If you want to see planets this month, you’re best to look in the eastern sky before sunrise. There you’ll see Mars, Venus, and Jupiter congregating and brightening over the next many weeks, along with a visit next week by the waning crescent Moon. But the planet Saturn still lingers in the western evening sky in early October after sunset, tangled among the claws of the fearsome scorpion represented by the constellation Scorpius and its next-door neighbor Libra.
Saturn is far past its best viewing position and size for this year. At a distance of 1.6 billion kilometers, the planet is just 14″ across and shines at magnitude +0.6. It lies low over the southwestern horizon after sunset, so it’s hard to get a sharp view of its rings and cloud bands in a telescope because of the thick, boiling air along the horizon. Observers in the southern hemisphere will see Saturn slightly higher in the sky and get a better view.
Just east of sand-colored Saturn lies the bright red supergiant star Antares, the 1st-magnitude star at the heart of the Scorpion. Low on the horizon in early October, Antares will twinkle aggressively in all but the steadiest skies and may reveal flashes of other colors as its light is scrambled by the churning evening air. About an hour after sunset, when the sky is sufficiently dark, grab a pair of binoculars and have a look at twinkling Antares. In the same field of view, just over a degree to the southwest, look for the faint smudge of the globular cluster Messier 4. At a distance of just 7,200 light years, it’s one of the closest of these ancient star clusters to Earth and the easiest to resolve into individual stars in a telescope. There are some 50,000 to 100,000 stars in this globular cluster, nearly all of them at least twice as old as the Earth and Sun.
Finally look for a fainter 2nd-magnitude blue-white star just east of Saturn. This is the star Graffias or β Scorpii. The star is one of the massive members of a group of stars near the plane of the Milky Way that is just 11 million years old and some 450 light years away. In a telescope, Graffias (also called Achrab) is a fine double star and easily splits at 50x or more. The main components of this star are a relatively wide 14” apart and shine at magnitudes 2.6 and 4.9. Each component is a hot and massive young star which will one day, like most bright stars in this part of the sky, explode as supernovae. Each component of Graffias is itself a close double star, and the brighter may have another star associated with it. So Graffias is a five-star system.
If you have the time, wait for darkness to fully arrive. If you are away from light pollution, you will see in this part of the sky the yellow-white band of the Milky Way thrusting straight up and perpendicular to the horizon and extending far overhead.
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