From city lights to deep space

PART 2 OF 4

In this second part of a series that looks at urban stargazing through four seasons, Rod Mollise reveals the wonders you can discover in the summer night sky…

When it comes to deep sky objects, in the summer the galaxies lying outside the plane of the Milky Way are setting and the marvels within our own Milky Way are on the rise. Urban observers don’t have as easy a time viewing the deep sky in summer, as the air tends to be humid and moisture scatters light pollution, making the sky brighter. Nevertheless, it is possible to see a multitude of distant and beautiful objects; you’ll just need to tailor what you are looking for according to the weather conditions. On humid summer nights look for bright star clusters, saving more difficult objects like galaxies for times when the weather is dry and the sky is dark.

Whatever the conditions, when deep sky observing there’s one characteristic that’s most important in the telescope you use – aperture, the size of its lightcollecting lens or mirror. A telescope with an aperture of at least 200mm is recommended in areas with light pollution. However, one with a 250mm aperture will be as portable and only a bit more expensive – the extra 50mm shows many more objects and gives more detail, even in bright urban skies.

There are a few different telescope designs to choose from:

Newtonian reflectors, which use a large mirror to collect light, are the least expensive when it comes to cost-to-aperture ratio. But, as the tube in a Newtonian is open to the elements, expect to clean its delicate mirrors regularly. Medium to long focal length Newtonian reflectors are not very portable in apertures above 250mm, and Newtonian mirrors need to be aligned to each other (collimated) frequently.

Refractors use a lens to collect light. Moving from the warmth of indoors to the cold outside deforms a reflector’s mirrors, so they take time to acclimatise. But the lens of a refractor is much less affected by changes in temperature. Refractors also rarely require collimation and their images can be very sharp. But you’ll often pay more for a 100mm refractor than a 250mm reflector.

Catadioptrics, like the Schmidt Cassegrain Telescopes (SCT), employ a combination of lenses and mirrors. Most SCTs come equipped with computerised drives that will reliably locate objects (which can be difficult to do ‘manually’ in light-polluted skies), as well as other high-tech features. But, while not as expensive as refractors per millimetre of aperture, they are more expensive than a Dobsonian reflector. The optics of SCTs also require collimation – though not as often as those of Newtonians.

From Left: A Newtonian reflector uses mirrors to collect and reflect light to an eyepiece for viewing; A refractor uses glass lenses to bend light and enlarge the image. They’re sharp, but expensive; Catadioptric telescopes are a combination of the other two types, and are lighter and more compact

Summer sights

The season’s stunning constellations to explore…

The Keystone asterism lies at the heart of Hercules

M13 in Hercules is a tight ball of stars, and needs a bigger scope

Hercules, the Hero

A well-known home of stunning globular clusters

Hercules may be on the dim side, but its premier deep sky object, M13 (mag. +5.8, 20’ across) is not. Even a beginner with a non-computerised telescope mount won’t have much trouble finding it, roughly midway between the line connecting the two westernmost stars of the Keystone – the lopsided square at the star-pattern’s heart. But, while M13 is bright, it is also compact. A 200-250mm telescope turns this ‘smudge’ into a beautiful ball of stars in the brightest skies.

Another notable globular star cluster is M92 (mag. +6.4, 14’ across), which lies away from the Keystone in the northern part of the constellation. While it is actually more tightly concentrated than M13, M92 seems easier to resolve – perhaps because the core isn’t as overwhelmingly bright as that of M13. In a 130mm telescope from a suburban location, M92 can appear resolved almost to its centre on a dry summer’s evening when the air is steady.

Lyra is meant to look like a lyre, a kind of harp popular in ancient Greece

Lyra, the Lyre

Discovered in January 1779, M57 is a luminous envelope of ionised gas

This constellation contains well-known wonders

Even in the worst urban light pollution, M57 (mag. +8.8, 3’48”x2’24”), the famous Ring Nebula, is a good target for a 80-100mm telescope, but its shape only becomes evident with an aperture of 150mm or more. A 250mm telescope shows brightness variations in its nebulosity and reveals a faint star embedded in the southwestern end of the ring’s disk. The holy grail is M57’s central star, the white dwarf left over from the dead sun that created the nebula.

Globular cluster M56 (mag. +8.8, 8’48”), in the eastern part of Lyra near the border with Cygnus, is often overlooked. Its small, dim stars are spread far apart, making it more like a rich but dim open cluster.

Lyra’s final urban wonder is not a deep sky object, but a multiple star, the famous Double-Double, Epsilon (ε) Lyrae, located 1°36’ northeast of Vega. At low power an attractive double star is visible, two suns separated by a wide 208”. Increase magnification to 200-250x, however, and both of the stars are revealed as doubles themselves. The northernmost pair, ε ¹, is separated by 2.78”, while the stars of ε ²to the south are 2.4” apart.

Cygnus is dominated by the Northern Cross asterism, which in northern latitudes is directly overhead at midnight in summer

Cygnus, the Swan

M27 was the first planetary nebula to be discovered, in 1764

Near midnight, the Swan rides high with her companions the Little Fox and Dolphin

One of the best planetary nebulae, NGC 6826 (mag. +8.8, 8’24” across), the Blinking Planetary, is located not far from the tip of Cygnus’s western wing. Even with a 100mm telescope, under light-polluted conditions it is impossible to see the round nebulae or its mag. +10 central star using direct vision. But practice averted vision – looking off to the side rather than directly at it – and the round nebulosity around the star springs into view. Alternate rapidly between direct and averted vision and the nebulosity blinks on and off.

Cygnus is home to two open star clusters. M29 (mag. +6.6, 10’ across) is located near the junction of Cygnus’ wings and body. North of M29 by 14°50’, and well away from the body of the Swan, is M39 (mag. +4.6, 31’ across).

The legendary Veil Nebula (NGC 6992/6960) is two separate arcs of nebulosity, only visible with an OIII filter. The first of the Veil’s two nebulous arcs, NGC 6960, often referred to as the Witch’s Broom Nebula, is easy to locate without a computer since this 3°30’ x 2°40’ streak of gas passes through a bright, mag. +4.2 star 52 Cygni. Although the given magnitude for the Witch’s Broom is +7.0, its large size means its surface brightness is extremely low. The other main component of the Veil,

NGC 6992, forms a 3°50’ x 2°40’ arc, 2°30’ northeast of the Witch’s Broom. This half of the Veil is referred to as the Bridal Veil Nebula thanks to its intricate, lacy structure. On a good night it can begin to give up significant detail to telescopes with an aperture of 250mm or more.

Vulpecula, the Little Fox, is unimpressive, but it is home to M27 (mag. +7.4, 8’ x 5’36”), the Dumbbell Nebula. Not only is M27 bright and bold, it is large for a planetary nebula, making it one of the best for Northern Hemisphere observers. Lower magnifications reveal substantial detail and make the Dumbbell a great target for smaller telescopes.

The main object of interest in the small constellation Delphinus, the Dolphin, is a globular cluster, NGC 6934 (mag. +8.8, 8’24” across). Even with a 300mm telescope, it appears as an unresolved fuzzball, but it is set in a field rich with stars. In the suburbs, a magnification of 187x begins to resolve individual stars around the periphery of the cluster.

Ophiuchus’s brightest star is mag. +2.7 Rasalhagu

Ophiuchus, the Serpent Bearer

M10 was originally described as being ‘without stars’

With Serpens, the Serpent, these constellations hold some real gems

Once thought to resemble Asclepius the physician, Ophiuchus looks like a lopsided house drawn by a child. Nevertheless, there are wonders aplenty here.

Ophiuchus’s main attractions are globular clusters, beginning with the famed Twin Globs. M10 (mag. +6.4, 20’ across) is a treat. With a 280mm telescope, it is resolved to its core and shows hordes of tiny stars. M12 (mag. +7.6, 16’ across), which lies northwest of M10, is bright and compact, but fewer stars and a looser structure make it appear dimmer than the magnitude figure suggests.

Serpens is actually two constellations, Serpens Caput (the head) and Serpens Cauda (the tail). While the tail is far to the south and doesn’t hold much of interest, the head is a different story, containing one of the best globulars visible from northern latitudes: M5 (mag. +6.6 and 23’ across). M5’s statistics alone hint at what a spectacle it is. While it is slightly dimmer than M13 and its concentration is about the same, M5 is easier to resolve in any telescope, and is just visible with the naked eye under perfect conditions.

Scutum

M11: a loose cluster in smaller scopes, at 100x a crowd of stars is shown

Home to one of the prime attractions of the summer sky: the Wild Duck Cluster

M11 (mag. +6.3, 14’ across), the Wild Duck Cluster, is a bright open star cluster. As its surface brightness is high, it is easy to make out with a 75mm telescope under poor city skies. Indeed, enough stars should be visible to make clear why this object is the Wild Duck – triangular in shape, suggesting a flight of ducks.

Scutum’s other Messier open cluster is M26 (mag. +8, 10’ across), a group of stars whose magnitude and size make it sound like a good catch. However, it turns out to be a major disappointment. A 150mm telescope shows a scattering of at most 10 faint stars and one slightly brighter sun.

Globular cluster NGC 6712 (mag. +8.7, 2’30” across), just to the east of Scutum’s stick figure, isn’t anything special in itself, but the field it is set in is incredibly rich – packed with stars, it is almost difficult to tell where the outer stars of the cluster stop and background stars begin. NGC 6712 is visible in the city, and a 300mm telescope may resolve some stars, but its beauty is reduced since the field is no longer as rich with suns as it is from darker skies.

Low on the horizon viewed from London, Sagittarius cannot be seen at all from Scotland. Its legs drop down below Corona Australis

Sagittarius, the Archer

M20, the Trifid Nebula, is named for its three parts: emission, reflection and dark nebulae

Barely above the horizon, this constellation holds a few legends

While the constellation doesn’t look anything like the centaur archer of mythology – it looks more like a teapot with a perfect lid and spout –it is home to some bright, legendary objects.

M22 (mag. +5.1, 32’ across), the Great Globular in Sagittarius, is an example of an object that makes a visit to the south of England worthwhile. Its highest altitude from the latitude of London is less than 15°, but it is large and easily visible in smaller telescopes. With a 280mm telescope its oval shape becomes evident and its loose central core becomes visible.

Elsewhere, the star cluster NGC 6530 is loose but attractive, and is composed of about a dozen bright stars. It appears to be embedded in the nebulosity of M8, the Lagoon Nebula, but is thought to actually be a foreground object.

Sagittarius’s other famous nebula, M20 (mag. +6.6, 30’ across), the Trifid Nebula, is badly hurt by its low altitude. Despite having a fairly high magnitude and being reasonably large, its faintness is due to the fact that some of the Trifid is reflection nebulosity, which is more difficult to see than emission nebulosity from light-polluted areas.

‘Uncle’ Rod Mollise is an American amateur astronomer and writer who lives near Mobile, Alabama. He is the author of Choosing and Using a New CAT

Tagged pkar