Stephen Tonkin’s Binocular Tour

December leads us into Cetus, where a cosmic question and an almost stellar galaxy await

Tick the box when you’ve seen each one

1 ZIBAL

Zibal (Zeta (c) Eridani) is an easy double star located 4.75º east of mag. +3.9 Azha (Eta (d) Eridani). Zibal is a pure white star that shines at mag. +4.8; look 5 arcminutes northwest to find its golden, mag. +6.6 companion. This is a line of sight pairing: the companion is intrinsically 1.6 magnitudes brighter, but appears 1.8 magnitudes fainter due to the fact it is four times farther away. SEEN IT

2 THE QUESTION MARK

We continue with a good star party piece. Identify mag. +2.5 Menkar (Alpha (_) Ceti) and move 7° west to Nu (i) Ceti. This mag. +4.9 yellow-white star is the dot at the bottom of a 2.25° ‘question mark’, which can be seen directly above when the Nu Ceti is in the south. The curve of the question mark is comprised of four yellowish 6th-magnitude stars, whilst the 7th-magnitude star in the middle of the upright is a brilliant white. SEEN IT

3 M77

Mag. +8.9 galaxy M77 is considerably more challenging to see, and ideally should be viewed under a dark, transparent sky. It is, however, easy to locate – look 52 arcminutes east of mag. +4.1 Delta (b) Ceti. Compact and nearly round, M77 could easily be confused for a globular cluster. It is a Seyfert galaxy, meaning that most of its light comes from an active nucleus where gas is spiralling into a black hole. This makes it quite easy to see the nucleus, but any light pollution will obscure the galactic disc. As a result the galaxy looks stellar, and thus is difficult to identify in binoculars. SEEN IT

4 MIRA

This star is the prototype of a whole class of objects, the Mira variable stars. Mira (Omicron (k) Ceti), the name of which derives from the Latin ‘mirabilis’ (meaning ‘amazing’), brightens from mag. +9.3 to +3.4 every 11 months, and will reach peak brightness once more in January. Mira variables are red giants in the late stage of their evolution. They pulsate in size; as they expand, they cool and radiate a higher proportion of their luminosity in infrared. As a result much less optical light is emitted and they disappear to the human eye. SEEN IT

5 37 CETI

If you look 1.5° northwest of the middle of a line between mag. +3.5 Deneb Algenubi (Eta (d) Ceti) and mag. +3.6 Theta (e) Ceti you will see a mag. +5.1 star that makes an isosceles triangle with the these two. This is the double star 37 Ceti. Its components are 49 arcseconds apart, which should be an easy split even using lower magnifications, but the magnitude of the companion is only +7.9, which is 13 times dimmer. This magnitude difference can make it tricky to split, so we’d recommend using 15x70s and mounting them for steadiness. SEEN IT

6 HD 3807 GROUP

Imagine a line joining Deneb Algenubi and mag. +3.6 Deneb Kaitos Shemali (Iota (f) Ceti); running parallel, approximately 5° north, is a 6°-long row of six 6th- and 7thmagnitude stars. The brightest of these is 13 Ceti. The star 1.5° to its east-southeast is HD 3807. This is the southern corner of a rectangular asterism with diagonals of about 1.25°. The rectangle is brighter on the southeastern side and has a void in the northern quadrant. HD 3807 itself is a double star, shining at mag. +5.9. This is 10 times brighter than the mag. +8.4 secondary that lies 63 arcseconds to the west. SEEN IT