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Most of the stars that you see in the night sky aren’t alone — they consist of two stars or more. They’re so far away, though, that their light merges into a single pinpoint.
But it can be tough to tell which stars are single and which aren’t. Consider Denebola, the star at the tail of Leo, the lion. It’s high in the southwest at nightfall, well to the upper right of bright orange Mars.
Seen through a telescope, Denebola has several close companions. Detailed study, though, shows that none of the stars are bound to Denebola — they just happen to line up in the same direction.
Astronomers use several techniques to figure out which stars have true companions.
The simplest is to track the stars over a period of years and see how they move. Stars that are gravitationally bound stay close together, and make a loop around each other.
Another way is to look for eclipses. If a system’s orbit happens to line up along our line of sight, then the stars may periodically pass in front of each other, causing the system’s light to fade a bit.
And for stars that are especially close together, astronomers use spectroscopy — they split the light of the target star system into its individual wavelengths. Each star’s spectrum produces a unique pattern, like a barcode. If two or more stars are orbiting each other, then the astronomers see two or more barcodes that shift back and forth a bit — the signature of close companions among the stars.
Script by Damond Benningfield, Copyright 2014