Skip to Main Content

Apply Now to the University of Wyoming apply now

Northern Rockies Skies for April

March 30, 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.

Our conspicuous winter constellations, Orion, the hunter, and Canis Major, with Sirius, the brightest star in sky, slowly are ushered out of the night skies.

Our April night skies belong to two familiar constellations, Ursa Major, the Big Bear, and Leo the Lion. Both are visible high and to the northeast and easily identified.

The Big Bear, known to most as the Big Dipper, allows us to spot the star Polaris by following northward a straight line from the two stars at the end of the dipper, called the pointer stars. Polaris, the North Star, is in the constellation Ursa Minor, which means Little Bear, and is known as the Little Dipper.

Leo the Lion, with its bright star Regulus, lies to the south of Ursa Major. To the east of Leo lies Saturn in Virgo this time of the year. Saturn is at its closest approach to the Earth and so it will be a great time to see its moons; the rings are barely visible.

Look toward the zenith and slightly west so you can see the constellation Auriga and its bright star Capella and the two Gemini Twins, Castor and Pollux.

April 2011 Northern Rockies Skies Interest: Stars: I. The Basics (best URL:

While the study of astronomy includes every aspect of the universe, the word itself -- from the Greek -- translates as star arranging, or star regulating. In fact, much of astronomy deals with the positions of stars in the three dimensions of space, and the physical laws governing stars.

The total number of stars visible to the human eye on a dark night, from south to north pole, is about 5,000. The number of stars in our Milky Way galaxy is roughly 100 billion. A recent coarse estimate for the number in the observable part of the universe is 300 sextillion (3 followed by 23 zeroes).

The surfaces of stars shine mostly in the visible part of the spectrum, although their radiation does continue to longer (radio, infrared) and shorter (ultraviolet, X-ray) wavelengths. Yet deep in the cores of stars the radiation is all gamma rays, released as a by-product of four hydrogen nuclei (single protons) fusing to make one helium nucleus, with two of the protons transmuting into neutrons.

Thus our primary source of energy on Earth -- the sun's light -- is a gigantic fusion reactor at the sun's center. Man-made fusion reactors have been developmental for more than a half century, and are now making steady progress. Fusion reactors, unlike their fission counterparts, do not make radioactive waste products.

In the 1860s, the famous Lord Kelvin (the absolute temperature scale was named in his honor) suggested that the sun shone by heat generated in comet impacts, and calculated an upper limit for the solar age of 100 million years, far short of the sun's estimated age of 4.5 billion years. Sixty years later Sir Arthur Eddington was the first to suggest and expound upon fusion as the sun's energy source. He showed that a star's size, color, surface temperature and lifetime all depend primarily upon its mass.

To be continued next time in Stars II: Mechanics.

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

1000 E. University Ave. Laramie, WY 82071
UW Operators (307) 766-1121 | Contact Us | Download Adobe Reader

Accreditation | Virtual Tour | Emergency Preparedness | Employment at UW | Privacy Policy | Harassment & Discrimination | Accessibility Accessibility information icon