Chi Cygni: A Wide-Ranging Variable

Partway up the base of the Northern Cross is Chi Cygni, whose orange-red tint will help you know you’re looking at the right star. It swings through a huge brightness range — from 5th to 13th magnitude and back — every 13 or 14 months. The numbers in italics are comparison-star magnitudes to tenths, with the decimal points omitted, for use when the star is brighter than magnitude 7.5. North is up, east is left.
Sky & Telescope diagram.
For more than three centuries, astronomers have been following the dramatic brightness changes of a star that lies partway up the base of the Northern Cross, about 8° from Albireo. In July 1686, Gottfried Kirch in Berlin noticed that Chi (χ) Cygni was missing from the sky. Not until the following October could the star be seen again. It takes 13.4 months, on average, to run through its complete brightness cycle.

Chi typically peaks near magnitude 5.2, after being only 13th magnitude just six months earlier. But the star can be up to two magnitudes brighter or fainter at maximum. For example, former Sky & Telescope editor Joseph Ashbrook found that Chi Cygni reached magnitude 4.6 on September 15, 1969. On at least one previous occasion it rose to 3.3, the brightest that any long-period variable has become except for Mira (Omicron Ceti).

In 2005 Chi Cygni came to maximum a little earlier than expected, peaking at about magnitude 5.3 in early July, judging from preliminary data of the American Association of Variable Star Observers (AAVSO). As of September 5, 2005, is was down to 7.9, still in reach of binoculars, and fading on schedule.

Use the chart here to keep watch on this wide-ranger! This star is one of those for which the AAVSO (www.aavso.org) seeks regular brightness estimates from its members — continuing an observational series that runs back for centuries.

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