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Digging a little deeper into the Big Dipper

The first star grouping that any beginning sky watcher north of the equator learns is the Big Dipper. And there is a good reason for this. Two of its seven stars serve as pointers toward Polaris, the North Star, which provides observers with nighttime orientation.

This important association is beautifully displayed on the state flag of Alaska, which was designed in 1926 by 13-year-old Bennie Benson.

For most of us, that is all we know about the Big Dipper and, in fact, in too many cases all we know about astronomy. In this column, I seek to dig a bit deeper into important science by talking about this single star grouping.

First off, you should notice that I avoided use of the word constellation in describing the Big Dipper. Although it is often referred to by that term, astronomers call it an asterism. An asterism is a smaller grouping. In fact, the Big Dipper stars form a part of a constellation containing many more stars called the Big Bear or Ursa Major. They form the tail and hip of that constellation.

Can you name any of the stars in the Big Dipper? If you can, you are already well past me in your astronomical knowledge. Numbering the stars from the tip of the Dipper handle, they are 1: Alkaid; 2: Mizar; 3: Alioth; 4: Megrez; 5: Phecda or Phad; 6: Merak; and 7: Dubhe. Several of those names derive from their position on the Big Bear; for example, Phecda means thigh.

When you look closely at the Big Dipper, you will see that those seven stars differ quite a bit in brightness. Alkaid, Alioth and Dubhe are the brightest, with Alioth the brightest of the three, and Megrez is the dimmest. Does that tell us anything about how much light those stars are emitting?

It turns out that it does not. And here is where I urge you to think more seriously about what you see overhead at night.

We see the stars in two dimensions, as though they appear on a distant sphere. But space is three dimensional and those stars are at different distances from us here on Earth. That this is obvious is demonstrated by eclipses. If the sun and moon were at equal distances from us, there would be a terrible accident every year or so.

The distances to the stars beyond our solar system are very great, so great that they have to be measured in so-called light years. One light year is the distance light would travel in one year: almost 6 trillion miles.

The closest of those Dipper stars to us is Merak and it is 62 light years away. The most distant is its neighbor Dubhe, at 124 light years twice as far from us.

Now let us return to the Earth for a moment to consider light here. It turns out that light is one of those properties (like gravity) that varies in intensity inversely as the square of its distance from us.

Examples will unravel that jargon: car headlights twice as far from you are only one-fourth as bright; three times, one-ninth, etc. This same concept applies to the stars.

What astronomers do to provide a comparison of the light and energy emitted by stars is calculate the magnitude of that light as though all of them are equally distant from us. This so-called absolute magnitude (as opposed to what we see, which is called visible magnitude) is chosen so that stars are compared as though they were all 32.6 light years away.

By that measure, Dubhe rather than Alioth is by far the brightest of the Big Dipper stars, but Megrez, which is about the same distance from us as Alioth, remains the dimmest. Interestingly, while the North Star is shown on the Alaska flag to be brighter than the Dipper stars, its visible magnitude is about the same, but its absolute magnitude, because it is farther away, is much greater.