Orion's Belt is a magnetic sight on February nights. Take the bait and revel in a bounty of double and multiple stars, nebulae, and more.
Orion's Belt, the most recognizable starry asterism in the sky, stands front and center at 8 p.m. on February evenings. Visible from virtually everywhere on Earth, the belt is everything: a naked-eye treasure, a handsome binocular sight aglitter with dozens of stars, and a rich hunting ground for telescope users.
The most famous dark nebula in the sky, the Horsehead, resides here, as do nine other nebulae, a unique star cluster, and the remarkable multiple star Sigma (σ) Orionis. Plus, one of the Belt stars is a double and the other a triple. The only thing the asterism lacks is a bright nebula or star cluster, but you can resolve that deficiency by descending 4° south to the Orion Nebula and friends.
Although the following observations were made with a 15-inch (37-cm) Dobsonian reflector, I'll note when objects can be seen in smaller instruments. Let's start with several fine double and multiple stars in order of increasing right ascension (RA). STF refers to a double star in the Friedrich Struve catalog:
- 31 Orionis: Beautiful orange primary paired with a delicate 11th-magnitude companion 12.7″ almost due east. The primary's color combined with the imbalance in magnitudes makes this a striking pair.
- Delta (δ) Orionis (Mintaka): The westernmost star in the Belt, Mintaka appears searingly white at low power. It's joined by a bright 6.5-magnitude companion 53″ due north. If you like sapphires, you'll love Mintaka.
- STF 734: A nice triple, though rough seeing for much of the winter has squelched my attempts at splitting the 7th-magnitude primary's 8.5-magnitude companion 1.5″ to the north. The third star, however, is very easy, shining 30″ to the west-southwest at magnitude 8.3.
- HV 118: Magnitudes 6 and 10 with the companion 28″ almost due west.
- STF 751: Magnitudes 8 and 9, separation 15″. Easy, low-power double 13′ northwest of the central Belt star, Alnilam.
- Sigma (σ) Orionis: A stunning quadruple with four stars like an elaborate earring easily resolved at 64×. The fainter triple star Struve 761 lies just 3.5′ to the northwest, making these side-by-side multiple stars a must-see in telescopes 3-inches and up. Stick to the lowest power required to split them to preserve their crisp, fiery beauty.
- Zeta (ζ) Orionis (Alnitak): The easternmost star in the Belt. The pure white, brilliant 1.9-magnitude primary dominates its dim 9.9-magnitude companion 58″ to the north-northeast. The pair resembles Mintaka but with a fainter secondary star. Increase the magnification to 200× or higher and you'll spy the magnitude-3.7 companion just 2.2″ southeast (P.A. 166°) of the primary, elevating Zeta from double to triple star status. A 4-inch scope should be up to the task on nights of good seeing.
Orion is home to the Orion Molecular Cloud Complex, an extensive group of bright and dark nebulae of which the Orion Nebula is only the brightest manifestation. Located between 1,500 and 1,600 light-years away and spanning about 25°, the cloud is hundreds of light-years across and home to thousands of new and "in utero" stars. Some of these suns light up patches of dust through reflection, others excite gases to fluoresce. We'll examine a variety of these clouds (and a lone star cluster) located within 1° of the Belt:
- NGC 2024 (Flame Nebula): Although listed as an emission nebula and seemingly a great candidate for enhancement with an O III filter, I discovered to my surprise that the view was much better without a filter. Either way, you have to deal with light trespass from nearby Zeta Ori. Use a medium magnifications of around 100×–125× both to darken the sky and better keep Zeta out of the field view. With this setup, the amount of detail visible was amazing. A prominent dark rift "rips" the nebula in two, with smaller dark rifts chopping up the eastern half of the mass. Viewed south up, NGC 2024 resembles the Greek letter pi (π). (Dimensions: 30′ x 30′, Type: Emission)
- IC 434 and the Horsehead Nebula (B33): IC 434 is the faint, diffuse emission nebula that provides the luminous backdrop for the shadowy Horsehead. The Horsehead was invisible without a nebular filter and, despite very dark skies, extremely difficult to pick out with the O III. But the hundred bucks I spent for an H-beta filter was well spent. Using a 142× eyepiece with the filter in place, the Horsehead appears surprisingly large (8′ × 6′ across) and opaque. With averted vision I could trace the neck and the head, and occasionally could tell what direction the nose pointed (north). Still, the Horsehead is vague and requires dark skies (you won't see it if you can't see the winter Milky Way) and averted vision.
When using an H-beta filter, allow time for your eyes to adapt to the extra-dark field of view. Observers with telescopes as small as 6-inches have dug out the Horsehead with an H-beta under dark skies, so give it a try! (IC 434 dimensions: 60′ × 10′, Type: Emission; B33 dimensions: 8′ × 6′, Type: Dark nebula)
- NGC 2023: Bright, diffuse emission-reflection nebula centered on a 7.6-magnitude star. Very easy to see with and without nebular filters, appearing like a luminous fog with a star shining through. At 124× without a filter the nebula was extended north-south with subtle variations in light and hints of dark mottling around the edges from encroaching dark nebulae. (Dimensions: 10′ × 10′, Type: Emission and Reflection)
- IC 435: This nebula is very easy to spot just 0.5° east of brighter IC 2023 and centered on an 8th-magnitude star. It's a little fainter than IC 2023 and only about half as large. (Dimensions: 4′ × 3′, Type: Reflection)
- IC 432: This largish, faint reflection nebula extends north and south of a 6.5-magnitude star. Some texture was seen. (Dimensions: 8′ × 4′, Type: Reflection)
- IC 431: A small, fairly dense ball of nebulosity centered on a 7.5-magnitude star. (Dimensions: 5′ × 3′, Type: Reflection)
- IC 426: A dim but obvious haze extending east and north of a pair of stars with magnitudes of 8.6 and 10.7. (Dimensions: 7′ × 7′, Type: Reflection)
- IC 424: Another small reflection nebula. Despite my best efforts, I could not confirm seeing it. (Dimensions: 2′ × 2′, Type: Reflection)
- IC 423: A large, diffuse, faint, and vague patch, like frosty breath on the verge of dissipation. Partially fills the southern half of a rectangular figure of stars. No condensation and no obvious stars associated with the nebula. (Dimensions = 6′ × 4′, Type: Reflection)
- Berkeley 20: This small patch of haze looks exactly like a faint comet at 64×. But using 224× I could make out four ~14.5-magnitude stars poking out of the mist. The cluster lies immediately west of a magnitude 11 foreground star. Be 20 is one of the most ancient open star clusters known with an age around 6 billion years. It's also one of the most distant — more than 27,000 light-years away. No wonder it's so faint! Photo here. (Diameter: 2′, approx. 20 stars total)