Brian Jones: Lyra
LYRA: THE CELESTIAL HARP
The tiny but conspicuous constellation Lyra is easy to locate at this time of year; its brightest star Vega being prominent a little to the west of the overhead point during September evenings after sunset. This group of stars was known to the Arabs as an Eagle of the Desert, to the Native North Americans as a Vulture and to the Romans as a Harp, although the most famous legend attached to Lyra is that of the Ancient Greeks who identified the group with the harp given to Orpheus by Apollo so that he could play music to the Argonauts.
The leading star in the constellation is the brilliant blue-white Vega. Shining from a distance of around 26 light years, making it one of the Sun’s closest stellar neighbours, Vega is destined to play an important role in the distant future, at least as far as Earth-based observers are concerned.
The north celestial pole is currently marked by the star Polaris, which is situated in the constellation of Ursa Minor (see previous blog on Ursa Minor). The Earth’s axis is currently pointing towards Polaris, which means that if you were stood at the North Pole, Polaris would be located directly overhead. The daily rotation of our planet on its axis makes the rest of the stars in the sky appear to travel around Polaris, their paths through the sky being centred on the Pole Star.
However, the position of the north celestial pole is slowly changing, this because of a ‘wobble’ in the Earth’s axis of rotation. This wobble is known as ‘precession’ and is similar to that of a spinning top which is slowing down. Precession is caused by the combined gravitational influences of the Sun and Moon on our planet. Each resulting wobble of the Earth’s axis takes nearly 26,000 years to complete, the net effect of precession being that, over this period, the north celestial pole traces out a large circle around the northern sky. For northern hemisphere observers, this results in a slow change in the apparent location of the north celestial pole. Polaris will eventually relinquish its position and Vega will become the Pole Star some 11,500 years from now.
Much further away than Vega is the blue-white star Sheliak which shines from a distance of around 900 light years. This huge distance means that the light we’re seeing from Sheliak set off on its journey towards us just a few years after the Battle of Hastings! Of particular interest in Lyra is the star Epsilon. If you have really keen eyesight, and the sky is dark and clear, you may notice that Epsilon is a double star. Binoculars bring out both components easily. However, larger telescopes reveal that each of the two stars making up the Epsilon system is actually double again. The fact that the Epsilon system contains four stars was first noticed by the astronomer Sir William Herschel in 1779. The two pairs are both binary stars, the members of which orbit each other. Epsilon is also known collectively as the ‘Double-Double’ star.
Delta is another naked-eye double with stars that are blue-white and orange-red in colour. Although these two stars are wide enough apart to be seen without optical aid, binoculars may be needed to bring out the individual colours of the stars. Binoculars may also be used to resolve the two stars forming the double star Zeta. Lyra lies quite close to the Milky Way and the whole area around Delta and Zeta is rich in faint stars and is a marvellous sight through binoculars, providing the sky is reasonably dark and clear.
So, if the warm summer evenings tempt you out to sample some of the wonders of the late-summer sky, check out Lyra. The Heavenly Harp certainly has a lot to offer and is well worth the efforts of the rambling astronomer! Happy stargazing!