THE NIGHT SKY - JULY and AUGUST 2010

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As darkness gathers on late summer evenings the seven stars of the Plough may be seen starting their descent towards the north-western horizon. Lower down and almost due west, the bright orange star Arcturus can be found by following the curve of the handle of the Plough. Just above the south-western horizon are the stars of Scorpius, led by fiery Antares. Due south is Sagittarius, marking the heart of our Milky Way galaxy, a splendid region of the sky which is unfortunately poorly seen from our latitudes. In the South is the brilliant white star Vega, brightest member of the so-called “Summer Triangle”. The other members of this trio, Deneb in the constellation of Cygnus and Altair in Aquila are further to the east. On dark, moonless nights the summer Milky Way should be clearly seen in this region of the sky. Above the eastern horizon, the Great Square of Pegasus stands, diamond shaped, leading the rising stars of autumn. In the north-east the “W” shape of Cassiopeia is also getting higher and down on the northern horizon lies the yellow star Capella. The planets Venus, Mars and Saturn, which have all adorned the evening sky during the first half of 2010 gradually disappear into western evening twilight during the summer months. Jupiter however will be a splendid sight in the otherwise faint constellation of Pisces. Finally, with the Moon out of the way prospects look good for the Perseid meteor shower which peaks on the night of August 12th/13th.

The Solar System in July and August

Venus reaches Greatest Elongation East on August 20 th at magnitude -4.2, although it will be very low down on the western horizon by then. Venus closes in on Mars during July and August and the two planets are just two degrees apart on August 20 th. By then however Mars, much the fainter of the two at magnitude +1.5, will be very difficult to see in evening twilight. The Moon is 5 degrees to the south of Venus on July 14 th and 6 degrees south of Mars on July 16 th.

Saturn, slightly brighter than Mars, completes the trio of planets that can be currently seen during evening twilight. It is just two degrees from Mars on July 30th and within three degrees of Venus on August 7th and 8th. The Moon is 7 degrees south of Saturn on July 16th and again on August 13th.

Binoculars or a telescope will be needed to see Neptune which comes to opposition at magnitude 7.8 on August 20th on the border of Aquarius and Capricornus.

Jupiter is now rising before midnight and shines brilliantly at magnitude -2.8 in Pisces. It is fascinating to watch the transits, shadow transits, eclipses and occultations of its four Galilean moons through a telescope. Single and double transits and shadow transits are very common. There are however no triple events during July or August.

Binoculars will be required to see magnitude 5.7 Uranus. The search will be aided by its current close proximity to Jupiter.

Mercury is currently too close to the Sun for easy observation.

As mentioned above, maximum for the Perseid meteor shower occurs in a moonless sky on the night of August 12th – 13th. Zenithal Hourly Rates of up to 80 may be achieved.

A total solar eclipse occurs on July 11th. The partial phase may be seen from parts of the South Pacific and South America. The path of totality lies almost entirely over water, but ends in the extreme south of South America. No part of this eclipse may be seen from the UK.

Constellation of the Month: Lyra

The “Summer Triangle”, consists of the stars Vega, Deneb and Altair. The brightest of these stars, blue-white Vega is also the brightest star in the small constellation of Lyra. Vega is the fifth brightest star in the sky but its brightness owes more to its proximity than its intrinsic luminosity. Vega is an ordinary main sequence star and at a distance of 26 light years, one of the closest of the first magnitude stars to us. In 1850 Vega became the first star ever to be photographed and in 1963 one of the first to have its diameter (three times that of the Sun) measured by interferometry. In 1983 it was discovered that Vega is surrounded by a disk of cold dark dust that may be in the process of coalescing into a planetary system much like our own Solar System.

Lyra has much more to offer than just Vega. Close to Vega for instance is the celebrated “double double”, Epsilon Lyrae. Binoculars and even keen naked eyesight will split Epsilon into two stars, but a telescope under high power will show that each component is itself a close double. It is thought that the four stars form a true physical system, gigantic in size compared with the Solar System with the two doubles taking about a million years to orbit each other.

Lyra also has a couple of very interesting variable stars. Third magnitude Beta Lyrae is the “type star” for the Lyrid variables. These are eclipsing binaries consisting of two giant stars that are so close together and orbiting so rapidly that they are pulling each other out of shape and material transfer is actually taking place between them. Seventh magnitude RR Lyrae is the type star for a class of variables very similar to the Cepheids but with shorter periods and lower luminosities. The RR Lyrae stars are often referred to as the “cluster variables” because they are very common in globular clusters and their regular period-luminosity relationship has been very useful in determining the distances to these objects.

Lyra possesses one globular cluster from the Messier catalogue (M56), but the showpiece deep-sky object is undoubtedly the planetary nebula M57, also known as the “Ring Nebula”. This is probably the best known example of this type of object although others (such as M27 in nearby Vulpecula) are brighter and larger.