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Our galaxy's heart

Map 20: South Equatorial 5
Libra to Microscopium
bug_nebula_noao_250
Planetary nebula NGC 6302 is located in the constellation Fornax.
Adam Block/NOAO/AURA/NSF

This star map will keep your telescope's drive humming through many observing sessions. When we look toward this sky region, we face our Milky Way's center, where star clusters and nebulae abound. In Sagittarius alone, we find 15 Messier objects — the largest number in any of the 88 constellations. Scorpius adds four more. We'll start our tour, however, in the small constellation of Norma the Carpenter's Square.

Several nice open star clusters populate the southeastern part of Norma, which lies at the bottom-right of Map 20. Norma continues to Map 24, where we find NGC 6067, a magnitude 5.6 naked-eye gem that displays about 100 stars when viewed through a 6-inch telescope. Adding to the appeal of this object is its placement within the Norma Star Cloud, a rich region of the Milky Way. Scopes above 14 inches in aperture will reveal several hundred additional stars.

Norma's other naked-eye open cluster, NGC 6087 (Map 24), shines at magnitude 5.4 and measures 15' across. Most of its light comes from the variable star S Normae. Every 9.75 days, this star's magnitude varies between 6.1 and 6.8. NGC 6087 contains more than 50 stars visible through 8-inch telescopes, but the field is so crowded with faint background stars that — except for a dozen brighter members — you'll have trouble determining which stars belong to the cluster.

Corona Australis the Southern Crown contains a bright globular cluster, NGC 6541. You can see this cluster, which glows at magnitude 6.1, without optical aid if it's high enough in the sky. A 4-inch telescope gives a great view, but, through a 12-inch, you'll see more than 100 outlying stars around a concentrated core.

IC 1297, a magnitude 10.7 planetary nebula, lies 1.5° east of Beta Coronae Australis. You'll need to crank up the magnification on this 7"-diameter object because, at low powers, it appears stellar. A nebula filter helps because it allows light from the planetary through, but it dims the light from surrounding stars. IC 1297's faint, bluish hue is difficult to see in telescopes with apertures less than 16 inches.

Telescopium contains a small cluster of galaxies, and the group makes a good target for a large telescope. The brightest member, NGC 6868, glows at magnitude 10.6. Other cluster galaxies include NGC 6861 (26' to the west) and NGC 6870 (7' to the north), but both are several magnitudes fainter than NGC 6868.

In Scorpius, you'll encounter a wealth of deep-sky objects, including four Messier objects: M4, M6, M7, and M80. This constellation offers so much more, however. Look 1° northeast of M4, and you'll find another nice globular cluster, although not nearly as bright. NGC 6144 shines at 9th magnitude and measures 9.3' across, about one-third the diameter of M4. You'll only see it to about half this diameter, however, unless you use a 12-inch or larger telescope.

Scorpius is awash in planetary nebulae. Among the best are NGC 6072 (magnitude 11.7, 40" across), NGC 6153 (magnitude 10.9, 25" across), and NGC 6337 (magnitude 12.3, 48" across). By far the best planetary in Scorpius, however, is the Bug Nebula (NGC 6302).

Find the Bug surrounded by the Scorpion's tail midway between Lambda and Mu Scorpii. NGC 6302 measures 2' by 1'. A prominent lobe with a tapered end makes up the nebula's western side. A faint extension protrudes from its eastern edge. At magnitude 9.6, NGC 6302 is a bright planetary; use an OIII filter to bring out its subtle details.

Under a dark sky, it would be hard to miss the Scorpius OB1 association, a huge group of hot stars. It begins at Zeta Scorpii and extends northward nearly halfway to Mu Scorpii. The brightest contributor — open cluster NGC 6231 — lies at the southern end only 1/2° from Zeta. This magnitude 2.6 gem spans 14' and displays more than 100 stars to 6-inch and larger telescopes. The tight grouping of stars at the cluster's center is particularly striking.

Two beautiful emission nebulae, each measuring roughly ½° across, are must-see objects. First, find the star-forming region known as the Cat's Paw Nebula (NGC 6334). The Cat's Paw actually comprises five individual nebulous patches. The one at the southeastern end is brightest.

After you've marveled at the Cat's Paw Nebula, move about 1¾° north-northeast to NGC 6357. While you'll see the whole extent of the Cat's Paw, only the west-central region of NGC 6357 — involved with a small star cluster — shines brightly enough for medium-size scopes.

Finally, we arrive at Sagittarius the Archer. For many Northern Hemisphere observers, Sagittarius lies too near the horizon to provide a quality observing experience. For those who live (or can travel to) where the constellation is high in the sky, even a whole season of observing won't be nearly enough to cover all of its deep-sky treats.

In addition to the Messier objects, Sagittarius contains a score of bright deep-sky gems, mainly nebulae and star clusters. One extragalactic target you can find is Barnard's Galaxy (NGC 6822). This object is somewhat of an observing challenge. It glows at a respectable magnitude of 8.8, but its light spreads over an area 19' by 15'. This combination means it's a low-surface-brightness object, so you'll need a dark sky to see it. Because NGC 6822 lies only 1.6 million light-years away, you can use a nebula filter with an 8-inch or larger telescope to see HII regions — vast clouds of glowing hydrogen that eventually will form stars.

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