DEEP-SKY TOUR
Globulars galore on a cluster-busting voyage around Delphinus
Tick the box when you’ve seen each one
1 M2
M2 is a bright, large globular cluster in Aquarius located approximately two-thirds of the way along a line joining Epsilon (¡) Pegasi to Beta (`) Aquarii (not shown on the chart). With a listed magnitude of +6.4 and a decent visual diameter, it’s an easy target for smaller instruments. A 6-inch scope shows it as a glowing haze with an almost stellar core. Upping the aperture to 10-inch increases light grasp and resolution, allowing most of the member stars to be resolved. Here the brightest part of the cluster appears elongated into an 8x6 arcminute oval. The general haze around its bright centre can be traced further than this using magnifications around 200x. SEEN IT
2 M15
M15 is even easier to find than M2. This globular cluster (which, like M2 was discovered by French astronomer Jean-Dominique Maraldi in 1746) is best located by extending a line from Theta (o) Pegasi to Epsilon (¡) Pegasi and extending that line for half the distance again. This is a lovely object which is interesting to compare and contrast with M2. M15 is far more compact, appearing approximately 5 arcminutes across through a 6-inch scope. Increased magnification hints at a mottled texture across the entire cluster. A 10-inch scope at high power reveals a small but definitely non-stellar core surrounded by a mass of resolved stars. Through a 12-inch instrument M15 is a delight, offering a one arcminute core surrounded by a bright five arcminute inner region, which itself is surrounded by a 12 arcminute outer halo. SEEN IT
3 NGC 7042
Our next target is significantly trickier to track down than the previous two. NGC 7042 is an Sb-type spiral galaxy in Pegasus, located 3° 52 arcminutes west and 1° 40 arcminutes north of M15. It has a listed magnitude of +12.8, making it best suited to larger apertures. Discovered by William Herschel in 1784, it forms one of a pair of galaxies, the other being the barred spiral NGC 7043. Of the two, NGC 7042 appears larger and brighter, with NGC 7034 being around mag. +14.6. These are distant objects estimated to be around 200 million lightyears away. NGC 7042’s apparent diameter is around 2 arcminutes. SEEN IT
4 NGC 7006
We hop next door into Delphinus, the Dolphin, for our next target, the mag. +10.5 globular cluster, NGC 7006.
This can be found 3° and 34 arcminutes to the east of the double star marking the dolphin’s nose, Gamma (a) Delphini. Compared to M2 and M15, this is a tricky globular. Through a 6-inch scope it appears rather small at just one arcminute across. NGC 7006 is around four times further away than M15 with a distance estimated at 160,000 lightyears. Smaller instruments tend to show it looking like a fuzzy star but larger apertures will begin to reveal its true globular nature and start to resolve its component stars. SEEN IT
5 ABELL 72
The Abell catalogue contains some very faint planetary nebulae, but fortunately Abell 72 is one of the brighter ones. Even so, it still requires a larger aperture to see well. Abell 72 is located 3° and 13 arcminutes east, and 40 arcminutes south, of Gamma (a) Delphini. It has a listed magnitude of +12.7 but this is spread across a relatively large disc approximately 2 arcminutes in diameter. Consequently the surface brightness of Abell 72 is relatively low. A 12-inch scope reveals a degree of irregularity at magnifications around 100x. The use of an OIII filter is highly recommended here as this helps add some contrast to the object. SEEN IT
6 NGC 6934
Our final target is another globular cluster located to the south of the main pattern of Delphinus, 3° and 55 arcminutes south of mag. +4.0 Epsilon (¡) Delphini. Another one of William Herschel’s discoveries (this one made in September 1785), NGC 6934 is about 50,000 lightyears from Earth and the cluster stars within it are estimated to be about 10 billion years old. A 6-inch instrument will show the mag. +8.7 cluster as a north-south elongated haze, 1.5 arcminutes across at its longest dimension. A 10-inch scope unsurprisingly gives a better view, as it partially resolves its globular nature at higher powers. A 9th magnitude star appears two arcminutes west of the cluster’s core. NGC 6934 remains unresolved through a 12-inch scope even at high magnifications. Through such an aperture, the apparent size of the globular is a fraction under two arcminutes across. SEEN IT