Astronomy Jargon 101: Eclipsing Binary

In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll be seeing double with today’s topic: eclipsing binaries!

Our galaxy hosts loads of binary stars. So much so that the majority of all stars in the galaxy are members of a binary system. Astronomers can only find most binary systems through intense scrutiny, either by having a telescope big enough to reveal two or more stars where we once thought there was only one, or by using spectroscopy to notice the wiggling motion of one star caused by the orbit of a hidden companion.

But sometimes the galaxy makes it easy for us, and advertises the existence of a binary. If the orbit of the binary star lines up just right – out of sheer coincidence – then the stars will periodically cross in front of each other, causing their combined brightness to dim. Astronomers call this situation an eclipsing binary, because the two stars are constantly eclipsing each other.

The best-known example of an eclipsing binary is the star Algol in the constellation Perseus. Medieval Arabic astronomers gave the star this name, which means “wonderous”, because they noticed its brightness occasionally changing.

Depending on the sizes of the stars, the brightness of the individual stars, and the size of the orbit, an eclipsing binary may dip in brightness once or twice. If two dips happen, the biggest dip is called the primary eclipse, regardless if it’s the bigger or smaller star causing the reduction in brightness.

Eclipsing binaries are fantastic tools for understanding stars themselves, because they reveal so much useful information. The light curve, which is a map of the change in combined brightness of the two stars, tells astronomers the nature of the orbit and the relative sizes of each star. Using the spectrum of the two stars, astronomers can then calculate their masses. Combining the two reveals the densities of the stars, which is an incredibly useful number used to help understand the nature of stellar interiors.

Since around 1995, astronomers have had telescopes big enough (we’re talking at least 8 meters across) to observe eclipsing binaries in other galaxies, including in the Large and Small Magellanic Clouds and Andromeda.