Changing Planet

Astronomers Clue in to Why Binary Stars Are So Bountiful

Above, an artist has depicted the Kepler-35b, a Saturn-size world orbiting its host binary star system.
Courtesy of Mark A. Garlick / space-art.co.uk

Everyone needs some alone time—even stars. Astronomers now think they have an explanation as to why so many stars are single or double stars.

While most stars are born in clustered stellar nurseries, the great majority we see across the cosmos spend their lives as loners or double stars.  A new study published this week in the Astrophysical Journal suggests why so many stars we see when we look up at the night sky wander away from their nests.

Nearly two-thirds of the stars within 81 light-years of Earth, it turns out, are either part of a binary or multiple system. In the study, astronomer Alan Boss from the Carnegie Institution of Washington combined observational databases and theoretical modeling to reveal that while most stars are indeed born within clusters, including the sun, many are simply kicked out of their home group because they cause gravitational instability for the rest of their family.

What Boss’s modeling shows is that magnetic fields emanate from the newborn stars and determine the extent of a star-forming cloud’s fragmentation. Above a certain magnetic field strength, single infant stars are formed, while below it, the clouds fragment into multiple ones.

And because of the obvious prevalence of binary star systems, this cloud fragmentation process is most likely what is part of the heritage of most stars we see around us.

“When we look up at the night sky, the human eye is unable to see that binary stars are the rule, rather than the exception. These new calculations help to explain why binaries are so abundant.” explained Boss, in a statement.

See for Yourself

Binary star systems are some of the easiest stargazing opportunities for novices and for those stuck within city limits. Here are a few of the finest examples, cinches to hunt down using nothing more than your eyes, binoculars, or small backyard scopes.

Mizar and Alcor: This famous, and easiest, of all doubles appears as the middle ‘star’ in Big Dipper’s handle.  Centuries ago, the ability to separate Mizar and Alcor was a test for good vision among hunters in ancient Persia and was known as the horse and rider.

This starchart show the Big Dipper in the lower northeast evening sky in early fall with the stunning Mzar/Alcor binary star system.  Credit: SkySafari
This star chart show the Big Dipper in the lower northeast evening sky in early fall with the stunning Mizar/Alcor binary star system. Courtesy of SkySafari

However, another surprise is in store if you train your telescope on the stellar pair.  Look closely at the brighter of the two, Mizar, and you will notice through the eyepiece that it itself is a binary, owning a fainter companion.

Incredibly, each of these stars are doubles themselves, making Mizar a quadruplet star system. But these little companions are out of reach of backyard scopes and can be detected only with spectral readings from big observatories.

 

This wide angle star chart shows the location of the famous double-double star system in the constellation Lyra, next to the bright star Vega, which marks one corner of the Summer Triangle in the early autumn evening sky. Credit: SkySafari
This wide-angle star chart shows the location of the famous double-double star system in the constellation Lyra, next to the bright star Vega, which marks one corner of the Summer Triangle in the early autumn evening sky. Credit: SkySafari

Double-Double in Lyra: Epsilon 1,2 Lyrae is a another quadruplet star system, a wide double-double pair of stars right next to Vega, the brightest star we can see near overhead in evening skies at the end of September across the Northern Hemisphere. This special binary should be easy to track down with binoculars even from cities, thanks to its proximity to the Summer Triangle formation.

And again, just like with Mizar and Alcor, if you look with a small telescope, each is double in itself when viewed at high magnification.  An amazingly pretty sight not to be missed.

Happy Hunting!

Andrew Fazekas, aka The Night Sky Guy, is a science writer, broadcaster, and lecturer who loves to share his passion for the wonders of the universe through all media. He is a regular contributor to National Geographic News and is the national cosmic correspondent for Canada’s Weather Network TV channel, space columnist for CBC Radio network, and a consultant for the Canadian Space Agency. As a member of the Royal Astronomical Society of Canada, Andrew has been observing the heavens from Montreal for over a quarter century and has never met a clear night sky he didn’t like.

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