2021, September 24:  Summer Triangle Up High

Astronomy, Sky Watching
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September 24, 2021:  The Summer Triangle – Vega, Altair, and Deneb – is high in the south during the early evening hours.

2021, September 24: The Summer Triangle is high in the southern sky two hours after sunset.
Chart Caption – 2021, September 24: The Summer Triangle is high in the southern sky two hours after sunset.

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by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:41 a.m. CDT; Sunset, 6:44 p.m. CDT.  Check local sources for sunrise and sunset times for your location.

With Venus, Jupiter, and Saturn dominating the early evening sky, the Summer Triangle – Vega, Altair, and Deneb – is high in the south.

Each star belongs to its own constellation.  Vega is part of Lyra. Altair is in Aquila. Deneb is in Cygnus.

Vega – “the falling eagle” – is a bright jewel on the harp.  The blue-white star is the second brightest star in the northern half of the sky, after Arcturus that is low in the west at this hour. (Northern half of the sky means north of the celestial equator.  Vega is the third brightest star visible from the mid-northern latitudes, when Sirius is included.  The brightest star is in the southern half of the sky, south of the celestial equator.) Vega is about 25 light years away and shines with an actual intensity of about 100 suns.

The constellation is small.  The musical instrument consists of a small triangle connected to a parallelogram. 

Epsilon Lyrae (ε Lyr on the chart) is an interesting star.  It is frequently known as “The Double-Double.”  Through a binocular, the single star is actually two stars – a double star.  Each of those single stars resolves into its own double star through a telescope.  If you attend a star party at the local astronomy club, ask to see “The Double-Double.” 

To spot Epsilon Lyrae through a binocular, sit in a chair or lie on the ground.  It’s very high in the sky at this hour and a challenge to see from a standing position.

Photo Caption – The Ring Nebula (NASA/Hubble Space Telescope).

In the parallelogram, on the short side opposite from Vega is a star that has shed its outer later into space revealing its hot core.  It is known as the Ring Nebula, that resembles a tiny puff of light through a telescope.  Photographs reveal the colors and the hot cinder at its center.

Altair – “the flying eagle -is in the center of three stars at the head of the constellation.  The three stars somewhat resemble Orion’s belt stars in their spacing. The constellation’s wings appear as a diamond shape, with its tail below.

Altair is the seventh brightest star in the northern half of the sky and it is about 8 light years closer than Vega.  Altair is less than 20 times brighter than our sun.

The final corner of the triangle is Deneb – “the hen’s tail.”  The constellation is a swan, flying southward along the Milky Way.  The star is over 1,000 light years away.

Along with Rigel, Orion’s Knee, Deneb is one of the most luminous stars that we can see without a telescope.  It shines with an intensity of over 90,000 suns. 

Visualizing a southward-flying swan, Albireo marks the head or the nose of the bird.  It is a double star and worth adding to your list of celestial wonders to see through a telescope, along with Jupiter, Saturn, and the Double-Double.

In a telescopic eyepiece, Albireo appears as a yellow star and blue star, two gems on the black velvet of the night sky.

While we’re looking at this part of the sky, one of the first accepted black holes, Cygnus X-1, was found among the Swan’s stars, over 6,000 light years away.  A black hole is revolving around a bright-blue star, pulling gasses from the star’s exterior.  The gas falls toward the black hole generating x-rays as it spirals toward that point of no return.

During the evenings of early autumn, step outside with a binocular and take a look at these sensations of the night sky.

Detailed Daily Note: One hour before sunrise, the moon (17.4d, 89%) is over halfway up in the west-southwestern sky.  It is 12.5° to the lower left of Hamal and 20.6° to the lower right of Alcyone (η Tau, m = 2.8), the brightest star in the Pleiades star cluster.   Three bright planets shine in the evening sky.  Forty-five minutes after sundown the trio is visible simultaneously.  Brilliant Venus is over 8° up in the southwest, 2.2° to the lower left of Zubenelgenubi and 24.0° to the lower right of Antares.  Farther eastward, bright Jupiter (m = −2.7) is 21.0° up in the southeast.  Saturn is slightly higher than Jupiter and 16.1° to the upper right of the Jovian Giant.  Two hours after sunset, Jupiter is nearly 30° up in the south-southeast, 2.9° to the lower right of μ Cap, 1.5° to the upper right of Deneb Algedi, and 1.7° to the upper left of Nashira.  At this hour Saturn is about the same altitude as Jupiter, but in the southern sky and east of the meridian.  The Ringed Wonder is 1.3° to the lower right of υ Cap.  The planets continue to retrograde. By three hours after sunset, the moon (18.1d, 82%) is nearly 10° above the east-northeastern horizon.

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