It’s August and suddenly the nights are getting a bit longer so what is there to see in our night sky this month?
Everyone should watch out for August’s amazing Perseid Meteor Shower. You probably know already that there are a number of regular meteor showers over the year but the Perseids are among the greatest and oldest (they have been observed for about two thousand years). The shower is spread over several weeks, reaching a sharp maximum between August 8 and 14 with a peak on 12 August. As the night unfolds (weather permitting) and the constellation Perseus climbs higher in the north eastern sky the number of falling meteors you can see will increase. During the peak, lucky observers in previous years have seen a hundred or more falling meteors per hour. Be prepared to stay up late though, as Perseus is best seen in the early hours of the morning.
As you watch this spectacle, think about what you are seeing. The meteoroids are tiny pieces of debris that are left behind from a comet called 109P/Swift-Tuttle (just Swift-Tuttle to her friends). Swift-Tuttle orbits the Sun every 135 years. Comets are largely made of ice and rocky dust. As the comet gets warmer as it approaches the Sun, some of the ice turns into gas spraying into space, carrying dust with it. As a result a stream of dust is left in a tail behind the comet. Over time the dust is smeared along the comet’s orbital path. As the Earth moves through the comet’s orbit our planet ploughs into the hapless dust particles causing a rain of meteors. Most meteoroids are no bigger than a grain of sand; however they burn up spectacularly as they shoot through our atmosphere at 59 km/s (37 miles per second).
The Perseids will reach their peak about 12 August however it is possible to see them for a number of days either side of the peak maximum. This year’s peak is unfortunately timed for skywatchers in the UK as it is predicted for 2-4.30 pm (BST) which is of course right in the middle of the afternoon. However if you go out on the evenings between 10 and 14 August you are sure to see some Perseids. This shower is called the Perseids because if you trace the path of the meteors they appear to radiate from the constellation of Perseus, named for the Greek mythological hero who killed the evil gorgon Medusa.
Meteor showers always have an element of unpredictability. Although the Perseids is quite a reliable meteor shower it can have quiet periods followed by flurries of activity. Usually we can hope for a ZHR or Zenithal Hourly Rate of about 80 meteors per hour but this year there is a possibility of a peak ZHR of up to 200 (ZHRs of this magnitude are usually only measured by experienced observers under perfect conditions) . We could witness this greater spectacle around midnight on the 11-12 August based on a prediction from NASA and our colleagues David Asher and Tolis Christou of Armagh Observatory who see the possibility of an additional brief minor outburst of meteors caused by the Earth barging through a denser swarm of meteoroids made of several overlapping recent debris trails from the comet.
To see the shower you will need a nice dark, clear sky so ideally try to find an area with as little light pollution as possible. Bring a deck chair or sun lounger and a warm coat or blanket for comfort and let your eyes adapt to the darkness for at least 15 minutes. Alternately on 11 August 2016 come along to the observing session and open night hosted by Armagh Observatory.
What else is there is see this month? The Moon won’t show its face in the evening skies of early August 2016 making it a good time to look out for (or even photograph) deep sky objects like the star clusters and nebulae discussed in previous months and the Milky Way. Wait until it is truly dark, probably after midnight about 5 August and look south. If it is clear and dark enough you will see the Great Sky River arching from horizon to horizon over your head. Look at it through binoculars and marvel at the multitude of stars. Then put the binoculars aside for a moment and find the Summer Triangle of stars we have discussed so many times here. The top left star in this asterism is Deneb, brightest star of Cygnus and indeed one of the most luminous stars in our part of the Galaxy. Cross-shaped Cygnus is said to represent that deeply detestable deity Zeus from the Greek Mythological Universe (GMU) as he posed as a swan on a mission to mess with mortals. Now you have found Cygnus, pick up your binoculars and use them to sweep through the rich starfields of the Milky Way, but watch out for a dark patch where it seems that stars are rare. This is the Cygnus Rift (sometimes alternatively called the Northern Coalsack), a deep lane of dark interstellar dust and molecular gas starting about three hundred light years away in our spiral arm which blocks the light from stars beyond it.
The Cygnus Rift is only part of a larger molecular cloud complex that extends down the sky through the length of Milky Way. This huge blotch is called the Great Rift and continues into Aquila (as the Aquila Rift; well-known to followers of that wonderful SF author Alastair Reynolds), through Ophiuchus, into Sagittarius (neatly blocking the Galactic Core from human eyes) and beyond the horizon to far Centaurus.
Lying in the Great Rift are the stars of the constellation of Vulpecula the Fox. Of course these stars really lie in front of the Rift which is further in the distance. Vulpecula is another of those fiddly little constellations created by Polish astronomer Johannes Hevelius (1611 – 1687). Again this is not a constellation named after a crafty character from the GMU, but something put up there by Hevelius in a flight of whimsy. Hevelius is quoted in Richard Hinckley Allen’s classic Star Names: Their Lore and Meaning (1899) as explaining (and remember, to him the constellation of Lyra was actually a vulture)
I wished to place a fox and a goose in the space of the sky well fitted to it; because such an animal is very cunning, voracious and fierce. Aquila and Vultur are of the same nature, rapacious and greedy.
And so the constellation of Vulpecula et Anser (Little Fox and Goose) was born!
Over time Anser the Goose has faded into almost complete obscurity on star charts but is still represented by Alpha Vulpeculae which is known as Anser or sometimes Lukida. Alpha Vulpeculae itself is an M class red giant about 300 light years distant. It is believed to be a member of the Arcturus Stream, a group of old and fast moving stars that are believed to have originated, not in our own galaxy, but in some ancient dwarf galaxy (a conclusion based on their chemical fingerprints). Through binoculars or telescope, Alpha Vulpeculae can seem to be the primary of binary system, but this is misleading; its “companion” is 8 Vulpeculae, a K class orange giant which lies on the same line of sight but, at about 485 light years distant, is much further away.
Another amazing object in Vulpecula is a well-known planetary nebula, M27, also known as the Dumbbell Nebula. About 1350 light years from the Sun, this object marks where a star underwent a prolonged demise, ejecting vast shells of gas during its ordeal. This rough globe of glowing gas is what we now see as the nebula, deep inside it is the dead but still white hot core of the star, an object known to astronomers as a white dwarf. Although dead as any dodo, the white dwarf will shine on, radiating its heat and light over trillions of years as it cools. M27 is a suitable target for back garden telescopes, and a pretty sight it is, but remember the limitations of the human retina, through the eyepiece expect it to look a greenish grey and not the vivid colours captured by imaging devices. If you do try to image it an exposure of 90 seconds or so ought to reveal it in reds and greens. These colours suggest M27’s alternative (and more appropriate in my opinion) nickname of the “Apple Core Nebula”.
August is the last month of the summer holidays for some but there’s still plenty to see up there. Make the most of the summer evenings!
(Article by Colin Johnston, Science Education Director. Article updated 3 August 2016)