According to Levin, pairs of orbiting stars locked together by gravity are called what?

• A. Binaries
• B. Planets
• C. Clusters
• D. Galaxies

Answer: (A)

Two stars bound by gravity that orbit a common center of mass are called binary stars. The gravity between the two stars pulls them into orbital motion around their shared barycenter, so they move in a pair rather than alone. This setup follows the same gravitational rules that govern other orbital motion, and studying their orbits lets astronomers measure the masses of the stars because the orbital size and period are directly related to the total mass. There are several ways we detect them: if the two stars can be seen separately, it’s a visual binary; if their spectral lines shift back and forth as they move, it’s a spectroscopic binary; and if one passes in front of the other from our viewpoint, causing eclipses, it’s an eclipsing binary. Planets orbit stars, not two stars together; clusters and galaxies involve many stars, not just a single pair. So the term for pairs of orbiting stars is binary stars.