Colorado Skies: What's up with the "Great Square"?

In the vast expanse of autumn sky between the Summer Triangle stars of Vega, Deneb and Altair and the bright stars and constellations of the winter sky one can only find a few stellar landmarks of any prominence.

One of these, the so-called Great Square of Pegasus, can be seen just to the east of overhead in this month’s early evening Colorado sky. Made up of a set of four second magnitude stars, the Great Square occupies an area of the sky roughly the same size as the Big Dipper. Even more interesting is the fact that the Great Square is almost perfectly aligned in a north-south direction with its western edge coincident with the “prime meridian” of the astronomical latitude-longitude coordinate system astronomers use to determine positions of celestial objects.

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As is the case with most constellations and star groups, the stars of the Great Square represent a diverse blend of stellar types located at a variety of distances. Three of the stars of the Great Square are hot, bluish stars ranging in age between 1 billion and 3 billion years, while the fourth star is a red giant star which is in an advanced stage of its life cycle. (An interesting binocular or small-telescope exercise is to observationally determine which of the four stars of the Great Square is indeed the red giant star.) Distances to the stars of the Great Square exhibit similar significant differences and range in value from 100 to 450 light years

Astronomers are particularly interested in this region of the sky because, like the Realm of the Galaxies in the springtime sky, the Great Square is relatively free of the obscuring gas and dust of the Milky Way, whose plane sweeps past the Great Square at a considerable distance to the north and west through constellations such as Cygnus, Perseus and Cassiopeia. As a result, astronomers are able to peer through this “Pegasus Window” into the “extragalactic space beyond the Milky Way, and in doing so, have discovered a vast and wondrous myriad of other Milky Way systems that we call the galaxies.

Over the past 90 years astronomers have explored ever deeper into extragalactic space through the Pegasus window and have recorded images of galaxies at the very edge of the observable universe, some 12 billion light years away. On the basis of the galaxy counts conducted in selected regions of the sky such as the Pegasus Window, astronomers estimate that some 300 billion galaxies exist throughout the entire universe, each of which is a system of stars, gas, and dust comparable in size to our own Milky Way Galaxy. Such is the awe-inspiring view through this amazing window to the edge of the universe!

Elsewhere in the sky: The planet Venus continues its gradual rise out of the evening twilight and is now easily visible to the southwest for about an hour after sunset. This splendid object passes between the planet Saturn and the much fainter and reddish Scorpius star Antares during the last week of October.

Farther to the east of Venus in the sky at sunset is the relatively bright ruddy planet Mars, which is currently located just to the north of the Sagittarius Teapot. Watch in upcoming weeks as Mars begins to sweep across the constellations of the zodiac almost as rapidly as does the sun.

The elusive planet Mercury has its finest pre-dawn apparition of the year during the first half of October. This speedy reddish-orange world will be readily visible above the eastern horizon for about 45 minutes or so before sunrise during the first two weeks of October.

After the first week of October, Mercury is joined in the morning sky by the planet Jupiter which begins October rising about a half-hour before the sun. By the end of the month, however, the Planet King will be rising over two hours ahead of the sun as a bright, yellowish-white object in the constellation of Virgo.

The planet Mars is too close to the sun to be easily seen this month.