Jimmy Westlake: Vanishing Star
Have you ever watched a star disappear? There are lots of stars that periodically fade and brighten, but there are only two that actually vanish from view, altering the shape of their constellation.
The first and best known is Mira, meaning the “Wonderful Star,” in the constellation Cetus the Whale. Mira serves as the prototype for its class of long-period variable stars.
The second, and the one that interests us today, is the Mira-type variable star named Chi in the constellation Cygnus the Swan. Chi Cygni is a red supergiant star nearing the end of its life. It is so bloated in size that it would spill over the orbit of Mars if placed in the middle of our solar system.
At its center, Chi Cygni has a contracting core of carbon and oxygen surrounded by shells of gas that are fusing hydrogen and helium. This situation makes a star very unstable. Throbbing like an overgrown heart, Chi pulsates with a period of 400 days, and in so doing changes its light output by a whopping factor of 10,000. This light variation takes Chi from easy naked-eye visibility to far below the naked eye limit. After shining brightly for a few weeks, the star literally disappears from view for many months.
Chi Cygni is putting on a memorable show this month, glowing brighter than it has in 148 years. It adds an extra star to the long arm of the familiar Northern Cross asterism that makes up most of our constellation of Cygnus the Swan.
Here’s how to locate the Northern Cross and Chi Cygni – after dark, go outside and face the eastern sky. High up in the east, you’ll spot the familiar Summer Triangle of bright stars: Vega, nearly overhead; Altair, to the southeast of Vega; and Deneb, to the northeast of Vega. Deneb is the bright, blue star that marks the top of the Northern Cross. From Deneb, form a line connecting to the star Albireo, located at the center of the Summer Triangle. This forms the long arm of the Cross. The shorter arm of the Cross is formed by three stars that intersect the long arm at a right angle up near Deneb. Midway between the star Albireo and the star at the intersection of the crossbar, there are two fainter stars along the same line. The star closer to Albireo is Chi Cygni.
Take a good look now, because in a few weeks, Chi will disappear from view and won’t be back for nearly a year.
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