July, 2021: During the early evening sky, Hercules is nearly overhead as evening twilight ends. The constellation has a spectacular star cluster.
by Jeffrey L. Hunt
Step outside near the end of evening twilight, about two hours after sunset during early July. Look high in the southern sky. Notice two bright stars. Arcturus, topaz in color, is high in the south-southwest. Bluish Vega is high in the south, approaching overhead.
Draw a line in the sky between the two stars. About one-third of the way from Vega to Arcturus is Hercules. The Hero is upside down. His body is known as the “Keystone.”
A magnificent star cluster, commonly known as Messier 13 (M13), is far beyond the stars that make the constellation’s keystone shape.
In a dark location, without bright streetlights, the star cluster resembles a fuzzy star. Through a binocular, the cluster looks clearly different from the pinpoint stars.
Through a telescope at low power, individual stars become visible, but they seem to merge into a clump at the center, but the stars there are light years apart.
The cluster is a magnificent globular cluster and is frequently referred to as “one of the most spectacular globulars in the sky.”
Unlike clusters like the Pleiades, Hyades, or Beehive, the globulars revolve around the galactic center outside the plane of the Milky Way. About a hundred years ago, they were mapped to demonstrate the location of the galactic center – behind Sagittarius – and the sun’s location is far from the galactic center.
Through a telescope, a globular’s first impression is that of a cotton ball. Then at a closer look individual stars are visible along with the central concentration of stars. These clusters could have a million stars in them.
In comparison, the more famously named clusters revolve around the galaxy in the galactic plane. The clusters may have several hundred stars in them and they seem to have lots of space between them. These clusters are named open clusters or galactic clusters.
To find M13, point your binocular between Zeta Herculis (ζ Her on the chart) and Eta Herculis (η Her).
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Categories: Astronomy, Sky Watching
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