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Wednesday, May 24

The most popular of the "unofficial" constellations, the Big Dipper makes an excellent signpost for locating two bright galaxies visible in binoculars and small telescopes.

Currently, the Dipper lies high in the northern sky, cup down, with its handle pointing more or less southeastward. It is on either side of the handle that our target galaxies lie: M 51 (the Whirlpool Galaxy) and M 101 (sometimes referred to as the Pinwheel Galaxy).

Begin with the star at the end of the handle, Alkaid. Just a little over 3 degrees to the south-southwest, in neighboring constellation Canes Venatici, lies M 51. In 7x50 binoculars, M 51 is a small, blot of light, similar to a slightly out-of-focus star. Binoculars in the 10x50- and 20x80-range resolve this into a pearl-shaped object. A telescope at medium magnification reveals that this galaxy is in fact composed of two galaxies.

The main disk has a distinct bright round core set within a surrounding film of glaucous light that exhibits just a hint of the spiral arms. Just off the main disk on the north we see a diffuse stellar point with flared regions of light on either side. This is NGC 5195. Tens of millions of years ago, these two galaxies passed near each other. The resulting gravitational tug-of-war pulled one of M 51's spiral arms "out of round," as well as disturbed the structure of the companion.

M 101 is more of a challenge. In apparent surface area, it is one of the largest spiral galaxies in the sky (two-thirds that of the full moon), and consequently quite faint. This galaxy forms a convenient isosceles triangle with Mizar, the star in the bend of the Big Dipper's handle, and Alkaid.

M 101 is a difficult object in 7x50 binoculars, but not impossible. Bear in mind that you're looking for a circular patch of light only slightly brighter than the sky background itself. Higher-power binoculars work better because they increase the sky-to-object contrast. A 6- or 8-inch (0.15- or 0.2-meter) telescope with low magnification will pick up the galaxy's compact soft core, but, aside from a slightly enhanced region west of the nucleus, the disk simply melds into the darkness without revealing much in the way of spiral detail.

This entire region of sky actually contains a number of very fine, bright galaxies, neighbors to our own Local Group of galaxies. I'll tell you how you can find them in Saturday's Skywatch column.

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Jeff Kanipe is the author of A Skywatcher's Year, an astronomy guide just published by Cambridge University Press. He is a former editor at Astronomy and StarDate magazines and a writer for the Earth & Sky radio series.

The images in Skywatch are produced by Starry Night software.