Stephen Tonkin’s Binoc­u­lar Tour

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

Sky at Night Magazine - - CONTENTS -

Tick the box when you’ve seen each one


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


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.


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


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


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

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