July Night Sky
Jupiter finally succumbs to bright evening twilight and is lost to view towards the end of July. Mars remains visible during the evenings, low in the SW, and is joined by the First Quarter moon on the 5th. On the 12th Mars lies just above Spica, chief star in Virgo. Telescopically Mars appears quite small with just a hint of surface detail visible. Saturn is well placed in the evening sky, visible across in the SSW midway between Mars and the conspicuous orange star Antares in Scorpius. Through a scope the stunning rings should be quite apparent – one of nature’s finest sights. Look for Titan, Saturn’s largest moon as a speck of light nearby, it circles around Saturn twice a month in an anti-clockwise direction. Saturn lies near our moon on the 7th. Mercury makes an appearance in the dawn sky after July 20th. Look for it low down in the NE just a few degrees above the horizon and lower left of much brighter Venus, which may serve as a guide. A slim crescent moon lies off to the right of Venus on the 24th. You will need to be up around 50 minutes before sunrise to spot them.
Moon Phases: 1st Qtr 5th. Full 12th. Last Qtr 19th. New 26th.
The shorter summer nights allow little opportunity for the casual observer to readily identify many of the constellations on show; indeed only a handful or so of stars are visible before most people have gone to bed! Just identifying these few stellar jewels scattered across the heavens can be tricky. Yet, not all are new, unfamiliar stars, some are old friends, but now located in a different aspect of the sky. Time then to put a name to the new arrivals and unmask our stellar friends’ disguise.
We’ll start high to the NE with the second brightest star currently visible in the night sky: Vega, chief star in Lyra, the Harp. Vega resides 27 light years away and may be considered a young, very hot adolescent star, a fact reflected by its lovely steely blue-white hue. Back in the 1980s a debris disk was discovered by infra red satellite surrounding Vega, which astronomers believe to be left-over material after the formation of Vega itself. This may eventually coalesce into a planetary system.
Our next star, Altair, is located across in the ESE in the constellation of Aquila the Eagle. Also considered a young star, Altair is a star in a spin, an unusually rapid one, rotating in less than six hours. As a result Altair is decidedly oblate – rugby ball shaped. Altair also has the distinction of being the nearest first magnitude star visible from Britain during the summer, a mere 16 light years away.
Both Altair and Vega play a prominent role in one of the few star legends to have come down to us from ancient China, the appealing story of the weaving girl and the herd-boy. According to this tale Vega, the weaving girl, and Altair, the herd-boy, were deeply in love, and lost in amorous dalliance neglected their duties. The two were suitably punished, eternally separated by the celestial river, the impassable Milky Way. Only once a year, on the seventh night of the seventh moon were the lovers allowed to meet when a bridge of doves temporarily spanned the river of stars. The two will embrace once again on 26 July. No voyeurs please!
Below Saturn over in the WSW, the bright spring star Spica, or alpha Virginis, is inexorably heading towards the western horizon. Spica is a close spectroscopic binary star (you won’t split them using a telescope) over 260 light years distant. Both components are larger and much hotter than our sun.
Capella, our next port of call, lies in the constellation of Auriga, the charioteer, and is actually visible all year round from our latitude (circumpolar). During the summer months Capella is located low above the N horizon, hence it is often mistaken for the North Star. Like Spica, Capella is an extremely close binary system, the two components separated by just 7 million miles. Both stars are similar in type, temperature and age to our own Sun, i.e. middle aged, but considerably larger. They are situated 46 light away.
Next up is the brightest star visible throughout summer and early autumn, Arcturus, presently located high in the SSW. In the distant past solitary Arcturus would have quite resembled our own sun, but has evolved into an orange giant star 30 times its original diameter. At 37 light years Arcturus is the nearest example of this type of star.
Arcturus would however be dwarfed by our next luminary, which is only briefly visible from our shores during midsummer. Deep orange Antares, the ‘rival of Mars’, is situated in the constellation of Scorpius, located very low to the south as twilight deepens. Classed as a red super giant, Antares has a diameter estimated to be around 250 million miles, ten times the size of Arcturus and lies over 350 light years away. Unlike Arcturus, Antares will eventually bow out in a blaze of glory, a fate that will definitely befall our final destination
Deneb, in the constellation of Cygnus is currently located mid-way up in the NE. Along with Vega and Altair, it forms the ‘summer triangle’ asterism. Of the stars visited, Deneb is the least bright, but appearances can be deceptive. Its immense distance of around 1,700 light years hide Deneb’s true nature, a genuine heavy weight amongst luminaries, a stellar superstar 25 times more massive and 140 thousand times more luminous than our Sun. In terms of true age Deneb is still very young, but is converting energy at such a prodigious rate that in less than 50 million years it will fully evolve into a star possibly twice the size of Antares, ending its days in cataclysmic fashion as a supernova. Then, for a few weeks Deneb will outshine all but the Sun and Moon and our descendants will marvel at the new light in the night . . . and day sky!
by Mark Lawson, Whitby & District Astronomical Society
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