SPACE.com -- Seeing Double

The double-double Epsilon Lyrae in Lyra and the contrasting star Alberio in Cygnus the Swan are two fine binary stars for the small telescope.

Acrux in Crux the Southern Cross is a beautiful close binary with components that share nearly the same brightness and color.

The inner solar system this week.

Top: The sky as seen from mid-northern latitudes; Bottom: The sky as seen from mid-southern latitudes. Both are at 9:30 p.m., facing south. The curved line represents the plane of our solar system, called the ecliptic.


	Seeing Double By Jeff Kanipe posted: 30 June 2005 08:01 am

Of the millions of stars that can be seen in the night sky, more than half possesses one or more companion stars. Fortunately for skywatchers, many are visible in small telescopes.

Double stars can either orbit about a common center of gravity, in which case they are called binary stars, or they may appear in chance alignments that make them appear associated when in fact they have very different distances. These are called optical doubles.

Double stars come in an infinite variety of colors, brightness, and separations. One such study in contrasts is Alberio, which marks the head of Cygnus the Swan. Alberio is often said to be the finest double star in the heavens. The primary member is bright yellow, while the fainter component has a bluish tint. The pair's generous separation of 35 arcseconds makes this an easy object for low-power telescopes. (One arcsecond is equal to the apparent diameter of a dime seen one mile away.)

In the Southern Hemisphere, Alpha Crucis is an example of two stars of nearly the same brightness and color, but with only a hair's separation between them. This star - thirteenth brightest in the night sky - marks the foot of the Southern Cross. Its components are tinted blue-white but have a separation of only 4 arcseconds.

Double and triplet stars are fairly common. In rare instances, however, double stars come with multiple members. One famous example is Epsilon Lyrae - known as the "Double-Double." Epsilon lies northeast of Vega, the brightest star in Lyra the Lyre. A 4-inch telescope at medium magnification splits Epsilon into two stars with a separation of 208 arcseconds (or 3.4 arcminutes). Increased magnification, however, reveals that each component has a companion separated by a little over 2 arcseconds, making this a challenging but exceptional quadruple star for the small telescope.

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