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Northern Rockies Skies for July

June 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.

It is that time of the year to get a spectacular view of the center and plane of the Milky Way galaxy. Using binoculars, you can discover its rich attractiveness of colorful stars, impressive star clusters and nebulae and contrasting dark and faint glows.

Go to the southern horizon and see the constellation Scorpius, identified by the red supergiant Antares. Above Scorpius "the serpent-bearer," Ophiuchus, is prominent, accompanied by the fainter and smaller constellation Serpens "the snake." These constellations are near the center of the Milky Way galaxy and with your binoculars you can see several star clusters and hundreds to thousands of stars that are grouped together by their own gravitational attraction.   

Overhead is the constellation Hercules, "the strongman," and to the east is Cygnus "the swan," sometimes called the Northern Cross. Cygnus lies along a major section of the plane of our Milky Way galaxy. Midway between Cygnus and Hercules is the tiny constellation Lyra, with the brilliant bluish-white star Vega.

On the northern horizon appears the great "W" in the sky, Cassiopeia. The plane of the Milky Way is brilliantly displayed through the constellations Scorpius-Cygnus-Cassiopeia. The full moon this month is on July 15. The early Native Americans called this the full buck moon since this is the time of the year the male deer start to grow their antlers.

July Interest: Stars IV - Sunspots (best URL:

When the sun is observed indirectly using a screen projection apparatus, dark spots are sometimes visible on its surface. (Never observe the sun directly, either with the unaided eye or with binoculars or telescope -- serious retinal damage is likely.)  These sunspots are areas of lower temperature, 3,000-4,500 degrees Kelvin (5,000-7,600 Fahrenheit), compared to the average photospheric temperature of 5,800 Kelvin (10,000 Fahrenheit). Sunspots usually travel in groups -- in fact in paired groups that are bound together -- because sunspot groups are bipolar like magnets.

The largest spots are roughly the size of Jupiter, 10 times Earth's diameter.

Sunspots were first regularly recorded more than 2,000 years ago by Chinese astronomers. Accurate counts of sunspots by Europeans began in the early 1600s with the invention of the telescope. In the 1700s it became clear that the number of sunspots peaked with an 11-year regularity.

In retrospect, the coldest part of the Little Ice Age (around 1700 A.D.) corresponded with the Maunder minimum, when the number of sunspots was very low. This may have been coincidence, or not: The variation in solar output from sunspot maximum to minimum is only about 0.1 percent (1.3 watts per square meter at the top of Earth's atmosphere); and there are several other proposed causes for the Little Ice Age. A maximum is now forecast for 2013, and it could turn out to be another record low maximum, as solar activity in 2010-2011 has remained below predictions. From analysis of geologic strata we know that the 11-year sunspot cycle has persisted for at least hundreds of millions of years.

Sunspots are concentrated magnetic flux channels from below the sun's surface into its atmosphere, that often give rise to solar prominences arching between the two spot polarities. Imaging by the Solar and Heliospheric Observatory reveals the internal structure of the spots to be depressions with rotating vorticity. Because the sun reverses its north-south magnetic polarity every 11 years, a full sunspot cycle is actually 22 years.

Next time we will discuss the sun's magnetic sphere of influence: The Heliosphere.

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

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