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Northern Rockies Skies for August

July 29, 2014

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.

One of the most beautiful nighttime visions is a full view of the hazy Milky Way on a clear, dark Rocky Mountain night. It is best seen during the absence of the light of the moon in August, so pick a dark site outside of town and away from nearby lights.

To locate the Milky Way plane, first go to the southern horizon where the constellations Scorpio (see star Antares) and Sagittarius are located. The location of these constellations is actually in the direction of the Milky Way’s center. The plane of the Milky Way then shoots up toward the zenith and north. The hazy light of the Milky Way passes through Aquila (star Altair) and Cygnus (star Deneb), Cepheus and Cassiopeia (the stretched-out “W”), and finally through Perseus on the northern horizon.  

Once you locate the Milky Way plane, try scanning it with a small telescope or binoculars. The hazy light is the accumulative light of distant, unseen resolved stars. With the binoculars or a small telescope, you can actually see more stars and, in many cases, small clusters of stars that are gravitationally bound to each other. Enjoy the Milky Way’s beauty.

Finally, you can see both Saturn and Mars on the southern horizon shortly after sunset. In the morning, before sunrise, you will see both Venus and Jupiter close together. It is quite an impressive sight.  

August 2014 interest: Are Earth-Like Planets Rare?

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The Fermi paradox, introduced last month, is that there are roughly 500 billion stars in the Milky Way galaxy, with some fraction of planets in those systems possibly capable of supporting advanced civilizations that could have developed technology for interstellar travel across the galaxy. In 1950, the nuclear physicist Enrico Fermi asked, “Why has no such civilization visited or made contact with Earth?”  That is, we have no clear, verified evidence of any contact. While we are far from a definitive answer, interesting bounds can be set on the first part of Fermi's paradox, the number of Earth-like planets in the galaxy.

Several factors reduce the number of stellar systems capable of supporting life, including: fraction of stars in a "galactic habitable zone" where radiation is not too intense and sufficient heavy elements exist; fraction of stars with rocky planets that also have the necessary composition (for example water, carbon and oxygen) to sustain complex life; and timescale, over which life can be supported on such planets without suffering  cosmic extinction events such as frequent large asteroid hits.

Astronomers have reasonably bounded estimates on many aspects that enter the calculations for these fractions and timescales. Other factors such as whether a large moon facilitates or hinders life; and whether a large gas planet such as Jupiter is beneficial for shepherding asteroids away from the Earth-like planet, are uncertain by a few orders of magnitude.

The need for a Jupiter is debated. The moon factor is invoked for its possible role in plate tectonics and the tides -- also widely debated. Guestimates for the (fairly unconstrained) large-moon factor range from 1 to 1/10,000. But using this range and all the other fairly well estimated factors give a range in number of Earth-like planets that could possibly sustain life for long periods of roughly 30,000,000 to 3,000 with the galaxy.

In a simplistic calculation, taking these planets to be distributed throughout the galaxy, with its radius of about 50,000 light years, yields an average separation between life-accommodating stellar systems of about 15 to 1,500 light years. Such distances would seem not too far to travel for civilizations with advanced technologies, necessarily much beyond our own rapidly progressing capabilities.

Thus, we see why Fermi asked his question. Even if we adopt our low estimate of 3,000 advanced Galactic civilizations, we might expect one of them to have colonized the galaxy. The answer to "why no contact?" would lie in some other aspect of the problem, including lifetimes of these civilizations or, perhaps, their behavioral norms, or in inherent physical limitations of space travel -- to be discussed next time.

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

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