We always try to over-deliver at One-Minute Astronomer. Today, instead of one dazzling sight, we present two: a pair of colorful double stars on display near the celestial equator.
• The nearby star 70 Ophiuchi is one of the best-known and widely studied binary star systems. It’s found in an odd V-shaped collection of five stars called Poniatowski’s Bull. This strange little asterism is easy to see with the naked eye from suburban skies; if you’re in the city, try your finderscope or binoculars. 70 Oph is on the east side of the “V”; it’s near the celestial equator, so it’s visible from the northern and southern hemispheres.
• The two components of 70 Oph have magnitude 4.2 and 5.9; the brighter star is a yellow-gold while the fainter looks orange-red, with some observers reporting a tinge of violet. Move your telescope out of focus just a touch to see the colors well. Each star has an intrinsic brightness only a fraction that of our Sun.
• The components complete a revolution about each other in only 88 years. And because the system is only 16 light years away, the stars are close enough for you to resolve in a backyard telescope. So this is one of the few double stars you can see revolve about each other during a human lifetime.
• The average distance between the stars is about the same as the distance between the Sun and Uranus. The stars were closest together in 1989. Since then, their separation has quickly increased from 1.7 arc-seconds to about 5 arc-seconds. You’ll need at least 100x to resolve them cleanly with a telescope.
A Deeper Look
• Nearby in the sky to 70 Ophiuchus, though physically much farther away, is alpha Herculi. Also known as Rasalgethi (Arabic for “The Head of the Kneeler”), this is another splendid star for visual observation. The components appear orange and blue-green… a very pretty sight.
• Again, you’ll need a scope at 100x to resolve these two: they’re separated by about 4.6 arc-seconds. Unlike 70 Ophiuchi, these stars take a long time to revolve around each other… nearly 3,600 years.
• The brighter star of alpha Herculi is an enormous red giant that’s 400 times the diameter of the Sun and some 800 times as bright. The stars have magnitudes of 3.5 and 5.4 roughly, and are some 380 light-years away. The fainter star is itself a closely-space double with an invisible white dwarf companion.
Good To Know
The brighter component of alpha Herculis is also a variable star with an irregular period that averages around 90 days. The magnitude varies from around 3.5 down to 4.0. See if you can notice the change in brightness over the course of a month or two by comparing it to the fainter component.
To the discerning observer, double stars are a delight. You don’t need dark skies to find and enjoy them, the color contrasts are sometimes beautiful, they provide an excellent test and challenge for your optics, and they’re a great measure of atmospheric seeing.