Stephen Tonkin's Binocular Tour
A wild variable, a spectral oddity and a deceiving line of sight pair lurk in the region of Canes Venatici
Tick the box when you’ve seen each one 1 M3
M3 is one of the best globular clusters in the northern sky. It’s found at the midpoint of a line from mag. +3.0 Seginus (Gamma (a) Boötis) and mag. +4.3 Diadem (Alpha (_) Comae Berenices), appearing as a severely defocused star. If you use averted vision by shifting your gaze to the bright star 0.5º to the southwest, you should notice that the defocused ‘star’ appears to brighten and grow. This is the glow of the globular’s half a million stars.
2 17 CANUM VENATICORUM
Find mag. +2.9 Cor Caroli (Alpha (_) Canum Venaticorum) and navigate just over 2.5° in the direction of Seginus. You’ll see a widely separated (4.6 arcminutes) pair of stars: mag. +5.9 17 Canum Venaticorum and mag. +6.3 15 Canum Venaticorum. This line-of-sight pairing illustrates how distance affects magnitude: although they appear to be of similar brightness, 15 Canum Venaticorum is six times farther away than 17 Canum Venaticorum, but 25 times as bright. 3 V CANUM VENATICORUM
Halfway between Cor Caroli and mag. +1.9 Alkaid (Eta (d) Ursae Majoris) there is a pair of 6th-magnitude stars separated by about 0.25°. Locate them, then pan 1.5° in the direction of 21 Canum Venaticorum where you should find, shining somewhere between mag. +8.5 and +6.5, V Canum Venaticorum. We are vague about the expected magnitude because, although it is often described as a variable with a period of 191.5 days and a range of +7.7 to +6.6, both its range and period have fluctuated wildly in recent years.
4 LA SUPERBA
Locate mag. +4.2 Chara (Beta (`) Canum Venaticorum) and pan 4.5° towards mag. +2.2 Mizar (Zeta (c) Ursae Majoris). You’ll find a pale orange star (it looks a deeper orange in larger apertures). This is Y Canum Venaticorum, a cool carbon star with a magnitude that varies from +6.3 to +4.7 in a period of about 160 days. Its common name, La Superba, may have led you to expect a more impressive sight, but it was not given because of its colour, but on account of its unusual spectrum, in which absorption lines from carbon compounds weaken much of the light from blue-violet end. 5 M94
Our next target is a galaxy. Return to Cor Caroli and imagine a line between it and Chara. At the halfway point, shift your gaze at a right angle for slightly more than 1.5º, towards Alkaid. Here you should find the faint glow of light that is mag. +8.9 spiral galaxy M94, though you may need to use averted vision. Viewing it is easier under dark, transparent skies, so if you are unable to see it in 10 50s either wait for better conditions or try larger binoculars. 6 UPGREN 1
We finish with the enigmatic Upgren 1: is it a very old open cluster, two open clusters in the same line of sight, or merely an asterism? Return to the line between Cor Caroli and Chara, and imagine it as being one side of an equilateral triangle with the third apex to the southwest. Just inside this third apex, slightly closer to Cor Caroli, you should see a group of six 8th- and 9th-magnitude stars spanning about 14 arcminutes. Whatever its nature, this sole member of the Upgren Catalogue is a very pretty binocular object.