Gamma Cephei II 
The art and science of astrometry is all about making precise measurements of tiny angles in the sky. Those angles are often described with analogies. Measuring a given angle might be like measuring the size of a quarter held 1500 miles away, or the width of a gnat’s eyelash from opposite shores of Lake Gitche Gumee. But that eyelash can be critical — it can tell us a lot about a star and its environment.
Consider Gamma Cephei, which is about 45 light-years away, in the northern constellation Cepheus. It consists of two stars and one known planet. But the technique that was used to discover the planet doesn’t reveal its mass. Nor does it reveal how the system is laid out — whether all three objects move in the same plane, as though they were all on a tabletop. That measurement is important for understanding how all planetary systems were born, including our own.
A team led by Texas astronomer Fritz Benedict has been using Hubble Space Telescope to plot the position of the larger of Gamma Cephei’s stars, which is the one with the planet. The star is pulled back and forth a tiny bit by the planet’s gravity. Measuring that shift, along with the orbit of the two stars, will reveal the mass of the planet and the system’s layout.
Even with Hubble, though, it’s a tough measurement to make. The researchers have to account for a lot of other motions — some of which aren’t so easy to isolate. More about that tomorrow.
Script by Damond Benningfield, Copyright 2012