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Northern Rockies Skies for August

July 27, 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.

August is almost always a particularly good time for viewing the Milky Way galaxy and the Perseids meteor shower.

The subtle, hazy band of stars high in the southern sky is the Milky Way. It is best explored with the naked eye and binoculars, where you will discover a myriad of faint stars, dark dust and many star clusters. Visible on the southern horizon around 10 p.m., one sees Scorpius, the scorpion, and its brilliant red star Antares, together with Sagittarius, the teapot. This marks the direction to the center of our galaxy. Overhead is Vega, one of the brightest stars in the sky.

In that direction the Milky Way traverses Cygnus the Swan, the northern cross, then plunges into Cassiopeia, the big "W" in the sky on the northern horizon. The Perseids meteor showers will last up to four days centered around its peak date, Aug. 13-14. You may see up to 60 per hour if you look in the direction of the constellation Perseus, in the northern sky after midnight.  However, this year, there is a full moon to compete with this spectacular event. You can see Saturn after sunset in Virgo, near Spica, Jupiter will rise after midnight and beats down on the sunrise sky around 3:30 a.m.

August Interest:  Stars V. -- The Sun's Heliosphere (

Besides the Sun's light, which could be seen by large telescopes from vantage points distributed across our Milky Way galaxy, there are two other "spheres of solar influence:"  The Sun's gravity dominates over that of the nearer stars out to the distance of the Oort comet cloud, about 50,000 astronomical units. (1 A.U. equals the earth-sun distance).

The Sun's magnetic field extends to only about 75-100 A.U., a much smaller domain named the Heliosphere.

Magnetic fields arise from moving charged particles. In the case of the Sun, circulation of ionized matter, or plasma (mainly electrons and protons), in a convection zone beneath the surface of the Sun gives rise to the magnetic field.  Because the Sun is not solid, it rotates differentially, taking about 25 days to complete a rotation at its equator and about 34 days near the poles.

The shearing effect of differential rotation pushes magnetic lines of force together, intensifying the field and resulting in eruption of plasma and field, called prominences, from the Sun's surface.

When the plasma eruptions disconnect from the surface, they give rise to the solar wind -- energized plasma and magnetic field traveling together, at initial ejection speeds of about 1 million kilometers per hour.

The rapidly moving charged particles are both a continuing source of magnetic field and concomitantly linked to the field since their trajectories are spirals around the lines of force.  When these plasma-field events overtake and interact with the Earth's magnetosphere (and those of the other planets in the solar system), they may precipitate charged particles onto the Earth's poles, where our magnetic field is anchored, causing auroral displays at high latitudes.

The plasma-field bubble continually blown by the Sun -- the Heliosphere -- depletes its reach far past the orbit of Neptune, coming into equilibrium with the interstellar magnetic field and gas.

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

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