Deep-Sky Tour
A series of objects that get smaller but more intense around the foot of Castor
Tick the box when you’ve seen each one
1 COLLINDER 89 This month’s Deep Sky Tour is based around the foot of the twin Castor in Gemini. This is marked by a distinctive curved line of stars including Tejat Posterior (Mu (+) Geminorum), Tejat Prior (Eta
(d) Geminorum) and 1 Geminorum. Balanced on Castor’s foot are numerous open clusters the largest of which, on paper at least, is Collinder 89. This has a visual magnitude of +5.7 but is large, sparse and best seen with a telescope using a low magnification, wide-field eyepiece. Even then it’s easy to overlook because it lacks any real concentration. It runs between mag. +6.1 8 Geminorum and mag. +7.0 12 Geminorum and has a line of 7th magnitude stars crossing its centre. The Milky Way runs through this region providing a rich background of faint stars. Ĵ SEEN IT 2 IC 443 If you imagine a line joining Mu and Eta Geminorum, our next target, the supernova remnant IC 443, occupies the half of that line closest to Eta. IC 443 is also known as the Jellyfish Nebula because of its resemblance to a swimming jellyfish in longexposure photographs. Visually, the object is quite challenging and requires the use of an 8-inch or larger scope with visual assist filters such as OIII and UHC to see convincingly. A good, dark sky is also highly recommended. Even then, it’s only the brighter portion that is visible, the arcing glow that occupies the northeast portion of the nebula. Through the eyepiece this appears like a faint glowing arc of light approximately 20 arcminutes in length. Larger instruments show the glow to be knotted in appearance. SEEN IT 3 M35 We mentioned how large Collinder 89 looks on charts and how sparse it looks through an eyepiece. Our next target, M35, looks smaller on charts but is far more impressive through a telescope. It marks the north-pointing right angle in a triangle formed with Eta and 1 Geminorum. At mag. +5.3 it’s visible to the naked eye as a faint smudge. M35 occupies an area around 0.5° across and contains upward of 500 members, about 120 being brighter than mag. +13. A number of orange stars contrast beautifully with the general blue-white colour of the majority. Numerous star arcs can also be seen. A particularly nice asterism in the cluster resembles the outline of the spaceshuttle coming in to land. SEEN IT 4 NGC 2158 Continuing the trend of reducing chart sizes and increasing concentration brings
us to our next target on this tour, the open cluster NGC 2158. This is easy to find, sitting 24 arcminutes southwest of the centre of M35. At mag. +8.5 it’s a lot fainter than its visually splendid sky neighbour but equally, it’s more concentrated. NGC 2158 is also considerably smaller than M35, roughly a sixth of the size at 5 arcminutes across. The cluster is best viewed with larger instruments as its surface brightness is quite low. Even low light pollution hides it from view. However, under dark skies it is possible to see NGC 2158 surprisingly well even with an 3-inch refractor. The cluster’s dimness and small apparent size comes from the fact that it’s around 11,000 lightyears away. M35 for comparison is 2,800 lightyears from the Sun. SEEN IT 5 IC 2157 Next we travel to another open cluster close to the southern end of Castor’s foot. IC 2157 is located 35 arcminutes to the west of NGC 2158 and 46 arcminutes north of 1 Geminorum. This object is visible in a small scope as a faint, round glow with several brighter stars overlaid across it. Like Collinder 89, IC 2157 can appear quite sparse through the eyepiece, an attribute not helped by the fact it’s set against a rich background of Milky Way stars. A 10-inch scope shows its ill-defined nature well, revealing around 15 stars in a 5 arcminute area, which is a similar size to NGC 2158. The brightest members are around mag. +10.5. SEEN IT 6 NGC 2129 Our final object this month is another open cluster in Gemini. This is mag. +6.7 NGC 2129 which lies 43 arcminutes to the west of the mag. +4.2 1 Geminorum, the star marking Castor’s toe. Incidentally, 1 Geminorum is notable because it is occulted by the Sun at the June solstice. Consequently it marks the approximate location of the most northern part of the ecliptic. The cluster is visible but unremarkable in a small telescope, showing two bright stars of magnitudes +7.4 and +8.2 aligned northsouth. A 10-inch scope reveals over 30 stars in a 5 arcminute area with subtle concentration. The two brighter stars are HD 250289 to the north and HD 250290 to the south. They share similar proper motions, suggesting they may be part of a binary system. SEEN IT