Looming large in the night sky, the Big Dipper has been an old friend to most of us for as far back as we can remember. Though the Dipper’s shape is now changing and will look different in just a few thousand years, for us it hasn’t changed a bit since we were kids.
Poetically, the Dipper reaches its annual apex in spring, the season of renewal. Then, during the first few hours of night, it hovers high in the north, almost overhead, a far cry from the horizon-hugging stance it adopts in the autumn. While over in the southwest the sky regales us with a dazzling display of diamondlike stars, in the north the Dipper floats forlornly before a dark and deso-late region of the heavens. This realm lies far from the Milky Way, out of the plane of our own galaxy. Hence the Dipper guides our eyes away from our own galaxy and toward the richness of the rest of the universe.
Because no foreground interstellar dust obscures the view, we can look past the Dipper to distant galaxies. Even amateur telescopes unveil an extragalactic potpourri that includes the awesome spirals M101 and M81 and the strange, turbulent galaxy M82. Each has more stars than could be tallied by a person counting nonstop since the telescope’s invention, and each is numbingly distant. By the time the light from one of these galaxies passes the Dipper’s stars, it has already completed 99.999 percent of the journey to our eyes.
Yet even those galactic empires are neighbors compared with a stupendous cluster of galaxies lying in the same direction. It’s so remote that light leaving its quadrillion suns just as the last dinosaurs gazed upward will not reach us for at least another half-billion years. The ancient glow we see, flushed like an old sepia print and altered by the expansion of the universe, brings us the latest news from an era that no longer exists.
That conglomeration of galaxies is known as the Ursa Major cluster, a name taken from the Big Dipper, the directional sign in front of it--for curiously, the Dipper is not a constellation but an asterism, a segment of Ursa Major, the Big Bear. Big is the right word: it’s the third largest constellation in the heavens, just behind Virgo and Hydra--the serpent--each of which promiscuously occupies a few more degrees of sky.
As for the individual lights of the Dipper itself, take a look at the second star of the handle: it’s really a pair of stars called Mizar and Alcor. The ability to discern this horse and rider was reputedly an old Arabic test for keen eyesight. Point a telescope at the pair and even a third star pops into view. Of course, the stars of the Dipper with the greatest claim to fame are those that guide our eyes to today’s North Star. On these spring nights, the pointers, the two stars at the end of the bowl, lie at the leftmost edge of the Dipper, pointing downward to a solitary star of their same brightness: Polaris, the polestar.
Most of the Dipper’s stars are gravitationally linked. They are a family of stars and not simply a random line-of-sight configuration like most constellations. Of the thousands of such star groups known, the Dipper is the nearest to Earth, which is why it appears so large.
But you don’t have to remember that tonight. Just gaze at the Dipper the way you did that night long ago, before such facts had any special importance. Such a dependable and lasting friend is worthy of a springtime salute.