• This evening, we zoom in on a variable star -- a star whose brightness changes -- near the bright star Vega in the small but distinctive constellation Lyra the Harp.

• Here's how to locate it. A dark sky brings out the four rather faint stars to the left of Vega on December evenings.

These stars form a parallelogram -- a four-sided figure with its opposite sides equally long and parallel to one another. Three fingers at arm's length are enough to cover over this small parallelogram of stars.
• The two stars shining on the left-hand side of the parallelogram are Sulafat and Sheliak. Sheliak - also known as Beta Lyrae - is the variable star.
• This star varies because -- although Sheliak looks single to the eye -- it's really a binary star, or two stars that revolve around one another. Sheliak is a special kind of binary star system, known as an eclipsing binary. One star in the Sheliak system blocks out the light of its companion star in regular periods, as seen from our earthly vantage point. This blocking of one star by the other causes Sheliak's brightness to dim every 6.5 days.

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