The Night Sky
May / June
Our starting point this month is Virgo, one of the constellations of the zodiac - its name is Latin for virgin. Lying between Leo to the west and Libra to the east, it is the second largest constellation in the sky (after Hydra). Virgo is quite easy to find by way of its brightest star, Spica - follow the arc of the handle of the Plough to Arcturus (a bright orange-red star in Boötes) and then onwards to the blue star Spica, with Jupiter sitting just alongside. Arc to Arcturus and spike to Spica, as the saying would have it.
The star 70 Virginis has one of the first known extrasolar planetary systems, with one confirmed planet 7.5 times the mass of Jupiter.
The star Chi Virginis has one of the most massive planets ever detected, at a mass of 11.1 times that of Jupiter.
The sun-like star 61 Virginis has three planets: one is a super-Earth and two are Neptune-mass planets. Markarian's Chain is a stretch of galaxies that forms part of the Virgo Cluster - the galaxies lie along a smoothly curved line. Charles Messier first discovered two of the galaxies, M84 and M86, in the year 1781. The other galaxies seen in the chain are first mentioned in John L. Dreyer’s New General Catalogue (NGC), published in 1888. It was ultimately named after the Armenian astrophysicist, B. E. Markarian, who discovered their common motion in the early 1960s. Member galaxies include M84 (NGC 4374), M86 (NGC 4406), NGC 4477, NGC 4473, NGC 4461, NGC 4458, NGC 4438 and NGC 4435.
Part of what makes this chain unique is its visibility through smaller telescopes. Larger telescopes can be used to view the fainter galaxies, while a smaller scope can show a wider range.
At least seven galaxies in the chain appear to move coherently, although others appear to be superposed by chance.
In the vicinity of Markarian’s Chain is the giant elliptical galaxy M87, one degree to the south-east – it is bright enough to be visible in binoculars. A little further east is M90 – larger telescopes will show hints of this galaxy’s spiral arms.
M61 (NGC 4303) is an intermediate barred spiral galaxy. It was discovered by Barnaba Oriani on May 5th, 1779. This was six days before Charles Messier observed the same galaxy, but he mistook it for a comet. M61 is one of the largest members of Virgo Cluster, and is designated to belong to a smaller section of the galaxy cluster known as the S Cloud. It has an active galactic nucleus and is classified as a starburst galaxy containing a massive nuclear star cluster with an estimated mass of 105 solar masses and an age of 4 million years. Seven extragalactic supernovae have so far been observed in M61, making it one of the most prodigious galaxies for such cataclysmic events.
Lastly, south of Spica is NGC 5170, an edge-on spiral galaxy that will require steady skies and a large telescope to be seen at its best.
Virgo contains the pretty double star Porrima. The two stars in this system revolve about each other in 169 years, which means you can see, in a good backyard telescope, the motion of the stars over the course of only a decade or so. Until 1995, a small scope easily split this pair. From 1996 through 2010, the two stars moved too close together to be resolved in a small telescope. But by 2010, they separated enough to once again split in a telescope on nights of very steady seeing.
To split Porrima, pick a night when the air is steady and the stars twinkle very little. You’ll need a 3″ or 4″ telescope or larger and a magnification of 200x. Each star in the Porrima system is about magnitude 3.6, and each is a white F-type main sequence star just a little bigger and brighter and hotter than our Sun.
24 Coma Berenices - this widely spaced double is a real showpiece, particularly when seeing conditions are good. The primary is a deep yellow-orange in colour with a rich blue secondary hovering to the west, sometimes described as colourful gemstones resting in their settings.
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.
95 Herculis is a lesser known double star in Hercules, but its colour contrast can be even more vivid than Rasalgethi. These are both evolved stars that have moved on to burning heavier fuels in their core. While separately the stars should appear white and yellow, their proximity tricks the eye into seeing more vivid colours. Admiral Smythe, a prolific 19th century amateur astronomer, described the pair as “apple-green and cherry red”.
Regulus (α Leonis) presents a widely separated pair of stars that share a common proper motion.
Algeiba (γ Leonis) is a beautiful slow-moving binary; both golden yellow.
Cor Caroli (α Canum Venaticorum) is one of the finest double stars in the sky for small telescopes – the colours have been described as white and slightly yellowish.
Kuma (ν Draconis) - the eyes of the dragon. This pair of bright, beautifully matched stars certainly qualifies as eyes and they are easily found in the head of Draco.
16 and 17 Draconis, twin to the Dragon’s Eyes and easy to split in binoculars (if you can hold them steady enough!). Miram (η Persei) is the ‘topmost’ star in Perseus and the one closest to Cassiopeia – a very fine double star, showing golden yellow and pale blue colours.
After a feast of galaxies in Virgo, let’s go for something completely different:
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.
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.
Don’t miss the globular clusters M3 and M13 – the finder chart for double stars shows them both.
The Eta Aquarid meteor shower will peak around May 5th and May 6th – the best time to catch the show is just before dawn.
The Eta Aquarids is one of two meteor showers created by debris from Comet Halley. The Earth passes through Halley's path around the Sun a second time in October. This creates the Orionid meteor shower, which peaks around October 20th.