Sky Object of the Month – July 2014

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Sky Object of the Month – July 2014

Rasalgethi (alpha [a] Herculis)  – Double Star in Hercules

by Glenn Chaple

          A number of years ago, I conducted a “Top Ten Doubles” survey for Deep Sky Magazine. One of the top vote-getters was Rasalgethi, the alpha (a) star in Hercules. Few would dispute its inclusion on the list.

For starters, it’s comprised of bright stars – the magnitude 3.5 primary being attended by a magnitude 5.4 partner. Secondly, a reasonably close separation of 5.0 arc-seconds puts it within range of small-aperture telescopes while making for an eye-pleasing sight in larger instruments. Most notable are the colors. Through my 127mm f/12 Maksutov/Cassegrain reflector and a magnifying power of 171X, they seemed to be orange-yellow and light blue – typical of the hues reported by other observers. The ruddy color of the primary is in keeping with its M5 spectral class; the bluish tint of the secondary is not. With a spectral class of G5, it should be yellow like the sun. The color discrepancy seems to be a result of the magnitude difference between the two stars. Quite often the secondary of a close unequal double star takes on a bluish tinge, regardless of spectral class.

Not only is Rasalgethi a noteworthy telescopic target, it’s a noteworthy system – period! The main star is a red supergiant with a diameter of 340 million miles. Were it put in place of our sun, it would engulf the entire inner solar system to beyond the orbit of Mars. Like many stars of its class, Rasalgethi varies in brightness. Its 2.7 to 4.0 magnitude fluctuations occur in a 4-month cycle superimposed over one of 6 years. The companion is no slouch, either. It’s a tight binary pair comprised of a four-solar-mass G5 giant circled every 52 days by a 2.5-solar-mass F2 dwarf. Hipparchus data place the Rasalgethi system at a distance of 360 light years. The actual separation between the two main components is roughly 46.5 billion miles. Their orbital period likely exceeds 3500 years.

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                        Jeremy Perez (


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