Northern Rockies Skies for September: Cygnus the Swan

August 27, 2013

A monthly look at the night skies of the northern Rocky Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.

Right after sunset, located directly overhead within the summer triangle, lies the constellation Cygnus the Swan. Sometimes referred to as the “Northern Cross,” this important constellation lies within the plane of the Milky Way.

Deneb is its brightest star. Albireo, located at the “bottom” of the cross, is one of its most interesting. It is a close binary star, with one blue and the other yellow. Cygnus is full of other significant stars, star clusters and nebulosities.

One of the brightest known X-ray sources is Cygnus X-1, most likely a black hole revolving around a blue super giant star. The black hole has a diameter of about 20 miles, with 9 times the mass of the sun packed into it.

Another star, 61 Cygni, was the first star whose distance was measured directly using the principle of parallax by Friedrich Wilhelm Bessel in 1838. 61 Cygni also is known as Piazzi’s “Flying Star” (1804) due to its very fast “proper motion,” which is its large change in position relative to all the stars.

Finally, Cygnus was chosen -- as one of the selected regions in the sky that was monitored -- by NASA’s Kepler satellite to search for extrasolar planets. As a result, there are many stars in Cygnus with recently discovered planets and planetary systems.

Planetary Watch: Early this month (around Sept. 8) on the southwest horizon right after sunset, you will notice the crescent moon moving past Venus (the brightest) and Saturn. Watch for Jupiter and Mars rising in the east in the early morning, around 3-4 a.m.

September Interest: Venus's Atmosphere

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Venus is often referred to as Earth's "sister planet," but this description applies in size and bulk composition only. Venus' diameter of 7,521 miles is 0.95 times Earth's (7,917 miles), and its mean density (5.2 grams/cc)  is similar to Earth's (5.5 grams/cc) as well. The similarity ends there.

Characteristics that distinguish it from Earth arise principally from Venus being about two-thirds the distance from the sun, or 67 million miles, compared to Earth's distance of 93 million miles. It follows from the inverse square law that Venus receives nearly twice as much solar flux illumination as does the Earth.

Over the eons, practically all the lighter atoms and molecules -- hydrogen, water, helium, nitrogen and oxygen -- were driven from Venus's atmosphere and into space by the sun's heat. Also, in the absence of water and, therefore, plant life, which incorporates carbon dioxide (CO2), Venus accumulated an atmosphere that is 96.5 percent CO2 and 3.5 percent nitrogen. (A definitive exposition on the interplay between oxygen and carbon dioxide throughout Earth's history is Nick Lane's "Oxygen: The Molecule that Made the  World"). 

Additionally, the surface atmospheric pressure on Venus is very high -- about 92 times that of the Earth, which is comparable to the pressure at a 1 kilometer depth in Earth's oceans. An extreme greenhouse effect resulted, with surface temperatures more than 860 degrees Fahrenheit, which is higher than temperatures used for sterilization.

Winds at the Venusian surface are only a few miles per hour but, when combined with the high atmospheric density, the wind force is sufficient to move stones. At the tops of the slightly cooler Venusian mountain peaks, a highly reflective substance has been imaged -- the analog of snow -- which is speculated to be a metal-like substance such as tellurium, with a melting/freezing point of 841 degrees Fahrenheit.

Since 1960, about 45 spacecraft from the Soviet Union, the United States and Japan have been sent to Venus to "fly-by” orbit, descend through the atmosphere and, in a few cases, land and survive on the harsh Venusian environment long  enough to send back detailed information about the surface. This will be discussed next time.

To view this month’s sky chart, click here.

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