Stephen Tonkin’s Binocular Tour

Start this month’s tour with a historical eye test DQG QLVK LW E\ SUDFWLVLQJ \RXU DYHUWHG JD]H

Tick the box when you’ve seen each one

1 MIZAR AND ALCOR

Before the invention of eye charts, stars were used to test visual acuity. When you could no longer see that the mag. +2.2 Mizar (Zeta (c) Ursae Majoris) and its mag. +4.0 companion, Alcor (80 Ursae Majoris), were separate stars, you needed spectacles. They are easy to split in binoculars, which also reveal a mag. +7.6 companion that lies 6 arcminutes south of Alcor and 8 arcminutes east of Mizar. Extend this line from Mizar through the fainter companion for 2.5° and you come to a reddish star, mag. +4.6 83 Ursae Majoris. SEEN IT

2 OYY123

Our next stop is the double star, OYY123, at the end of a 4° long chain of stars that extends west from Thuban (Alpha (_) Draconis). At mags. +6.6 and +7.0, the components are of a similar brightness that, combined with a separation of 69 arcseconds, makes them easy to split with hand-held binoculars. The ‘OYY’ designates Otto Wilhelm von Struve’s catalogue of double stars. SEEN IT

3 PHERKAD AND PHERKAD MINOR

Pherkad (Gamma Ursae Minoris) is one of the ‘Guardians of the Pole’ (the other is Kochab). It shines at mag. +3.0 and is the southernmost star of the ‘bowl’ of Ursa Minor’s Little Dipper asterism. The mag. +5.02 pale orange Pherkad Minor (Gamma-1 Ursae Minoris) is easy to spot, 17 arcminutes to the west. The stars are not a true binary, they’re not even gravitationally bound to each other: Pherkad is 487 lightyears away while Pherkad Minor is 398 lightyears away, and they’re moving in different directions.

4 POLARIS ASTERISM

Many astronomers see Polaris (Alpha (_) Ursae Minoris) merely as a convenient marker for the North Celestial Pole (NCP), oblivious to the asterism of which it is part. 10x50 binoculars reveal that the mag. +2.0 Polaris blazes in a ring of mostly 8th and 9th magnitude stars, nearly a degree wide. One of the stars in the circlet is slightly displaced from Polaris, which bisects the line joining it and the NCP, enabling this more precise determination of its location. SEEN IT

5 KAPPA DRACONIS ASSOCIATION

Kappa (g) Draconis is a hot (14,000K) B-type star that’s 540 times more luminous than the Sun. To the north is a pair of orange K-type stars, the brighter of which is the mag. +4.9 6 Draconis, which is only about 300 times as luminous as the Sun. To the south is a star with a similar luminosity, the long-period pulsating variable (mag. +4.9 to +5.0) 4 Draconis. This is a cool M-type star whose surface temperature is a ‘mere’ 3,940K. SEEN IT

6 M81/82 GALAXY PAIR

In the north polar region of the sky you can find the galaxy pair M81 (Bode’s Nebula) and M82 (The Cigar Galaxy). Take a line from Phecda (Gamma (a) Ursae Majoris) through Dubhe (Alpha (_) Ursae Majoris) and extend it the same distance to the northwest. The galaxies should be at the end of this line; M81 is the brighter of the pair. The two galaxies are a useful target upon which to practise averted vision: as you direct your gaze at one, the other appears more clearly. SEEN IT