The Big Dipper and Great Spring Arch
If you sit back in a lawn chair facing northward after twilight on these late Marshall spring evenings and tilt your head way up toward the overhead zenith, the Big Dipper will leap out at you, hanging diagonally by its long-curved handle. You’ll probably be more comfortable if you sit in a reclining lawn chair because of how high the Dipper is in the sky. Save your neck and your back! The great thing about the Big Dipper is that it’s almost completely light pollution proof. You really to be looking from a lit-up environment not to see it!
As prominent as the Big Dipper is, it’s not one of the official human-made constellations seen from Earth. It’s actually the rear end and tail of the official constellation Ursa Major, otherwise known as the Big Bear. As you can see in the diagram the stars of the Big Dipper are far brighter than the stars that make up the rest of one of the great beasts of the sky.
The Big Dipper is a great tool to help you find other constellations both near and far. It’s a great pointer. For example, if you draw a line in your mind’s eye from the star Merak to the star Dubhe, on the side of the Big Dipper’s pot opposite the handle, and then extend that line you’ll run right into Polaris, otherwise known as the North Star. Clench your fist and hold it out at arm’s length. Three of those fist widths at arm’s length is about the distance you have to go from Dubhe to get to Polaris.
As I wrote about a few weeks ago in Starwatch, Polaris the North Star is what I like to refer to as the “Lynch pin” of the heavens. Since Polaris shines almost directly above the Earth’s North Pole, every celestial object in the sky appears to revolve around Polaris once every 24 hours, in response to the Earth’s rotation below the celestial dome.
The Big Dipper’s handle, or the tail of the Big Bear, can also be used as a pointer. In fact, the Big Bear’s tail is the first leg of what’s called the giant arch of spring, which goes on well beyond Ursa Major and takes you to two of the brightest stars in the spring evening skies.
The middle star in the Big Dipper’s handle is a great celestial eye test for you. That star is called Mizar, and if you look very carefully at Mizar with just your naked eyes (eyeglasses count), you’ll see that it has company. Just to the lower left of Mizar is a much fainter star, Alcor. Mizar and Alcor are what astronomers call an optical double. Those are double stars that don’t have anything to do with each other physically. They just happen to lie in the same line of sight from Earth. Mizar is 78.2 light- years away and Alcor is 81.2 light-years away. If you’re new to Starwatch, a light-year is the distance a beam of light travels in a year’s time in the vacuum of space. That’s a heck of a long ways, with just one light-year equaling nearly 6 trillion miles!
There are many other fainter double stars in the celestial dome that are gravitationally related to each other. In fact, more than half the stars seen in our sky that appear to be single shiners are actually systems of two, three, or even more stars revolving around each other. A lone star like our sun is the exception rather than the rule.
If you continue along the great arch of spring, beyond the arc of the Big Bear’s handle, you’ll run right into Arcturus, a super bright star and, in fact, the second brightest night time star we see around here. Just remember the old stargazing saying, “arc to Arcturus.” Arcturus is the brightest star in the spring constellation Bootes the Hunter Farmer. To me, and many others, Bootes resembles a giant kite rising on its side in the eastern sky with Arcturus at the tail of the kite. Arcturus is a bloated orange super giant star more than 25 times the diameter of our sun. That would give it a girth of well over 20 million miles. Even with the naked eye you can easily see its orange-ish hue. Arcturus is 37 light years away, making it over 214 trillion miles from your backyard. Since it’s 37 light years away, the light we see from Arcturus this spring left that bright star 37 years ago in 1982, the same year the movie “E.T. the Extra-Terrestrial” was released.
If you continue the great arch of spring beyond Arcturus you’ll bump into the bright star Spica, in the large but faint constellation Virgo the Virgin. Just remember another old stargazing saying, “speed on to Spica.” Spica isn’t as large as Arcturus, with a diameter of just under 5 million miles, but it’s a much hotter star, so hot that it kicks out 5,000 times more light than our sun. In comparison, Arcturus only emits 1,200 times more light than our sun. Spica doesn’t shine as brightly as Arcturus in our sky because it’s much farther away in our Milky Way galaxy at a distance of 263 light-years. The light we see from Spica left that giant ball of glowing gas in 1756, about 20 years before the signing of the Declaration of Independence. Whenever you gaze upon any star remember that not only are you peering out over a tremendous distance, you’re also looking back into time.
Enjoy the spring arch and everything available overhead as you stargaze on these much warmer evenings.
Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul. He is also the author of “Stars: a Month by Month Tour of the Constellations,” published by Adventure Publications and available at bookstores and at adventurepublications.net.