The Evening Sky in June 2015
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The Evening Sky in June 2015
Venus, brilliant and silver, and golden Jupiter appear in the northwest quarter of the sky after sunset. Venus sets soon after 8 pm at the beginning of the month. Jupiter sets two hours later. Through the month Venus moves a little higher in the evening sky while Jupiter sinks lower. This brings them together. By the end of June they are only a full-moon's width apart. The thin crescent moon will be near them on the 20th. Though low, Jupiter is worth viewing in a telescope. Even binoculars will show one or two of its moons looking like stars close to the planet. Venus looks like a featureless first-quarter moon, similar in size to Jupiter. The close separation of the planets is a line-of-sight effect. At the end of the month, when the two planets appear close together Venus is 78 million km away and Jupiter is 910 million km away on the far side of the Sun.
Low in the west at dusk is Sirius, the brightest true star. It sets around 9 pm mid-month, twinkling like a diamond. Canopus, the second brightest star, is in the southwest. It is a 'circumpolar' star: one that never sets. Sirius appears bright both because it is 20 times brighter than the sun, and because it is relatively close at nine light years*. Canopus, the second brightest star, is higher in the southwest sky, circling lower into the south later on. Canopus is 310 light years away and 13,000 times brighter than the sun.
Arcturus is a lone bright star in the northeast. Its orange light often twinkles red and green when it is low in the sky. It sets in the northwest in the morning hours.
Saturn, midway up the eastern sky, is the same brightness as Arcturus but cream-coloured. To its right but fainter, is orange Antares, the brightest star in Scorpius. A small telescope shows Saturn's rings and its biggest moon, Titan, about four ring-diameters from the planet. Other smaller moons appear as faint stars closer to Saturn. Saturn is 1350 million km away mid-month.
Crux, the Southern Cross, is south of the zenith. Beside it and brighter are Beta and Alpha Centauri, often called 'The Pointers' because they point at Crux. Alpha Centauri is the closest naked-eye star, 4.3 light years away. Beta Centauri and many of the stars in Crux are hot, extremely bright blue-giant stars hundreds of light years away. They are members of a group of stars that formed together then scattered. The group is called the Scorpio-Centaurus Association.
Antares, marking the scorpion's heart, is a red giant star: 600 light years away and 19 000 times brighter than the sun. Red giants are much bigger than the sun but much cooler, hence the orange- red colour. Though hundreds of times bigger than the Sun, Antares is only about 20 times the Sun's mass or weight. Most of the star's mass is in its hot dense core. The rest of the star is thin gas. Red giants are dying stars, wringing the last of the thermo-nuclear energy from their cores. Antares will end in a spectacular supernova explosion in a few million years. Below Scorpius is Sagittarius, its brighter stars making 'the teapot'.
The Milky Way is brightest and broadest in the southeast toward Scorpius and Sagittarius. It remains bright but narrower through Crux and Carina then fades in the western sky. The Milky Way is our edgewise view of the galaxy, the pancake of billions of stars of which the sun is just one. The thick hub of the galaxy, 30 000 light years away, is in Sagittarius. A scan along the Milky Way with binoculars will find many clusters of stars and some glowing gas clouds. Relatively nearby dark clouds of dust and gas dim the light of distant stars in the Milky Way. This makes them look like holes and slots in the Milky Way. There is a well-known dark cloud called The Coalsack by the Southern Cross. It is around 600 light years away. The dust, more like smoke, comes off old red stars. These clouds eventually coalesce into new stars.
The Clouds of Magellan, LMC and SMC, in the lower southern sky, are luminous patches easily seen by eye in a dark sky. They are two small galaxies about 160 000 and 200 000 light years away. The Large Cloud is about 5% the mass of the Milky Way; the Small Cloud is about 3%.
*A light year (l.y.)is the distance that light travels in one year: nearly 10 million million km or 1013 km. Sunlight takes eight minutes to get here; moonlight about one second. Sunlight reaches Neptune, the outermost major planet, in four hours. It takes sunlight four years to reach the nearest star, Alpha Centauri.