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Designations:
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M15, NGC 7078
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Object Type:
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Constellation:
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Pegasus
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21 hrs 30 min
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+12 deg 10 min
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6.3
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Size:
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18 arcminutes
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Distance:
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30,600 light years
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Discoverer:
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Maraldi II, 1746
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Nearly a twin of M2 in Aquarius, this glittering gem in the winged horse,
Pegasus, is one of six beautiful globulars brighter than 7th
magnitude that grace the northern sky
(the others are M2, M3, M5, M13, and M92). The Great Pegasus Cluster, M15, can be spotted without
difficulty as a "fuzzy star" with the unaided eye, lying just 4° northwest of the topaz
(Type-K2 I)
2nd-
magnitude star Epsilon (ε) Pegasi. M15 is some 30,000 light years distant (16,600 light years
farther away than M13) and measures up to 160
light years in diameter. Like M13, it contains many
red-giant stars. But because of its greater distance, M15 appears fainter and more compact than M13.
At low power the cluster hides inside a triangle of three 7th- to 8th- magnitude stars. Hazy, spiderlike arms are already apparent, and the cluster brightens rapidly toward the center. Otherwise, like M13, most of Ml5's stars evade direct gaze and require averted vision. William Herschel rated this good test object for resolution. But subtle details do show through. For example, it has a definite asymmetry. The late Harvard University astronomer Harlow Shapley first confirmed this by noting the oblateness at the cluster's central bulge, which is surrounded by a spherical shell of stars. M15 also displays dark patches. One obvious dark feature appears next to a detached string of stars on the northeast edge of the cluster's inner shell. Another, tighter arc of stars to the east of the nucleus makes the entire central region appear warped in that direction, something noticed by d'Arrest in the nineteenth century. According to Webb, "Buffham, with a 9-inch [mirror] finds a dark patch near the middle with two faint, dark lines or rifts like those in M13." I did not notice these. My view is more like the one Isaac Roberts described at the turn of the century, in which stars are arranged "in curves, lines and patterns." The cluster has an unusual resident, a 14th- magnitude planetary nebula, Pease 1, on its northeast side. In fact, M15 is one of two globular clusters known to contain a planetary; the other is a 10" X 7" object, GJJC-1, in M22. F. G. Pease discovered the M15 planetary in 1928. But measuring only 1" in diameter and awash in a sea of stars, Pease 1 is nearly impossible to detect in backyard telescopes. Its central star shines at magnitude 15.0. The nebula and the star eluded my gaze in the Genesis. The cluster is also host to a wealth of variable stars (nearly 100 are known). Jones points out that M15 ranks third behind M3 and the famous southern globular Omega Centauri in the number of variable stars it contains. In 1974, M15 was discovered to be a source of X-ray energy, which, together with the cluster's apparent tightness, led some astronomers to conjecture that a black hole lurked at its center. The Hubble Space Telescope, however, disproved that by resolving the cluster virtually to its core and revealing nothing extraordinary about it. Instead, astronomers now believe that the X-ray energy might be coming from one or more supernova remnants. |
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Telescope:
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Focal Length:
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1200mm
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Mount:
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Camera
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Guider:
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Exposures:
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9 5-Minute exposures (Luminance) 4 7-minute exposures for each color (red, green, and blue)
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Location:
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Cicero, IN
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Software:
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CCDSoft for image acquisition, processed with CCDStack and Photoshop CS2
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