Astronomy

The Big Dipper



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"The Big Dipper"
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The Big Dipper is one of the most recognizable and easily located of the major star constellations. Also known in Britain as the Plough (and less commonly the Cleaver), the Dipper has been given a variety of names in non-English-speaking countries, even within Europe, where it is also known as a saucepan (Dutch), a fishing net (Finnish), and the Great Sages (India).


FINDING THE BIG DIPPER

The Big Dipper is a particularly visible component of a larger star constellation visible throughout the northern hemisphere, and known by its ancient Roman name, Ursa Major (the Great Bear). A planisphere, or star chart, can be a helpful guide in locating this and other constellations. If you find yourself without such a device (and you probably are without one as you read this), fortunately it is relatively easy to find. The Big Dipper is easiest to find by searching for the long curve of three bright stars that form its "handle"; four adjacent stars make up the "bowl."

The non-handle end of the Big Dipper's bowl marks a direct line pointing from Polaris, the North Star, an especially bright star which also marks the position of a similar star formation, the Little Dipper. Because these constellations can be seen pretty much year-round in the northern hemisphere (and the North Star, which can be found by looking up and directly north, is always in more or less the same position in the sky at any given time of the night), they are fundamental marking points for stargazers. Once you can quickly find the Big Dipper and the Little Dipper, you can use them as orientation points, around which the various other constellations can be located.

Seven major stars make up the Big Dipper: Dubhe, Merak, Phecda, Megrez, Alioth, Mizar, and Alkaid.


BEYOND THE BIG DIPPER

For example, once you have located the Big and Little Dippers and Polaris in relation to one another, you can continue on the Big Dipper-Polaris line toward the W-shaped constellation Cassiopeia. Continue on that line, and you will reach a four-starred square called Pegasus, one corner of which marks the current position of Andromeda. (In billions of years, our galaxy will collide with the Andromeda Galaxy, which will have uncertain repercussions for stars in both galaxies.)

If you move in the other direction from the Big Dipper, following the curve traced by its "handle" section, you'll instead reach another set of stars and constellations. First is Arcturus, once within the ancient constellation of the shepherd (known from cave paintings in Africa). Next, along the same curve, you'll reach Spica, within the Virgo constellation.

A couple of final orientation lines also extend from the Big Dipper. First, the line traced by the handle-side corners of the "bowl" points down to Regulus, in the constellation Leo (the lion); pointing upward, the same lijne reaches Cygnus, the swan (actually a cross, today).


WHERE'S THE BEAR?

Originally, the Big Dipper formed the tail and rear end of Ursa Major, the Great Bear. While we take the name from the Romans, earlier Greek mythology explained that on one of Zeus's many dalliances with human women, this one Callisto, his jealous and long-suffering wife Hera turned her husband's new mistress into a bear. Later, Callisto was very nearly killed tragically by her own sun, the hunter Arcas, but Zeus saved her by turning Arcas into a bear as well, and sending them both hurtling into space, where they became immortal star constellations. Later, Hera moved them into their current position, from which they never set - so that the pair would never be able to sleep, the Greeks explained.

The Big Dipper is easily recognizable today - much less so the "bear" which it supposedly forms a part of. This is because, despite the appeal of the seeming shapes in the sky formed by stars like those in the Dipper, what we are seeing is usually not anything related to the actual positions of those stars relative to each other. The seven major stars in the Big Dipper, for example, vary from 80 to 125 light-years away from Earth. (By contrast, the closest star to Earth is 4 light-years away, and the farthest planet in our solar system, Neptune, is just 4 light-hours away.)

Plus, all of these stars are constantly moving with their own independent velocities - as is our Sun. As a result, a constellation which seemed to make perfect logical sense to the Romans 2000 years ago can seem utterly nonsensical to us today. (By the same token, some constellations which might seem entirely obvious today would have puzzled a stargazer in Roman times.) The closest of the Big Dipper stars, Merak, is an average yellow dwarf star a little hotter than our own Sun; the farthest, Dubhe (also known by the Latin designation Alpha Ursa Majoris) is a red giant, a type of old, dying star which our sun will become when it runs out of core hydrogen in several billion years.

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