2021, September 16: Double Vision

Astronomy, Sky Watching
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September 16, 2021:  The famous Double Cluster is high in the northwest before the beginning of morning twilight.  The cluster pair is easily observed without a binocular’s optical assist in dark locations.  A binocular is needed in town.  The Great Galaxy in Andromeda is nearby.

2021, September 16: A pair stellar bunches, known as the Double Cluster, is found between the shapes of Perseus and Cassiopeia. The Andromeda Galaxy (M31) is to the lower left of the Queen.
Chart Caption – 2021, September 16: A pair stellar bunches, known as the Double Cluster, is found between the shapes of Perseus and Cassiopeia. The Andromeda Galaxy (M31) is to the lower left of the Queen.

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by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:32 a.m. CDT; Sunset, 6:58 p.m. CDT.  Check local sources for sunrise and sunset times for your location.

During mid-September, morning twilight begins about 90 minutes before sunrise.  Bright stars in the Orion region of the Milky Way gleam in the eastern sky.

Farther west, the “M” shape of Cassiopeia is nearly two-thirds of the way up in the sky in the northwest.  Some sky watchers might note that the Queen has a “W” shape.  At some hours of the night, the “W” is upside down to make an “M.”

Perseus is higher in the sky.  The main portion of the constellation looks like a fish hook.

Seemingly, between the shapes is a large, cloudy patch of light.  This fuzzy cloud is made of two star clusters that the eye cannot resolve into individual stars.

The cluster duo is within the boundaries of the starry Hero and frequently named “the Double Cluster in Perseus.”

Photo Caption – The Double Cluster in Perseus as from a sky survey at the upper left and the close up from the Hubble Sapce Telescope. (Credits: Ground-based image: Digitized Sky Survey (DSS); Hubble image: NASA, ESA, and S. Casertano (Space Telescope Science Institute); Processing: Gladys Kober (NASA/Catholic University of America)

Cataloged as NGC 869 and NGC 884, the clusters are easily visible in a dark location to the unaided eye. They fit into a binocular field nicely. 

They are magnificent in the lowest power telescopic eyepieces, and they may spill outside the field of view.  In his book Deep Sky Wonders, Walter Scott Houston described the clusters as the “jewel of the night.” The variety of star color is magnificent.

The clusters are about 7,000 light years away and a few hundred light years apart.  At this distance, the stars are similar in luminescence to Orion’s Betelgeuse and Rigel.  At the clusters’ distances, a star with the sun’s characteristics is visible only in very large telescopes.

Earlier in the month, the moon toured several clusters, Pleiades, Hyades, Messier 35, and Beehive, that are similar to the Double Cluster.

Photo Caption – The Great Galaxy in Andromeda (Messier 31) from the Galaxy Evolution Explorer (Photo Credit: NASA/JPL/California Institute of Technology)

The Great Galaxy in Andromeda (M31 on the chart above) is to the lower left of Cassiopeia in the western sky and well-within human eyesight in a nicely dark sky.  Like the Double Cluster, the galaxy has a fuzzy appearance. A binocular is always helpful and necessary in town.  This spiral galaxy’s distance was first measured by Edwin Hubble to demonstrate that other galaxies are out there beyond our Milky Way galaxy.

With the moon in the evening sky, take a look at the Double Cluster and a nearby galaxy.

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