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Форум » Досуг » Космос » NGC 21
NGC 21
ingvarr
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Дата: 11.09.2016, 11:08:38 | Сообщение № 1
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NGC 21 (другие обозначения — NGC 29, IRAS00082+3304, UGC 100, ZWG 499.66, MCG 5-1-48, KAZ 19, PGC 767) — галактика в созвездии Андромеда.

Этот объект занесён в новый общий каталог несколько раз, с обозначениями NGC 21 и NGC 29.

Галактика NGC 21 была открыта немецко-британским астрономом Уильямом Гершелем 26 ноября 1790 года. Позднее она наблюдалась Льюисом Свифтом 20 сентября 1885 года. Такое двойное открытие выяснилось не сразу, именно поэтому галактика получила 2 номера в каталоге: NGC 21 (Свифт) и NGC 29 (Гершель).
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ingvarr
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ingvarr
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NGC 21 (= NGC 29 = PGC 767)
Discovered (Nov 26, 1790) by William Herschel (and later listed as NGC 29)
Also observed (Nov 11, 1827) by John Herschel (and later listed as NGC 29)
Recorded (Sep 20, 1885) by Lewis Swift (and later listed as NGC 21)
A magnitude 12.7 spiral galaxy (type SAB(s)bc?) in Andromeda (RA 00 10 46.9, Dec +33 21 10)
Historical Identification: Per Dreyer, NGC 21 (Swift list II (#5), 1860 RA 00 02 25, NPD 57 34.1) is "extremely faint, small, a little extended". Swift's position precesses to RA 00 09 38.5, Dec +33 12 39, but there is nothing there. However, as noted in the discussion of NGC 6, this is one of several galaxies discovered by Swift on the same night which mostly share an average error of 70 seconds of time too far west and 8' 12" too far south. Applying that correction, Swift's position for his II-5 precesses to RA 00 10 48.9, Dec +33 20 53, on the southeastern rim of the galaxy listed above and the description fits, so the identification is considered certain. The duplicate entry is due to the fact that with such a large error in Swift's position, there was no way for Dreyer to realize that it was an observation of a previously discovered nebula.
Note About Frequent Confusion With NGC 19: NGC 21 is almost due north of NGC 19, and Swift's incorrect declination for NGC 21 lies about halfway between them. By an unhappy coincidence, two major catalogs very rapidly put together in the 1970's misidentified the southern galaxy as NGC 21. It was only fairly recently that studies of the original observations of NGC objects showed that it was actually NGC 19. So although almost all modern references have corrected the mistake, a warning about a possible misidentification of NGC 19 as NGC 21 is still appropriate.
Physical Information: Based on a recessional velocity of 4770 km/sec, NGC 21 is about 220 million light years away, in good agreement with redshift-independent distance estimates of 200 to 270 million light years. Given that and its apparent size of 1.55 by 0.6 arcmin, it is about 100 thousand light years across.
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