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Moon, Mercury, and the 'Seven Sisters' -- a gorgeous conjunction

By / April 25, 2009

The sky map shows Sunday evening's conjunction as it would look from Chicago (minus the tall buildings and light pollution). The map shows the sky for around 8:30 p.m. Central Daylight Time. The conjunction appears about 10 degrees above the horizon. The tiny cluster of stars beneath the image of the moon are the Pleiades.

Sky map based on Stellarium software.

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Sunday evening, April 26, load "Mercury" from Gustav Holst's "The Planets" onto your iPod, grab some binoculars, and feast your eyes on as pretty a conjunction of cosmic stuff as you're likely to see.

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In the western sky, shortly after sunset, the moon, Mercury, and the Pleiades will be huddled closely together, roughly 10 degrees above the horizon. The earlier you look, the higher in the sky the conjunction will appear; but you may need the binoculars to pick out Mercury through the twilight.

The Pleiades -- also known as the Seven Sisters for the seven stars most readily visible to the naked eye -- contains more than 1,000 stars. The nine brightest stars are named for seven of the 12 daughters born to Atlas and Pleione of Greek mythology and for the parents.

The cluster is about 440 light years away. Roughly 25 percent are thought to be brown dwarfs, stellar shrimps that never gained enough mass to ignite their fusion furnaces.

The Pleiades's stars form what astronomers call an open cluster. The cluster formed out of the same interstellar cloud of dust and gas. But the stars are so far apart that they are not bound by their mutual gravitational attraction. Astronomers estimate that the stars in the Pleiades, also known as Subaru in Japanese, formed within the past 100 million years. In about 250 million years, they will begin to disperse.

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