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Southern Cross - August 1996

• Constellation of the Month - Lyra

Constellation of the Month - Lyra

Albert Brakel

Low in the north at this time of year there is a pretty little constellation that hasn't made it into this column for at least 5 years. It is Lyra the Lyre, and is easily found because it is a distinctive group that contains Vega, the fifth brightest star in the sky. In legend it is said to represent the instrument invented from the shell of a tortoise by Hermes for his half-brother Apollo, who passed it on to his son Orpheus, the musician of the Argonauts.

Though small, Lyra contains within its borders quite a number of interesting targets for the telescope, and is a paradise for small scopes. Vega itself has a wide, 10.5 magnitude field companion 63" to the south, which is not physically connected. It is difficult to see in the glare of Vega. The angle and separation between the two is changing because Vega, lying only 27 light years away, has a relatively large proper motion.

Epsilon Lyrae (RA 18h 44.4m, Dec. 39 40'), about 1.5 degrees NE of Vega, is well known as the "double double". It is easily resolved even with binoculars into two components 3.5' apart. If you have very keen eyesight, you could test it by trying to see them as a pair. Each component can in turn be split into two with a moderate telescope, transforming this into a quadruple system. With the C8 at 117x both appeared elongate, but they were split with 154x. The northern pair (mags. 5.1 and 6.0, separation 2.5") is aligned N-S, while the southern pair (mags. 5.1 and 5.4, 2.4") lies E-W. The seeing was better than average on this occasion; on poorer nights I have been unable to resolve the two pairs. You can therefore use Epsilon Lyr as a test for the seeing conditions. One of the southern components has been resolved into a close pair by speckle interferometry, making this a quintuple system.

The most distinctive asterism in the constellation is the parallelogram SE of Vega. The NE corner is Delta Lyr (18h 53.7m, 36 58'), a fine double star for finderscopes, binoculars, or the unaided eye. The beautiful contrasting yellow-orange and blue-white stars are separated by a huge 10.5'. Their magnitudes are 4.5 and 5.5.

Diagonally opposite we have Beta Lyr (18h 50.1, 33 22'), also known as Sheliak. Beta is a celebrated eclipsing variable with a period of 12.9 days, during which time it goes through two unequal minima of mag. 3.8 and 4.3, between maxima of 3.4. The binary is too close to resolve, but there is a third component to the system that does not pose a problem, a 7.2 mag. companion lying a wide 46" to the SSE.

Just under half way along the southern side of the parallelogram is Lyra's most famous inhabitant, the Ring Nebula M57 (18h 53.6m, 33 02'), one of the sky's brightest planetary nebulae. At 77x it appeared as a small grey disk about 70" across, showing a hint of a central "hole" with averted vision. 117x and higher powers up to 235x made the central "hole" more obvious. The "hole" is now thought to be the centre of an hourglass shape seen end-on, and the bright greyish "ring", which is distinctly elliptical, is the greatest thickness in our line of sight. With an OIII filter the "ring" appeared very bright, but the central "hole" also brightened so that the contrast with the "ring" was less, giving the object the less impressive look of a disk with a bright edge. Unless you use a big telescope, you will not be able to see the 15th mag. central star.

Burnham 648 (18h 57.1m, 32 54') is a binary close to Gamma Lyr on the same side as M57. The mag. 5.4 and 7.5 components vary their distance from 0.4" to 1.2" over a 61 year period. The stars are now closing, but still theoretically resolvable with 20 cm aperture, although in practice the seeing just wasn't good enough when I tried.

If you now look about 3 deg. NE of Gamma, around 19h 09m, 34 41', you'll come across the unusual sight of two remarkably similar and well-separated doubles in the same field of view. The northern one is Struve 2470, separated by 11' from Struve 2474. They are mags. 6.6 and 8.6 at 14", and 6.7 and 8.8 at 16" respectively, both are easily split with 50x, and both are oriented E-W. The striking similarity is only challenged by the northern pair being white, and the southern pair yellow. Even if you don't usually bother with double stars, don't miss these two.

Eta Lyr (19h 13.8m, 39 09'), towards the eastern border, is a white mag. 4.4 and 9.1 binary, so widely separated at 28" that it is resolvable with 50x, and yet another easy target for small scopes. In the same field to the SW is another easily split pair, white and orange in color, and dimmer. Look carefully to the east about the same distance for a third double in the field, dimmer and closer together, but again easily split.

Open clusters are not a feature of Lyra, but there is one 2 deg. SE of Eta and E of Theta Lyr. NGC 6791 (19h 20.7m, 37 51') is faint on the chart and hard to find. At first I saw what seemed to be a loose scattering of field stars, and it took a while to convince myself that I was in the right area. One observing guide describes it as an "unusually rich star cluster", but this is a bit off-beam for our latitude, where the low elevation doesn't help. The brightest members are 13th mag., and apparently at the higher elevations obtainable from the northern hemisphere, large telescopes show "a rich blend of tiny stars strewn across the entire field". I saw only a dozen faint stars in an area about 15' across.

In the north of the constellation, culminating only 11 deg. above the horizon, lies the semi-regular variable R Lyr (18h 55.3m, 43 57'). It fluctuates between mags. 4.1 and 5.0 over about 46 days, and like most stars of this type, it's red.

Lyra has one more gem for you to enjoy, in its southeast. This is M56 (19h 16.6, 30 11'), a small globular cluster 7' across with a fairly bright core. It was unresolved at all powers I tried between 77x and 235x, although with 235x some mottling could be detected around the edges with averted vision. The degree of resolution will be sensitive to the seeing conditions.