Mark Armstrong
Deep Sky: In search of winter nebulae
NGC 2359: The mighty Thor
Let’s begin our tour in ne fashion by visiting NGC 2359 in Canis Major, a sober-sounding
designation for a spectacular nebula that’s universally known as or’s Helmet. It’s gained such a
heroic nickname primarily from its appearance in deep images, in which it doesn’t require much
imagination to spot a resemblance to the helmet, replete with wing-like appendages, of the Norse
god.
e main body of or’s Helmet is a superb, multi-structured ‘bubble’, a molecular cloud that has
been blown out and sculpted by incredibly powerful winds from the massive and hot central star
WR7 (HIP 35378, magnitude +11.4), an exotic Wolf– Rayet star – a type of star characterised by
prodigious mass-loss driven by the aforementioned winds. ese pre-supernova stars are rare as they
live fast and go out with a bang; only a few hundred have been identi ed in our Galaxy.
Canis Major, the Greater Dog, is a small but brilliant constellation lying entirely south of the
celestial equator. is is dazzling Sirius’ stomping ground, so its location just to the southeast of
Orion is easy to pin point despite its low elevation from UK shores. or’s Helmet lies just under
nine degrees north-east of Sirius, in Canis Major’s extreme north-eastern corner. It forms a 30–40-
arcminute-sided triangle with HIP 35998 (magnitude +5.8, to the south-west) and HIP 36134
(+6.9, to the east-north-east).
At mid-month, NGC 2359 transits the southern meridian (culminates) soon after 11.30pm GMT,
at which time it has climbed to an altitude of around 25 degrees.
or’s Helmet is a prized imaging target, especially for those taking advantage of remote-observing
facilities. e stunning 8h image in this article is a cocktail of narrowband oxygen-III and
hydrogen-alpha data and broadband RGB data. If you’re looking for a target that demonstrates
dramatically the great advances made in imaging hardware and processing techniques in the past 25
years, then you’ll nd it here. Historically, the nebula was catalogued as having an apparent diameter
of 10’ × 5’, but, as with the Dumbbell Nebula (M27) in Vulpecula, modern images show swathes of
faint nebulosity surrounding the bubble.
Visually, owing to its low altitude in UK skies, you’ll likely need the light grasp of at least a 200mm
(eight-inch) telescope, 0° operating at low powers (say, around 100×) and with an O-III or UHC
lter, to catch a glimpse of the main bubble. Upgrading to a 300mm (12-inch) aperture can reveal
one of the ‘horns’. Try to observe NGC 2359 within an hour of culmination on a transparent and
moonless night.
NGC 2440: a diminutive planetary
We now head for something completely different for the second stopping-off stage on our tour.
NGC 2440 in Puppis is a small planetary nebula, spanning a mere 74′′ × 42′′ at its maximum extent,
but, as an image by the Hubble Space Telescope reveals, size isn’t necessarily everything, as it is a
beautiful and intricately-structured object.
Puppis, the stern, or poop, of the ship Argo Navis, borders Canis Major to the east. It is known
primarily for hosting the splendid duo of close-lying open clusters Messier 46 and 47, which are
located in Puppis’ north-western corner, close to its boundary with Canis Major (and Monoceros to
the north). NGC 2440 lies around 3.5 degrees south of Messier 46, the more easterly lying of the
clusters. At around mid-month, NGC 2440 culminates at about midnight GMT at an altitude of
around 20 degrees.
NGC 2440 shines at magnitude +9.4 and is not hard to nd through an 80mm (~three-inch)
telescope, though you’ll struggle to see it as anything other than stellar-like at low power. rough a
250mm (ten-inch) aperture 3 +30° it appears with an overall apparent diameter of around 20′′ to 30′′,
with a brighter centre. e magnitude +14.3 central white dwarf remnant is tough to spot from UK
shores. It is well worth making a real effort to see it though, as astronomers believe it’s one of the
hottest-known white dwarfs, with a temperature of 200, 000 degrees. NGC 2440 lies around 4,000
light years distant.
NGC 1977: the ‘Running Man Nebula’
e next three targets on our tour lie in mighty Orion, more familiar and accessible territory for
deep-sky enthusiasts. First up is NGC 1977, a popular target for deep-sky imagers who know it well
as the Running Man Nebula, sprinting through the stars about half a degree to the north of the
awesome Orion Nebula.
NGC 1977 is a 40’ × 25’ sized complex of primarily blue re ection nebulosity with some red
emission nebulosity divided by dark dust lanes. Together with the brighter Orion Nebula, it is part
of Orion’s giant molecular cloud complex. NGC 1977 is located in Orion’s Sword, a short (1.5-
degrees- long) row of three ‘naked-eye stars’ hanging about three to four arcminutes south from
Alnilam (epsilon [e] Orionis, magnitude +1.7), the central star of Orion’s Belt. Of course, the
middle ‘star’ of the Sword appears fuzzy as the Orion Nebula.
e Sword’s northernmost ‘star’ is actually a pair of massive stars, 42 Orionis (magnitude +4.6) and
45 Orionis (+5.2), that form part of the Orion OB1 association. ese two stars are embedded
within the brighter part of NGC 1977, with deep and detailed images showing the gure of a
running man just to their upper-right (north-west). A sparse star cluster (20’ × 10’ in size) consisting
of at least a dozen stars exists here.
NGC 1977 has two further NGC designations, 1973 and 1975, both found by Heinrich d’Arrest in
the 1860s. Observing with today’s superior equipment we know that what he saw was merely the
brightest parts of NGC 1977, which was found by William Herschel in 1786.
e brighter portion of NGC 1977 can be spied through a pair of binoculars on ne night, but a
small telescope at low power provides splendid views. Observing through a 100mm (four-inch)
telescope, veteran US visual observer Stephen James O’Meara asserts ‘discerning eyes should see
NGC 1977 as three distinct, and near parallel, banks of bright nebulosity, sliced and separated by
dark streams of obscuring matter’. NGC 1977 culminates at a healthy 35 degrees or so altitude at
about 10pm GMT, mid-month.
NGC 1788: in a spooky eld
We stay in Orion to visit NGC 1788, another pretty re ection nebula that lies a bit remote from the
constellations’ premier deep-sky action contained within its familiar bright-star outline. However, it’s
easy to nd as it lies just 1.8 degrees north of Cursa (beta [β] Eridani, magnitude +2.8), which itself
lies 3.5 degrees north-north-west of brilliant Rigel (beta [β] Orionis).
NGC 1788 is a bright, dusty re ection nebula that mirrors the Orion Nebula as a star-forming
region. Blue light from its young, energetic stars buried deep with the nebulosity is re ecting off the
dust grains and illuminating the nebula. NGC 1788 extends at its fullest extent, seen in images, to
8’ × 5’ and can be seen through a small telescope. Wide- eld images spanning around three degrees
show NGC 1788 forms part of the western wing of a giant cosmic bat, sometimes referred to as the
‘Cosmic Batman’.
NGC 1788 is comfortably placed on January evenings at mid-northern latitudes, culminating at
about 9.30pm GMT when it peaks at around 35 degrees in elevation.
NGC 2024: the spectacular Flame Nebula
Let’s head back to the centre of Orion, the Hunter, and locate his famous ‘Belt’, which is formed
from three bright stars, Mintaka (delta [d] Orionis, magnitude +2.2), Alnilam (epsilon [e] Ori, +1.7)
and Alnitak (zeta [z] Ori, +1.7), which are orientated north-west to south-east. Our target is
Alnitak, from where it’s easy to track down the location of NGC 2024, the famous Flame Nebula
(Sh2-277), which lies very adjacent to its east-north-east.
e Flame Nebula is a large, fan-shaped emission nebula, or H-II region, spanning half a degree
across, that is being energised by a erce torrent of ultra-violet light spewing from Alnitak, a hot
blue supergiant star. It’s a spectacular nebula that needs no hyping up, though there’s no doubt its
stock has risen owing to its proximity to the iconic Horsehead Nebula (Barnard 33); marvellous
wide eld images of the region often encompass both objects.
e Flame is not an easy target to observe or image owing to the close proximity of Alnitak.
Observers often gauge the prospects of tracking down the more elusive Horsehead by whether or not
they can see the Flame. Provided you can nd a way to keep Alnitak out of the eld of view, a
150mm (six-inch) telescope should be sufficient to reveal the two distinct areas of nebulosity, which
are separated by a spectacular, three-arcminute-wide dividing dust lane, which can be seen extending
to 12 arcminutes long through a 250mm telescope.
Alnitak and the Flame Nebula cross the southern meridian at about 10pm GMT at a healthy 37
degrees altitude from London mid-month.
Messier 1: The Crab supernova remnant
With our brief sojourn in Orion at an end, we now move on to Taurus, the Bull, another
heavyweight of the winter sky. Our target is the iconic Crab Nebula (NGC 1952), in a class of its
own 0° 0 as the most famous supernova remnant in the entire sky. It also has the honour of being
the rst listing in Messier’s catalogue.
Messier 1 could well be the most storied and studied deep-sky object of all. e story of the massive
star that blew itself to smithereens in 1054 and the subsequent appearance of the resulting
supernova, shining six times brighter than Venus, which -10° Chinese astronomers rst recorded
seeing as a ‘guest star’ during the daytime, has become part of astronomical folklore down the
centuries. It’s an amazing fact that no human has clapped eyes on another naked-eye galactic
supernova to this day.
Messier 1’s Crab moniker was penned initially by the British astronomer William Parsons, the third
Earl of Rosse, who sketched the nebula in 1844 and thought it resembled the form of the common
crustacean. English physician John Bevis is credited with its discovery, in 1731, while Messier
independently ‘discovered’ it in 1758.
Train a humble pair of 10 × 50 binoculars in the direction of magnitude +3 zeta Tauri, the star
marking the tip of the Bull’s southern horn, on a dark and moonless night and you should be able to
detect Messier 1 as a faint misty patch of light shining with an integrated magnitude of around +8.
An 80mm (~three-inch) telescope will give an impression of the Crab being orientated south-east to
north-west on the sky; powers of around 100× can show the brighter parts of the diffuse haze in an
’S’ shape. Although you won’t see the Crab’s complex and colourful lamentary structure that’s
become so familiar from the avalanche of images of it available on the Internet, a 200mm (eight-
inch) telescope operating with an O-III or UHC lter may hint at it on a particularly good night.
e famous Crab pulsar, the rapidly spinning neutron star remnant that was discovered in 1968, is
beyond visual recognition in all but the largest amateur instruments.
IC 443: A Jelly sh in Gemini
e penultimate target on our tour is IC 443, another galactic supernova remnant that lies across the
border in Gemini. ird-magnitude Propus (eta [h] Geminorum) marks Castor’s left foot and lies a
just under nine degrees east-north-east from zeta Tauri. Adjacent to Propus, under half a degree
directly east in fact, is the Jelly sh Nebula (IC 443, Sharpless [Sh2] 248), a very different denizen of
the deep to the Crab Nebula.
e Jelly sh is believed to be the remains of a massive star that went supernova between 3,000 and
30, 000 years ago. In common with the Crab, a pulsar has been detected there in X-rays. Visually, the
Jelly sh is much fainter than the Crab Nebula; you can get an idea of how faint as it was discovered
photographically in 1892. Its 50’ apparent diameter dwarfs the Crab too.
Astronomy Now’s deep-sky expert Owen Brazell has seen the Jelly sh through a half-metre (20-inch)
telescope on several occasions, but it should be visible through a 300mm (12-inch) from a dark-sky
site on a ne night. Owen recommends employing low powers and the addition of an O-III or
UHC lter. Imagers have a much easier time of it; some of the wide- eld images of its environs are
truly breathtaking.
NGC 2359, popularly known as Thor’s Helmet, is a superb Wolf–Rayet nebula.
Let’s go hunting for some visually fainter though still well-known imaging nebulae,
interspersed with brighter quarry to sooth strained eyes and stimulate photon-starved
sensors, writes Mark Armstrong.
NGC 2359 lies in Canis Major, just under nine degrees north-east FR of scintillating Sirius (alpha [a] Canis Majoris), the brightest star
in the sky.
NGC 1788 lies in Orion, close to its 29 boundary with Eridanus, some ve β κ degrees north-north-west of Rigel.
NGC 7538 lies east of Cepheus’ ‘house’-shaped asterism, LZ ζ Kruger 60 right on Cepheus’ boundary with Cassiopeia to the east.
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January 2021
Astronomy Now