On 15 June 2014 look south about 10.30pm. The sky will be far from dark, but with a little effort you ought to spy the triangle topped by the star Arcturus and with Saturn and Mars at its base. By the time an hour has passed, around 11.30 pm, this trio will have moved into the south west and you should be about to see the star Spica slightly below the Saturn-Mars line.
By this time almost directly south and very low in the sky will be Antares (Alpha Scorpii). This red supergiant is the brightest star in the constellation Scorpius and is of awesome size. If you were to exchange the Sun as the centre of the Solar System with Antares it would encompass the orbits of all the inner planets and most of the main belt asteroids, of the naked eye stars only Orion’s Betelgeuse is larger. It is about 550 light years from our Sun. The name Antares means, roughly, “Rival to Mars” in Greek and references the pair’s similarity in colour and brightness. Scorpius is an ancient constellation; it was said to represent a scorpion by the astrologers of Babylon three thousand years ago. Centuries later the Greeks identified it as the scorpion responsible for delivering a fatal sting to legendary hunter Orion. From the north of Ireland we see little of it; most of Scorpius will be hiding under the horizon.
Looking due east at 11.30 will be the famous and familiar Summer Triangle of Deneb, Vega and Altair low in the sky. If you are outside early and have the time you can watch the trio come into view as the sky darkens from blue to black. The rich star fields of the Milky Way run through the Summer Triangle, explore them with binoculars and you will probably be reminded just how many stars there are in the galaxy. By the way, when you look towards Cygnus, you are looking spinward, that is to say in the direction our galaxy is rotating.
Above the Triangle is the large but faint constellation Draco which extends up the sky, its head is just above Vega, then the constellation snakes between Ursa Major and Ursa Minor. Like the Bears, Draco is circumpolar for northern hemisphere observers, meaning it never sets below the horizon. Thuban (Alpha Draconis ) is an impressive star of magnitude 3.65 but was once, about 5000 years ago, Earth’s pole star. The constellation of Draco contains the star Kepler-10 homestar to the recently announced mega-earth planet Kepler-10c. At magnitude 11, Kepler-10 is too dim to be visible to the unaided eye.
Returning to Vega (which will be the pole star in about 12 000 years from now), follow an imaginary line between it and Arcturus, this line will cross two more constellations, faint and sprawling Hercules and that celestial smile Corona Borealis. Hercules, commemorating the mythical strongman, has never been a spectacular constellation to me but is home to one of my favourite objects, the justly celebrated M13, the Hercules Globular Cluster. Look at it with binoculars or a telescope if you have them (sadly it is all but invisible without them) and think about what you are seeing. M13 is 25 000 light years away, almost as far away as the Galactic Core, and is several hundred thousand stars crammed into a ball less than 200 light years across. If current thinking is correct M13 and other globular clusters were the cores of dwarf galaxies consumed by the Milky Way in ages past. If this is the case, M13 may have a large black hole at its centre.
Corona Borealis is a sweet little constellation, resembling a cheery cartoon grin but there is little there to excite observers without telescopes. Its brightest star is Alphekka (Alpha Corona Boreais) , a 2nd magnitude star. Actually an eclipsing binary star system, Alphekka is 75 light years from Earth. Corona Borealis means “Northern Crown” and the constellation has been said to represent legendary regal headgear from Greek and Welsh myth cycles.
The short summer nights have some subtle sights to enjoy, why not seek them out?
(Article by Colin Johnston, Science Education Director)