If you are outside just after dark this week and aim binoculars low toward the northeastern sky – about midway between the constellations Cassiopeia and Perseus – you might think you're seeing double.
That's because you are.
You're looking in the direction of the famous double star cluster of Perseus, one of – or should I say two of – the most beautiful showpieces in the heavens. Under a clear, dark sky, you might be able to see the pair with the unaided eye as a single fuzzy patch of light.
Prehistoric stargazers almost certainly spotted this sight, but it wasn't until around 150 B.C. that it was officially cataloged. Both Hipparchus and Ptolemy wrote about the pair but referred to them only as a “nebula” or “cloudy spot,” one of the half-dozen or so they knew at the time.
Modern astronomers identify the double cluster as “h and chi Persei,” as well as by the catalog numbers NGC 869 and NGC 884. NGC 884, the easternmost of the pair, is slightly larger but contains fewer stars – about 150 or so; NGC 869 is smaller but contains about 200 stars.
At a distance of little more than 7,000 light-years, these two clusters lie within only a few hundred light-years of each other. They seem to be relatively young – most likely less than 13 million years old – and travel through space tethered by gravitation.
If you are unable to find the double cluster right away, try a “star-hopping” technique to help you out. First, find the constellation Cassiopeia. This was imagined by some of the ancients as a lovely queen who was so smitten by her own beauty that, when the gods placed her among the stars, they positioned her so she would spend half of her time upside down. If you watch her over an entire night – or through an entire year – you will easily understand the origin of this idea.
Now you can certainly search for the celestial queen if you like, but you will probably have more luck finding a “W” outlined by its five brightest stars – now standing nearly vertically in the northeast during early evening hours. Simply trace its two nearly vertical stars, and extend that line downward about the same distance. Aim binoculars or a small telescope in this direction, and you will be rewarded with a double cosmic treat.
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