In August the Summer Triangle continues to dominate the seasonal sky. This classic asterism is defined by three bright stars of the constellations Cygnus, Aquila and Lyra, these are the distant but dazzling Deneb (Alpha Cygni), Altair (Alpha Aquila) and Vega (Alpha Lyrae).
Running through the Triangle is the pale misty band of the Milky Way. The Milky Way is the vast and slowly rotating spiral galaxy which is home to the Sun. When we look towards the Summer Triangle we are gazing spinward, in the direction the Galaxy is rotating, deeper into the Local Arm (also known as the Orion Arm, or even the Orion-Cygnus Arm by some), one of the Milky Way’s great spiral arms, significant to humanity as it is our arm. Viewing the Milky Way with any kind of telescope, a privilege enjoyed by a tiny minority of all the humans who have ever lived, shows it to be full of countless stars. Based on results from the Kepler satellite, we are now pretty sure that every one of those stars has at least one planet circling it. Capturing the Milky Way on a photograph with even a short exposure reveals structure, the vast but tenuous lanes of cool dark dust threaded through the spiral arms.
Blue-white Vega is of course one of the brightest stars in our night sky lying about 25 light years from us in Lyra. At magnitude 0.0, Vega is used as the ‘standard’ star against which all others are compared. Also in Lyra is the double star Beta Lyrae (Sheliak). This is in fact a close eclipsing pair of B-class stars about 960 light years from the Sun and was only the second eclipsing binary system to be discovered after Algol. The two stars are sufficiently close that both stars are deformed into ellipsoids by their mutual gravitational pull and matter is being drawn from one of the stars by the influence of its partner. This stolen starstuff forms an accretion disc which is spiralling into the attracting star. Systems like this are known as interacting binaries. A considerable amount of matter is exchanged between the stars, equivalent to about six times the mass of the Earth every year!
This dramatic situation in the Beta Lyrae system has been known about for a long time. It has inspired memorable artworks of the twin stars surrounded by great glowing spirals of red-glowing hydrogen seen in the skies of hypothetical neighbouring planets painted by famed astronomical artists Chesley Bonestell (1888 – 1986) and David A. Hardy (b1936). Sadly both artworks have dated somewhat, not quite matching our current understanding of accretion discs and their associated polar jets. Bonestell’s painting seems to have inspired a story by the well-known SF author Larry Niven (b1938). The Soft Weapon (part of his popular Known Space sequence) is set on “Cue Ball”, a fictitious Beta Lyrian planet with a skyscape after Bonestell. This tale was adapted into The Slaver Weapon, an episode of the almost forgotten Star Trek animated series (1973).
Beta Lyrae by Chesley Bonestell
Planet of Beta Lyrae (from The New Challenge of the Stars by Patrick Moore & David Hardy 1977)
If two stars are not enough for you, how about a quadruple system? Epsilon Lyrae (also known as the “Double-Double Star” lies close to Vega. To the unaided eyes of many, it appears as a nice double star (most of us need binoculars to see this), but viewed in the eyepiece of a telescope, each of the pair can be seen to be double stars too! This is a pair of binary systems orbiting each other about 0.16 light years (10100 AU) apart, the whole system being about 160 light years from the Sun. There actually seems to be a fifth star in the system but it is too close to one of the stars to be individually resolved being discovered with instruments in the 1980s. Some astronomers have suggested that there could be up to ten stars in the Epsilon Lyrae system!
Second brightest star in Lyra is Gamma Lyrae (Sulafat). It looks a good match in colour to Vega but is dimmer, this is hardly surprising as Gamma Lyrae is about 25 times as distant as Vega, some 620 light years from the Sun. A giant star, Gamma Lyrae is believed to be leaving its main sequence prime behind, and is now heading towards its red giant star phase followed by a graceful decline into a white dwarf surrounded by a planetary nebula. A sort of preview of Gamma Lyrae’s fate can be seen between it and Beta Lyrae in the dainty form of the Ring Nebula (M57). This is indeed a planetary nebula and it is justly famous for its beauty in astrophotographs. You will need a telescope and a high magnification to see it and do not expect a Hubblesque multi-coloured extravaganza to fill the eye, instead find a tiny grey smoke ring against the darkness and remain yourself that you are witnessing the end fate of a star a couple of thousand light years away.
Much, much closer to home, the annual Perseid Meteor shower reaches its peak between 12 and 13 August 2014. This is a stream of tiny gritty particles shed by the Comet 109P/Swift-Tuttle falling into the atmosphere at an eye-watering 59 km/s (37 miles per second) and meeting incandescent destruction about 100 km overhead. If you go out after midnight when your region of our planet is facing into the meteor stream and look north east you ought to see some of the bright streaks of the falling meteors radiating from the constellation Perseus. This is usually one of the best meteor showers to observe, at the peak about 90 fast and bright meteors per hour might be observed but this year the light of a bright and waning gibbous (almost full) Moon could overwhelm the spectacle of the meteors. It is still well worth watching though!
(Article by Colin Johnston, Science Education Director)
It’s (almost) decision time | stargazing newbie · August 8, 2015 at 03:32
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