the parallax, the closer the star is to Earth. Right now, astronomers have measured a rough parallax for the new star, but, by early next year, they hope to have a precise one. Then we’ll know exactly how close our new neighbor is. — KEN CROSWELL, BERKELEY, CALIFORNIA