Readers of these observing articles will know that I have a fondness for observing double and multiple stars. These may take the form of a stellar pair bound in a gravitational embrace (a binary), or two or more unrelated stars at greatly differing distances from Earth that happen to lie in the same line of sight (an optical double or triple).
Some doubles might be so close that it can be a challenge to resolve them into two separate stars, but their appeal often lies in the beautiful contrasting colours they present in the eyepiece. In an earlier article and its follow-up I led you to some showcase multiple stars of the summer sky, but here are some more to be enjoyed even in deep twilight, moonlight or from light-polluted areas.
You may click on the finder chart above or click here to download a high-resolution finder chart suitable for printing and use at the telescope. Beside each of the simulated views below you will find J2000.0 epoch coordinates for entering into the hand controller of a GoTo telescope or mounts equipped with digital setting circles (DSCs).
Alpha (α) Herculis aka ‘Rasalgethi‘
α = 17h 14.6m, δ = +14° 23’ (J2000.0)
Magnitudes: +3.5 & +5.4
Separation: 4.6 arcseconds
Currently high in the southwest of the UK at local midnight, Rasalgethi (or more prosaically, Alpha Herculis) is a fine binary star for small telescopes at about 120× magnification with components separated by 4.6 arcseconds. The brighter star is an orange-red semi-regular variable star ranging from magnitude +3.1 to +3.9 over a period averaging 90 days, while the magnitude +5.4 companion looks greenish in hue. The Rasalgethi system has an orbital period of about 3,600 years and its distance from Earth is in the region of 370 light-years.
61 Cygni aka ‘The Flying Star‘
α = 21h 06.9m, δ = +38° 45’ (J2000.0)
Magnitudes: +5.2 & +6.0
Separation: 31.7 arcseconds
Sixty-one Cygni is an easy 31.7-arcsecond double at 30× magnification with magnitude +5.2 and +6.0 components that lie 8 degrees southeast of Deneb, the brightest star in the constellation of Cygnus. Both the primary and companion of 61 Cygni appear orange-red. This double possesses the largest proper motion of any star just visible to the naked eye, leading Giuseppe Piazzi to call it the ‘Flying Star’ in 1792. Large proper motions are usually indicative of proximity, hence 61 Cygni was the first star system to have its distance measured by Friedrich Wilhelm Bessel in 1838. Some 11.4 light-years away, the pair orbit one another every 650 years and are the twelfth nearest star system to Earth.
Σ 2816 (Cepheus)
α = 21h 38.9m, δ = +57° 30’ (J2000.0)
Magnitudes: +5.6, +7.7 & +7.8
Separation: 11.8 & 19.9 arcseconds
Using Deneb as our starting point, head 15 degrees north to locate this glorious triple star in the constellation of Cepheus. Struve 2816 (written Σ 2816 in catalogues) actually lies near the heart of IC 1396, an open star cluster associated with a large, faint region of nebulosity commonly known as the Elephant Trunk Nebula. While the nebula is largely an astrophotographic target, the embedded open cluster makes an attractive backdrop, particularly when you catch sight of this beautiful trio of stars in the centre. While in the vicinity, look a further 1¼ degrees to the northeast of Σ 2816 to find Mu (μ) Cephei, a glorious deep-red variable better known as Herschel’s famous ‘Garnet Star’.
Mu (μ) Draconis aka ‘Alrakis‘
α = 17h 05.3m, δ = +54° 28’ (J2000.0)
Magnitudes: +5.6 & +5.7
Separation: 2.6 arcseconds
Finally, a very close but beautiful double star with almost identical components that’s a high-power treat. With a combined magnitude of +5, Alrakis, or Mu (μ) Draconis, presently lies almost overhead near the head of Draco around midnight as seen from the UK, some 22 degrees (or the span of an outstretched hand at arm’s length) to the upper right of Vega in Lyra. A genuine binary, the two stars of μ Draconis have an orbital period of 672 years and lie 90 light-years from Earth. With 10-cm (4-inch) aperture ‘scopes and larger, use powers of 150× or more when seeing permits to view this yellowish-white pair like distant car headlights on a country road.