Auriga the Charioteer (January 25-31) holds more fascination than just ancient star myths. The pentagonal-shaped figure is home to a trio of bright star clusters that, under certain sky conditions, can be seen with the naked eye.
Our galaxy contains hundreds of millions of star clusters, of which we can see mere thousands from Earth. Most star clusters are too faint to be seen without optical aid, but a handful can be glimpsed as misty patches between the stars. Lying in the star-thronged winter Milky Way, Auriga is bedecked with several prominent star clusters, and when the constellation is high in the sky, as it is this week after sunset, its brightest centerpieces, M36, M37, and M38, are easy to locate with just your eyes or binoculars.
These three clusters lie in a row across the southerly half of the constellation. To find them, you must observe in the darkest location you can find, away from the glare of streetlights and buildings. A site in the outer suburbs is okay, but the country is better. The sky must also be free of a bright Moon and high thin cloud cover or haze.
First, locate Capella, the brightest star in the constellation. In the evening sky this time of year it is nearly directly overhead for people in the Northern Hemisphere. Next, go due south from Capella until you reach the star lying in the opposite corner of the pentagon. This is El Nath. (Auriga shares this star with Taurus the Bull, in which it marks the tip of the bull's northern horn. El Nath is from the Arabic Al Natih,, the butting one.)
Now let your eye move northeast along the second-longest side of the pentagon toward the star Theta (9) Aurigae. About halfway to this star, you should come across three concentrated patches of misty light running southwest to northeast. Two will lie within the pentagon, but one will be just outside of its southeastern border. This easternmost cluster is M37; the next one toward the west is M36; and the westernmost cluster is M38.
You may find locating these clusters easier with a pair of binoculars. The narrower binocular field of view helps increase the contrast between the clusters and the darker sky background, and with a little magnification you should also notice some differences in the shapes and richness of these clusters. M37 is large and densely packed, with a bright yellow-orange star near its center. It is the 'grainiest' of the three clusters. M36 is much brighter than M37, but smaller and not as rich. M38 is as large as M37, but is fainter and coarser.
The distances to M38 and M37 are about the same, around 4,400 light-years. M36 lies only a little closer, about 4,100 light-years. In the scale of astronomical things, a couple of hundred light-years difference doesn't matter too much. These clusters are neighbors. If M36 were 4,000 light-years nearer, it would appear as bright and as large as the Pleiades.
The clusters differ quite a bit in age. M36 is very young, about 25 million years old, while M37 and M38 are far older, 300 million years and 220 million years, respectively.
While you're in this region of sky, scan with your binoculars northeast and southwest of Auriga. You'll see many more open clusters and star groupings, each containing hundreds, even thousands, of stars. Don't worry about what they are called or how far away or old they are. You can, if you wish, always learn these details later. For now, just enjoy them for their beauty and for giving this region the reputation for being one of the richest areas of the winter Milky Way.
Also this week:
• The trapezoidal constellation Corvus the Crow rises around 7:30 p.m. in the Southern Hemisphere; 11 p.m. in the Northern Hemisphere. In mythology, Corvus was the beautiful white bird of Apollo. The hapless fowl was turned into a crow, however, after he failed to fetch Apollo a beaker of water.
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