The Scorpion shows a rich variety of objects

Press, 4 August 1987, Page 37

The Scorpion shows a rich variety of objects

August is a good time to view the constellation Scorpius. It will be in an excellent evening viewing position. It is one of the few constellations that closely resembles its name, Scorpion. A sweep through this group with binoculars will show a wide variety of celestial objects. The brilliant first magnitude star, Antares, dominates this constellation. It marks the heart of the Scorpion. Its name means "Rival of Mars,” because its distinctive red colour is similar to that of the red planet.

The ancient Chinese knew Antares as the “Fire Star.” It was one of the Royal Stars of Persia in 3000 B.C. It is a supergiant at a distance of about 430 light years. Its size is enormous. It is almost 300 times the size of the sun. If it was possible to place Antares where the sun is, its outer boundary would extend far beyond the Earth.

The luminosity of Antares is roughly 5000 times that of the Sun. However, its absolute magnitude is slightly fainter than that of the sun. The absolute magnitude of a star is the brightness it would have if it was at the standard distance of 10 parsecs, which equals 32.6 light years. It is the star’s huge size that makes its absolute magnitude so faint, because the output of radiation from each square metre of its surface is comparatively small. Antares has a much fainter, greenish companion. This requires a telescope of at least 15cm aperture to show it. Antares is often occulted by the moon. These events give the best views of the companion during the few seconds the primary is hidden whilst the companion is not. The next such occultation visible from New Zealand will occur on September 1. The companion is a peculiar hot star that gives out radio emission, probably because of matter being ejected from it.

The head of the Scorpion is marked by a line of three second-magni-tude stars, of whch Delta Scorpii is the brightest, with Beta Scorpii slightly fainter. These two stars form an equilateral triangle with a very fine globular cluster, Messier 80.

This cluster lies at the apex of the triangle, situated halfway towards Antares. The cluster is just a mass of faint stars. In 1860 there blazed forth in its centre a nova, T Scorpii.

Another fine globular cluster can be found very close to Antares. Although the two objects appear close together in the sky the cluster is very much more distant than Antares. Alongside this cluster can be seen an area devoid of stars. Interstellar dust hides the stars beyond in this region. Globular clusters are generally spherical in shape and most contain many thousands of stars. It is estimated that the densest of such clusters contains 10 million stars, compressed into a diameter of about 100 light years. These clusters are where the oldest stars are found.

Scorpius abounds in old novae. The term nova, meaning new, is misleading, because a star already existed. The original stars was faint and a member of a binary system. Suddenly such stars erupt, increasing in brightness a thousand times, or even by ten thousand times in a day or two. Gas is blown off the star at terrific speeds. After erupting the star fades either rapidly in a few weeks, or slowly in months and even years. The fraction of the star blown away during an eruption is small so that ultimately the star returns to its original state.

This constellation also includes a recurrent nova, one that has had more than one outburst. This star, U Scorpii, had a brief eruption earlier this year. A sweep through Scor-

pius with a pair of binoculars will show many double stars. A number of these are not true doubles as they are not physically connected. It just happens that two stars, at vastly different distances lie in the same line of sight so they appear to be double. Beta Scorpii is an exception. It is a multiple system consisting of three components. The brighter two stars, of second and fifth magnitude, contrast in colour and can be easily separated. An unusual object in this constellation is the brightest X-ray source in the sky. This is known as Sco X-l and is identified with a faint star. This star, VBlB Scorpii, varies in brightness between magnitudes 11.6 and 13.2 in a period of almost eighttenths of a day. Most X-ray stars are binaries. Optically we usually see only the giant star of these pairs, while the X-rays are emitted by the compact companion. Sco X-l is an exception as its compact star outshines the giant. The changes in brightness occur in the same period of the orbital period as one star revolves around the other.

Another type of star cluster can be found just below the tail of the Scorpion. Here binoculars will pick out two very beautiful open clusters, Messier 6 and 7. The former has been described as like a butterfly with outstretched wings. These open clusters do not contain nearly as many stars as the globular clusters. They usually have one or two hundred separate stars. They are not condensed so it is possible to easily separate the individual stars. The stars are often of contrasting colours, indicating differences in their surface temperatures. Scorpius is a fine example of how many different types of celestial objects can be viewed with binoculars. It is a very rich region full of interest. It is also a happy hunting place for those amateur astronomers who search for novae.

August is not a good month in which to see all the naked-eye planets. Mercury and Venus reach superior conjunction on August 20 and 23 respectively. Mars will be in conjunction with the Sun on August 25. These three planets will therefore be invisible this month. Jupiter, in Pisces, rises just after midnight on August 1 and around 11 p.m. on August 31. It will be visible to advantage only during the morning hours. Saturn, in Ophiuchus, is well placed for evening

viewing. It will set at 4 a.m. on August 1 and at 2.20 a.m. on August 31. The bright star, Spica in Virgo, can be found close to the Moon on the evening of August 1. Spica will be occulted at 1 a.m. on August 2 but this will not be visible from New Zealand as the star will have set before the occultation occurs. The Alpha Capricornid meteor shower will reach its maximum on August 2. This is not a rich shower, but is remarkable for producing very bright, yellow fireballs.