Keeping Tabs 
The twins of Gemini are dropping feet-first toward the western horizon as night falls this month. One of those feet is marked by a small, faint smudge of light — the star cluster M35. It’s more than 2500 light-years away, and it contains about 150 bright stars.
The cluster is an important target for a team of McDonald Observatory astronomers. They use the positions of M35’s stars to calibrate observations made with Hubble Space Telescope.
The team uses Hubble to precisely measure the positions of stars. As Earth orbits the Sun, the viewing angle to a star changes a bit, and measuring that angle reveals the star’s distance. This technique — known as astrometry — also makes it possible to measure the masses of planets orbiting other stars by measuring the motions of the parent star.
The measurements are extremely precise, but they also require the Hubble instruments to be precisely calibrated — and that’s where M35 comes in. The positions of the stars in M35 are well known, so the Texas astronomers compare the known positions with what they see through the telescope.
BENEDICT: So you can look and say, “All right, here’s the position of this star, this star, this star, this star.” We know what their positions are, but we measure this. So we measure this, and if the position is different than what the catalog says, our instrument is making it different. And so to make the instrument give us the same answer, that’s the calibration. And so we apply the calibration, and then we measure those stars again and they agree perfectly with the catalog.
So keeping tabs on the stars of M35 helps the astronomers track many other stars throughout the galaxy.
Script by Damond Benningfield, Copyright 2013
Find out much more about Hubble Space Telescope and its role in this research on our special web site,Galileo to Gamma Cephei .