Ursa Major … and You
And that inverted bowl we call The Sky, Whereunder crawling coop’t we live and die, Lift not thy hands to It for help – for It Rolls impotently as thou or I. - from The Rubayyat
Ursa Major …
The constellation Ursa Major, the Great Bear, houses its most recognizable portion, the asterism called the Big Dipper. This month we’ll see what astronomical principles and facts we can glean from the Dipper and the Bear’s other parts. As in previous months, UB leaves some of the digging for you to do in projects below (that’s the … and You part).
Credit: Roberto Mura. Creative Commons Attribution-Share Alike 3.0 Unported license
First, we should appreciate the vast area the Great Bear occupies in our night sky. It’s the third largest constellation in that respect, and it’s difficult to imagine two larger ones. Greek mythology tells us that the Bear is carrying out a life sentence trundling around the north star once a day, every day, ever since mighty Zeus made eyes at sweet Callisto. Zeus had a mighty jealous wife Hera who made the verdict and imposed the sentence. Callisto is now the Bear. For company she has what was her son Arcas, now the Litttle Bear Ursa Minor which houses the north star Polaris.
To spot Polaris you face due north and look up an angle equal to your latitude on Earth. For most of my readers that’s about halfway from horizon to zenith or 45 degrees. The Dipper is very high overhead in late spring. The leading edge of its cup with the two “pointer stars” will also help you find Polaris. As the night progresses the Bear apparently will trundle westerly in a counterclockwise circle over Polaris. I say apparently because it’s our Earth moving and not the sky. [The diagram below is oriented for facing north northeast early evening in spring.]
Credit: Bonc. Creative Commons Attribution-Share Alike 3.0 Unported license.
Credit: Sidney Hall, 1825. Restored by Adam Cuerden. Public Domain
Why the long tail? Many cultures recognize Ursa as a bear, but bears don’t have long tails as the Dipper’s handle suggests. The Housatonic tribe of North America explained it with a tail (oops, tale) about three hunters chasing the bear through spring and summer. In fall they hit their target and the Great Bear’s blood colored the maple and other leaves.
While we’re on the tail, we should mention that you might be able to see the bend in the handle as two stars. The tribe thought that the middle "hunter" carried a large pot in which to cook the bear. This optical double star was used by many Arab and Native American tribes as a test of a warrior’s visual acuity. Can you see two? In a telescope you might see three.
Why the long legs? Three of the Great Bear’s legs extend far south toward Leo. The paws will be almost straight up (at the zenith) in May, so try not to fall over backwards finding them – best to get a chaise, hammock, or just a blanket to lie on. The legs each end in a pair of stars, and those six are known as the Gazelle Leaps. I can almost hear them: clip-clip, clip-clip, clip-clop. We might call the Leaps another asterism. They are highlighted in ovals in the chart above.
Burnham reports that the Dipper won’t be recognized as such many thousands of years in the future. The culprits are the two end stars classified as alpha and eta, with names of Dubhe and Alkaid, and their relative motions. The five central stars will pretty much hang together, but the handle’s end will droop severely while Dubhe at the lip heads directly away from the bucket. Burnham rejects theories that suggest that, with all this moving about, Ursa looked more like a bear in ancient times. Our sympathies go to those theorists: the changes could not have been that great even over the span of human history. Instead, Burnham cites a wise crack by one Will Cuppy who suggests that it is possible that bears looked more like the constellation a long time ago. Ha!
… and You
Projects.
1. Angular Distances. Make your own sketch of the seven main Dipper stars. [Refer to the chart and not the bear drawing.] Label the pouring edge, from Dubhe to Merak, as 5 degrees in length. Label the bottom of the Dipper bowl as 10 degrees. When you observe, extend your hand or hands to see what spans of fingers or palms correspond to those angular distances. You now have a gauge to carry with you. Use it to “measure” and label distances in the rest of your Dipper sketch, and while you're at it, estimate these:
a. The Dipper length from lip to Alkaid, the handle’s end. b. The distance from Dubhe to Polaris. c. The distance between Castor and Pollux, the Gemini twins. d. The length of Leo from Regulus to Denebola at the tail. Can you use your gauge to estimate the elevation of Polaris (your latitude)? My estimates can be found below.
2. Can you find the names of the two constellations that have areas greater than Ursa Major? Where are they?
3. Get the details on the Mizar / Alcor system. Are the two principles bound gravitationally? How many stars buzz around these two. Do we know the period of Alcor’s orbit, if there is one?
Galaxies M81, 82
Credit: Keesscherer. Creative Commons Attribution-Share Alike 4.0 International license.
4. We cannot access Ursa’s galaxies without a good-sized scope or monster binos, but they make for interesting reading, none more so than a very disturbed M82 [on the right, above]. This gal is a mere 38 minutes (about half a degree, or half of your pinkie width, yes?) from M81, a well-behaved, button-down, spiral galaxy. What are the latest thoughts on M82’s turbulent past?
OK, I lied. M81 did have an outburst called Supernova 1993J. Details here.
My distance estimates: a. 25 degr b. 30 c. 5 d. 25
Sources: Burnham’s Celestial Handbook, Audubon Field Guide