The Night Sky

August / September

The Summer Triangle is an imaginary triangle drawn from Deneb (in the constellation of Cygnus), to Vega (the brightest star in Lyra) and Altair (in the constellation of Aquila). This asterism will be your guide to the night sky from now until November.

The term was popularized by American author H.A. Rey and British astronomer Sir Patrick Moore in the 1950s. The name can be found in constellation guidebooks as far back as 1913. The Austrian astronomer Oswald Thomas described these stars as Grosses Dreieck (Great Triangle) in the late 1920s and Sommerliches Dreieck (Summerly Triangle) in 1934. The asterism was remarked upon by J. J. Littrow, who described it as the "conspicuous triangle" in the text of his atlas (1866), and Bode connected the stars in a map in a book in 1816, although without a label. These are the same stars recognized in the Chinese legend of The Weaver Girl and the Cowherd, a story dating back some 2,600 years, celebrated in the Qixi Festival. In the mid- to late-20th century, before INS, GPS and other electronic/mechanical equipment took their places in military aircraft, United States Air Force navigators referred to this asterism as the "Navigator's Triangle".

The Summer Triangle

Albireo (Beta Cygni) is, with some justification, the double star against which all others are measured. Firstly, because the pair is reasonably easy to split - a magnification of around 30 will do the job. (Binoculars probably won’t show you Albireo as two stars). And secondly, notice the striking colour contrast between the two, with the brighter star gold and the dimmer star blue.

The summer triangle contains two well known planetary nebulae, M27 (the Dumbbell nebula) and M57 (the Ring nebula):

A planetary nebula is an emission nebula consisting of an expanding, glowing shell of ionized gas ejected from old red giant stars late in their lives. The word "nebula" is Latin for mist or cloud, and the term "planetary nebula" is a misnomer that originated in the 1780s with astronomer William Herschel because when viewed through his telescope, these objects resemble the rounded shapes of planets. Herschel's name for these objects was popularly adopted and has not been changed. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.

At the end of a star's life, during the red-giant phase, the outer layers of the star are expelled by strong stellar winds. After most of the red giant's atmosphere is dissipated, the ultraviolet radiation of the hot luminous core ionizes the outer layers earlier ejected from the star. Absorbed ultraviolet light energises the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula.

Planetary nebulae likely play a crucial role in the chemical evolution of the Milky Way by expelling elements to the interstellar medium from stars where those elements were created. Planetary nebulae are also observed in more distant galaxies, yielding useful information about their chemical abundances.

M57, which is also known as the Ring Nebula and has been described as a celestial smoke ring, is located south of the Vega between Beta (ß) (Sheliak) and Gamma (γ) Lyrae (Sulafat), making it an easy target.

It is best observed using a telescope with an aperture of at least 20 cm (8 in), but even a 7.5 cm (3 in) telescope will reveal its elliptical ring shape. The interior hole can be resolved by a 10 cm (4 in) instrument at a magnification of 100×, while larger instruments will show a few darker zones on the eastern and western edges of the ring, and some faint nebulosity inside the disk. The central star, at magnitude 14.8, is difficult to spot. M57 is 2,300 light years away.

M27, which is known as the Dumbbell Nebula, is brighter and larger than M57 and closer to us at 1,360 light years. It is easier to spot than M57 and was in fact the first planetary nebula to be identified. It has been calculated that the original red giant star threw off its outer layers about 14,500 years ago, leaving its core behind as a white dwarf star that is the largest such star discovered to date.

Brocchi’s Cluster Under a dark sky, Brocchi’s Cluster (Collinder 399) can be seen with the naked eye as an unresolved patch of light; binoculars or a telescope at very low power are usually needed in order to view the "coat-hanger" asterism. It is best found by slowly sweeping across the Milky Way along an imaginary line from Altair toward Vega. About one third of the way toward Vega, the Coat-hanger should be spotted easily against a darker region of the Milky Way.

The asterism is made up of 10 stars ranging from 5th to 7th magnitude which form the conspicuous "coat-hanger", a straight line of 6 stars with a "hook" of 4 stars on the south side. An additional 30 or so fainter stars are sometimes considered to be associated as well.

The asterism and its immediate surroundings are a useful gauge for determining the faintest stars visible in a small telescope as there are a wide range of stellar magnitudes within the cluster easily viewed in one small location of the sky.

Double Stars

Double star ε Boötis (Izar) presents striking gold and blue colours and inspired its discoverer, F. G. W. Struve, to nickname it Pulcherrima – “the most beautiful.” Requires a magnification of 100X or more to split.

ξ Boötis shows very distinctive yellow and red colours.

β Scorpii (Acrab) is a beautiful double star, easy in small scopes. The brighter component is white; the secondary appears blue or blue-green.

μ Draconis rejoices in the wonderful name Arrakis - ‘the dancer’. Informally known as Dune, it is a fictional desert planet featured in the Dune series of novels by Frank Herbert. Herbert's first novel in the series, 1965's Dune, is popularly considered one of the greatest science fiction novels of all time and it is sometimes cited as the best-selling science fiction novel in history.

In reality it is a binary star system whose separation is very slowly increasing. The two lie at the theoretical limit of resolution for the common 60mm (2.4-inch) refractor. A magnification of about 120X and optimum seeing conditions are required.

36 Ophiuchi is low in the south at the moment, with Saturn close by. Another binary system, but with a wider separation that makes these golden yellow stars easy to resolve in small-aperture instruments. At a distance of “only” 19.52 light-years, 36 Oph is a relative neighbour of ours.

Rasalgethi (α Herculis), which means “Head of the Kneeler” is a star system that shows nicely in a small telescope. The brighter of the two is a red giant star, and the dimmer a yellow giant. But visually, in a telescope, the fainter yellow star appears almost green against the red giant - it’s a most striking contrast.

ε Lyrae is the celebrated “Double-double“. These two pairs – each a slow binary system - are 210 arc seconds apart, a separation that eagle-eyed individuals can discern with the unaided eye. All four stars can be glimpsed in a 60mm refractor at 120X.

β Cygni, Albireo has already been described in the Summer Triangle.

Deep Sky

Kemble's Cascade - Location Kemble's Cascade (Kemble 1), located in the constellation of Camelopardalis (the Giraffe), is an asterism - a pattern created by unrelated stars. It is an apparent straight line of more than 20 colourful 5th to 10th magnitude stars over a distance of approximately five moon diameters, and the open cluster NGC 1502 can be found at one end.

Locating the asterism is relatively straightforward, although Camelopardalis does not contain any distinctive stars: Locate Cassiopeia and draw a line across the ‘top’ of the W. Extending this line the same distance again brings you to the cascade.

It was named by Walter Scott Houston in honour of Father Lucien Kemble (1922–1999), a Franciscan friar and amateur astronomer who wrote a letter to Houston about the asterism, describing it as "a beautiful cascade of faint stars tumbling from the northwest down to the open cluster NGC 1502" that he had discovered while sweeping the sky with a pair of 7×35 binoculars.

Kemble's Cascade Houston was so impressed that he wrote an article on the asterism that appeared in his Deep Sky Wonders column in the astronomy magazine Sky & Telescope in 1980, in which he named it Kemble's Cascade.

Father Lucian Kemble was also associated with two other asterisms, Kemble 2 (an asterism in the constellation of Draco that resembles a small version of Cassiopeia) and Kemble's Kite (an asterism that resembles a kite with a tail which is also in the constellation of Camelopardalis). In addition, an asteroid, 78431 Kemble, was named in his honour.