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Cluster light, cluster bright: April evenings are ideal for viewing nearby star clusters.

The name of the month April may come from its once being the "aperture" (opening) of the year. Or perhaps it was the month of Aphrodite (Venus).

The latter idea is particularly appealing every eight years when April begins with one of my favorite recurring celestial events: the close meeting of Venus, the loveliest naked-eye planet, with the Pleiades or Seven Sisters, the loveliest naked-eye star cluster (see page 48).

In fact, April evenings are a time when all the great naked-eye star clusters are on display.

The Ursa Major Group. The five central stars of the Big Dipper are the core of a "moving group" of at least 16 stars that are drifting through space in the same direction. These stars were all born together, but they're gradually drifting apart because their mutual gravitational attraction isn't very strong. It's the group of physically related stars closest to Earth--centered about 80 light-years away. Its members are spread across at least 30 light-years in space and 23 [degrees] of sky.

The Coma Cluster. At about 280 light-years, the Coma Star Cluster is the third-closest, after the Ursa Major Group and the Hyades. It's centered about halfway between Beta Leonis (Denebola) and Alpha Canum Venaticorum (Cor Caroli). It represented the tuft of Leo the Lion's tail until the third century BC, when its name was changed to mark the amber tresses of Queen Berenice, wife of Ptolemy III of Egypt. The surprising full story of the queen and her locks is brilliantly told in Guy Ottewell's historical novel Berenice's Hair, available from www.universalworkshop.com.

The Coma cluster is an irregular scattering--beautifully disheveled hair--of a few dozen stars spread across about 5 [degrees] of sky. Binoculars are needed to see the cluster in light-polluted skies, but at least five of the stars are brighter than magnitude 5.5.

M44, the Beehive Cluster. Cancer's M44 spans more than a degree of sky. Ten of its stars shine between 6.3 and 6.9. About seventy more are brighter than 10th magnitude. M44 is just north of the ecliptic and thus makes a fine big target for the Moon and planets. M44 is about 580 light-years from Earth.

The Alpha Persei Association. Alpha Persei (Mirfak) shines at magnitude 1.8. It's surrounded by a loose grouping of somewhat fainter stars spanning more than 3 [degrees]. Many of these are visible to the unaided eye, but binoculars and ultra-wide-field telescopes give the best view. These stars are roughly 550 light-years away and are all quite young, born about 50 million years ago.

The Hyades and Pleiades. The Hyades include at least 16 stars brighter than magnitude 5.5 and span about 6 [degrees]. They're much more tightly bound than the Ursa Major Group, so they're often considered the closest true star cluster, some 150 light-years distant.

The Pleiades, roughly 400 light-years distant, are even more tightly bound. Nowhere are so many naked-eye stars packed into such a small area of sky--eight stars brighter than magnitude 5.5 within a 1 [degrees] circle.

The Pleiades Cluster is young, roughly 100 million years old, as attested by the preponderance of bright, hot, bluish stars.

The Pleiades play hostess to Venus in early April. But next month, after passing close to Beta Tauri, the planet begins a retrograde loop that is partially hidden by solar glare. When Venus finally starts to emerge from bright twilight at dawn in late June, it has--surprise!--moved into the Hyades.

Fred Schaaf has authored 13 books on astronomy.
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Title Annotation:Northern Hemisphere's Sky
Author:Schaaf, Fred
Publication:Sky & Telescope
Geographic Code:1USA
Date:Apr 1, 2012
Words:586
Previous Article:Planetary almanac.
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