This month’s deep-sky tour visits objects in Orion, including a number ’37’ star formation
1. Collinder 69
Recommended equipment: Small/medium telescope
Orion’s head is marked by mag. +3.4 Meissa (Lambda (λ) Orionis). This marks the northern vertex of a small naked-eye triangle, together with mag. +4.4 Phi 1 (φ ) Orionis and mag. +4.1 Phi (φ ) Orionis. A small scope at low power shows numerous fainter stars in the region, many belonging to cluster Collinder 69. Look out for the attractive line of seventh magnitude stars running south of Meissa.
Telescopes over 250mm may detect faint nebulosity around the cluster. This is Sharpless 2-264, an object made easier to see with an Oxygen (OIII) filter. It occupies a roughly circular area 6.5˚ across, centred on Meissa and has low surface brightness, with the brightest part west of Meissa. Long exposure images reveal the Angel Fish Nebula; a pink emission nebula that resembles a sideways-on fish.
2. NGC 2022
Recommended equipment: Small/medium or large telescope
Our next object appears within the boundary of Sharpless 2-264, 1.9˚ eastsoutheast of Meissa. Here lies the brightest planetary nebula in Orion, NGC 2022, which isn’t saying much as it shines with an integrated magnitude of +12.4! It has an apparent diameter of 20 arcseconds, with small scopes revealing a star-like object at low magnification. Upping the power won’t show detail, but should reveal the elliptical shape of this planetary.
Larger instruments show a grey ellipse with a darker region in the centre that creates the appearance of a ring. The central star has a mag. +14.9 and requires at least 400mm of aperture to see properly.
3. Abell 12
Recommended equipment: Large telescope
The Abell catalogue of planetary nebulae lists 86 objects, many of which are faint and require large scopes. Abell 12 adds to the challenge as it’s located near Mu (μ) Orionis. Mu shines at mag. +4.1 while the integrated magnitude of Abell 12 is +13.9, over 8,000 times dimmer than the star. Abell 12 is around 0.5 arcminutes from Mu and falls within its visual glare, making the planetary difficult to see, even with a large instrument. Nicknamed the ‘hidden planetary’, the trick is to employ a large aperture and an OIII filter.
4. NGC 2141
Recommended equipment: Small/medium or large telescope
Open cluster NGC 2141 sits 0.8˚ north and a bit east of Mu Orionis. When the term ‘open cluster’ is mentioned, the mind’s eye image is of a group of stars in the eyepiece. However, not all are bright enough to show such a view and NGC 2141 falls into this category. Despite being listed at mag. +9.4, smaller instruments are unable to resolve any of the cluster’s stars; you will see a mistiness behind a number of brighter foreground stars. You need a 300mm aperture to show the 20 individually resolved stars.
5. NGC 2194
Recommended equipment: Small/medium or large telescope
Located 3.5˚ northwest of NGC 2141 and half a degree northwest of the mag. +5.4 star 73 Orionis, is the open cluster NGC 2194. This has a similar size to our previous target, but is slightly brighter with a listed integrated magnitude of +8.5. A 150mm scope shows a misty glow sprinkled with six or so individual stars. A 250mm scope doubles the number of individual stars seen and reveals a granular texture to the mistiness, while a 300mm scope allows you to drill down into the cluster’s core where several tens of stars can be individually resolved.
6. NGC 2169
Recommended equipment: Small/medium or large telescope
The stars in our final object appear to form the number ‘37’ or perhaps less attractively, a shopping trolley! Listed at mag. +5.9, a small scope will show NGC 2169, containing 16 stars in a 5 arcsecond area. The count increases to around 20 cluster members through a 300mm telescope. Look for the contrast in colour between most of the stars and the two which mark the join between the vertical and horizontal components of the ‘7’. This pair is double star Struve 844 and looks orange compared to the white and blue-white colour of the others. The brightest star in the top of the ‘3’ is double star Struve 848, with mag. +6.9 and +7.8 components separated by 2.6 arcseconds.
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