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Drinking from the Big Dipper

Greetings, stargazers.

The Big Dipper is one of the most recognizable asterisms in the night sky. However, it is only a part of the constellation Ursa Major, which is one of the largest. As a reminder, an asterism is simply a recognizable pattern of stars, whereas a constellation is a well-defined region of the sky.

Ursa Major is circumpolar. This means if you are far enough north it is visible year-round. Except for a couple of stars, this is the case in the Four Corners as well, but trees and mountains make it impractical for most of us. On winter evenings it is grazing the northern horizon, but now that summer is approaching it is high in the sky and easy to see, with its ladle looking like it is spilling its contents.

The name Ursa Major means greater bear. It is a bear in the stories from cultures in many parts of the world, for at least as long as recorded history. I find this especially interesting since the bear often has a long tail represented by the handle of the dipper. That is unlike any mammal I have seen. Although it is outside my area of expertise, I doubt I am the only one to speculate that these stars have been called a bear for longer than humans have lived in the Americas.

The seven stars of the dipper are all very similar in brightness, having magnitudes between 1.76 and 2.44.

Duhbe and Merak, the two stars on the western side at this time of year, are the ones useful for navigation. They form a line pointing toward Polaris, the North Star. Following the same line, but in the other direction, they point toward the constellation Leo, which is currently near the zenith, or the point straight overhead.

The middle star in the handle is Mizar, along with its faint companion Alcor. Together they are called the horse and rider, and being able to resolve the pair was used historically as an eye test. On an astronomical scale, Mizar and Alcor are close to each other and moving in the same direction, but it uncertain if they are gravitationally bound in a single system. Through a low-power telescope Mizar is easily seen as a double star, with Alcor well separated from the closer pair. Interestingly, all three of these stars are spectroscopic binary stars. That means they all have companions that are too close to be seen individually. The companions can be identified by a wobble in the absorption lines of their spectra.

Since it is positioned well away from the Milky Way, the background sky in Ursa Major is relatively free of interstellar dust. This provides a clear view of many distant galaxies. Several bright ones, such as M81 and M101 are easily seen in a small telescope. The clear background sky was the target of the Hubble Deep Field image, showing that no matter where you look, there are thousands of galaxies out there.

Useful links

This month:

If you are willing to stay up past midnight, the summer Milky Way will be rising in the east. We are fortunate enough to live in a place that makes this view spectacular.

After setting, the sun stays close enough to the horizon at this time of year that many satellites can be seen after sunset and before sunrise. This month has numerous opportunities to watch the Space Station, the brightest satellite, cross the sky. There are even a few rare nights when multiple orbits can be seen. For example, Monday night it can be seen beginning at 8:51, and then again at 10:51, although the second crossing will be very low in the northern sky.

We are in a year of maximum solar activity. Besides showing some great prominences last month during the eclipse, solar activity can lead to auroral activity in the Earth’s atmosphere. It is rare for aurorae to be seen this far south, but not unheard of. These next few days before the quarter moon should provide the best chance if there happens to be an appropriate solar event. If there is one, let’s hope it just provides a nice visual display and not a huge communication disruption.

Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory.