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Northern Rockies Skies for May

April 28, 2011

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.

May provides a preview of the summer constellations.

Orient yourself with the Big Dipper directly overhead at dusk. The cup of the dipper is opened northward and its handle starts a great arc southward, arriving first at Arcturus, a bright orangish star, sometimes called the "guardian of the bear." The arc ends at the bluish gem Spica, the brightest star in the constellation Virgo. Arcturus is the brightest star in the kite-shaped constellation Bootes, the herdsman or "bear driver."

Later in the evening, rising above the eastern horizon, the three bright stars of the "summer triangle," Vega, Deneb and Altair, will start their summer trekking. On the western horizon Regulus, the brightest star in Leo the Lion, and the Gemini twins, Castor and Pollux, say goodbye to the springtime nights.

A May morning spectacular: Throughout the entire month of May look to the east an hour or so before sunrise. A close grouping of Mercury, Venus and Jupiter will occur, the closest happening on May 11, the conjunction. These three planets will form a 2-degree line. Additionally, Mars is nearby about 5 degrees to the east. So if you get out your binoculars you can see the two brightest planets in the sky, Venus and Jupiter, the red planet Mars and that elusive master Mercury all in one setting. It is a rare, May morning spectacular event.

May, 2011 Interest: Stars: II. Mechanics (best URL:

Stars, the closest example being our sun, are self-gravitating spheroids of plasma -- ionized matter. Stellar masses range from roughly 1/10 to 100 times the mass of the sun. The sun is about 300,000 times as massive as Earth. Its diameter is 1.4 million kilometers, about 10 times the diameter of Jupiter, and 109 times the diameter of Earth.

During most of a star's lifetime its energy source is fusion reactions, the simplest being conversion of hydrogen into helium, with release of gamma rays -- high-energy photons -- that are the primal energy source.

The resulting heat released in the fusion reactions is effectively a pressure from the inside pushing out on the plasma material of the star, counter balancing the weight of the star's overlying outer shells. Without internal pressure a star would collapse under its own weight to a much smaller, denser object (a white dwarf, neutron star, or black hole).

The fusion process involves two of nature's fundamental forces, the strong nuclear and "weak" forces. Unlike gravity and the electromagnetic force -- infinitely long-range, inverse square-law forces -- the strong and weak forces operate only within the confines of a nucleus. The strong force is roughly 100 times as attractive as the repulsive electromagnetic force between two protons -- two hydrogen nuclei -- and brings the protons close enough for the weak force to come into play as well. The net effect is to transform four separate hydrogen nuclei into a bound helium nucleus, two protons and two neutrons.

The pressures are high enough to drive protons to fuse only in the inner 25 percent of the sun. In this core energy-producing zone and the directly overlying radiative zone, energy is transported outward, mostly by radiation.

The further-out thermal convection zone transports energy to the surface, much like a pot of boiling water. Next time we will consider some details of physical processes in the nearest, best observed star, our sun.

To view this month's skychart, click here.

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