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Spring: galaxies and double stars mark spring skies.

SPRINGTIME CAN be frustrating for urban and suburban stargazers. The Milky Way, with its bright stars, star clusters, and nebulae, lies along the horizon during evening, where it's effectively invisible. So the prime deep-sky targets of spring are galaxies, which are notoriously difficult to view in light-polluted skies.

Fortunately, Ursa Major, the Great Bear, with the striking Big Dipper marking his body and tail, flies high on spring evenings. Several of the sky's most light-pollution-resistant galaxies lie in and near Ursa Major. It also contains the magnificent double star Mizar, which is extraordinarily easy to locate and observe.

Mizar is the next-to-last star in the Bear's tail. It marks the bend in the Big Dipper's handle. Mizar has a medium faint companion called Alcor, which is visible without optical aid to people with sharp eyesight under modestly dark skies. Mizar and Alcor are related--fairly near each other in space and moving in the same direction--but they probably don't constitute a true, gravitationally bound binary star.

In the early 17th century a number of people, including Galileo, observed this pair through telescopes and discovered that Mizar is itself a double--the first true binary star ever discovered. The pair is easily split at 30x or higher, and forms a striking telescopic pattern with Alcor.

Cor Caroli, the brightest star inside the crook formed by the Bear's tail and hind leg, is another superb, easy double star. It looks quite similar to Mizar except that the secondary star is a bit fainter and there's a color difference: the stars are very pale yellow and blue.

Cor Caroli is the brightest star of the constellation Canes Venatici, the Hunting Dogs. Its companion dog is called Chara, marked by the constellation's second brightest star. Cor Caroli and Chara form a squat, nearly isosceles triangle with the galaxy Messier 94. M94 has a tiny, bright core, making it unusually easy to spot in light-polluted skies. M94 tends to look like a slightly fuzzy star surrounded by a small, round halo. The darker your skies are, the bigger the halo will appear.

Messier 51 (shown on page 69) is by far the most impressive galaxy in Canes Venatici when viewed under dark skies. You can find it 3W[degrees] from Alkaid, the tip of the Bear's tail, toward the center of Canes Venatici. But M51 can be tricky to spot in heavy light pollution. Look for the distinctive little quadrangle that it makes with three faint little stars very near it, as shown on the chart at right.

Star formation is unusually vigorous in M51's spiral arms, perhaps due to a past close encounter with its companion galaxy NGC 5195. So it's probably easier to see M51's spiral structure through a telescope than any other galaxy's. But in a typical city or suburb, you will only see the bright central regions of M51 and NGC 5195, perhaps embedded in a barely detectable common cloud of light. But even that is a remarkable sight.

M63 and M106 are the other two Canes Venatici galaxies that were cataloged by 18th-century French comet hunter Charles Messier. They're comparable to M51 in terms of visibility, but a little harder to locate, because they lie farther from any bright star.

M63's core is small and bright, though not as intense as M94's, and its elliptical halo is fairly easy to make out. M106 is similar to M63, but its core is somewhat more diffuse, making this galaxy a little harder to spot in bright skies.

At the opposite end of Ursa Major, well beyond the front of the Big Dipper's bowl, lie M81 and M82, the sky's brightest galaxy pair. Sometimes I find them easily by proceeding from the star Phecda through Dubhe diagonally across the Dipper's bowl, and continuing in nearly the same direction for a slightly greater distance. If that fails, I look 2[degrees] toward Mizar from the star marked 24. And if light pollution makes 24 Ursae Majoris invisible, I follow the line from Upsilon (u) through the star marked 23 to find Rho (p), Sigma1 (o1), and Sigma2 (a2) in my finderscope or telescope, and star-hop from there. By the way, 23 Ursae Majoris is a lovely double--a bright star with a faint reddish companion that's well separated at 100x.

M81 is the second-brightest northern galaxy after M31 in Andromeda (see page 64), and its core is big and bright. That makes it easy to see in all but the worst skies. But its spiral arms, which show beautifully in photos, are extremely hard to see by eye even in perfect conditions.

M82, by contrast, is bright throughout, showing more detail than any other galaxy in heavy light pollution. Look for the dark dust lane that divides it nearly in half. M81 and M82 fit together in a single field of view at 50x, but you'll get a better view of M82's details at higher power.

Beware of the galaxies M109 and M108 and the planetary nebula M97 (the Owl). Beginners often look for these because they're close to familiar stars of the Dipper's bowl, but they're quite hard to spot in light-polluted skies. Move Phecda out of the field of view before looking for M109, otherwise the star's light will probably overwhelm it. For M97 and M108, a slightly more detailed chart is on page 57.

M97 is fairly large and diffuse, making it the hardest of the four Messier planetary nebulae to spot in bright skies. A nebula filter (also called a narrowband or line filter) makes the job much easier.

M101 is a classic example of a galaxy that's hypersensitive to light pollution. Huge but very diffuse (spread out), this galaxy is magnificent under dark skies. But its spiral arms are too faint to see from typical suburbs, and its core is diminutive and not bright at all. So it might just be the most difficult Messier object to spot in lit-up skies.
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Title Annotation:Observing: Deep-Sky Seasons
Author:Flanders, Tony
Date:Jan 1, 2013
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