Jimmy Westlake: When planet meets star
May 23, 2011
Saturn is the slowest moving of the naked-eye planets, requiring nearly 30 years to wander through the 12 constellations of the zodiac. It is presently traversing the constellation of Virgo and is prominent in our early evening sky. Saturn entered Virgo in September 2009 and won't leave this huge constellation for good until September 2013.
Virgo's brightest star is Spica, a blue-hot beauty whose name means "the ear of wheat." Yellowish Saturn is nearly identical in brightness to Spica and is located about a hand-span to Spica's upper right this month. Look for the prominent duo due south and about halfway up in the sky at about 10 p.m. in late May.
Once you've identified Saturn, you'll notice immediately that there is a second fainter object very close to it. This is the star Gamma Virginis, also known by its proper name Porrima, after a Roman goddess of prophecy. During the past four months, Saturn has been slowly backtracking toward Porrima and the two now form a striking "double star" to the naked eye. Today, you can barely squeeze a full moon between the two, but by June 8, when they are closest, that separation will be cut in half. Of course, the apparent closeness of the planet and the star is a line-of-sight illusion. Porrima is 38 light years from Earth, while Saturn is only 75 light minutes away — 266,000 times closer.
Porrima is recognized as one of the finest binary stars in the sky. What looks like a single star to the unaided eye is really a pair of twin suns, almost identical in brightness and pale yellow color. Porrima's two stars require 169 years to complete an orbit of each other. They are 43 astronomical units (AU) apart, on average, but their elliptical orbits carry them from a maximum distance of 81 AU apart to a minimum of only 5 AU. For comparison, Earth is 1 AU from the sun; Jupiter, 5 AU; Pluto, 40 AU. When closest together, as they were in 2005, the twin stars seem to merge into one, and it is very difficult to see both stars distinctly except through a large telescope. By now, they have separated enough that any telescope 3 inches in diameter or larger should resolve the pair.
As Saturn creeps closer to Porrima in the coming nights, both objects should be visible at the same time in a telescope at low power. Seeing a planet and a bright star together in a telescope is an unusual sight anytime, but if that planet is the magnificent ringed planet Saturn and the star is a stunning binary like Porrima, the view can only be described as cosmic. But even if you don't have a telescope, take a moment to look up in wonder, with nothing more than your two eyes, as planet meets star.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all around the world. His "Celestial News" column appears weekly in the Steamboat Today newspaper. His "Cosmic Moment" radio spots can be heard on local radio station KFMU. Also, visit Westlake's website at http://www.jwestlake.
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