天鹅座的疏散星团M39
影像版权与提供：Heidi Schweiker, WIYN, NOAO, AURA, NSF
说明：刚好在人类裸视极限的M39，是北方天空中一个很美丽的大型疏散星团。位于北天天鹅座内的M39，离我们大约有800光年远，他内部为数不多的恒星占住了比满月略大的天区。上面这张M39的影像，是由33张WIYN望远镜照片所组成出来的，拍摄地点是美国亚利桑那州的基特峰(Kitt Peak)。 M39成员恒星的年龄大约都是三亿年。远比太阳的五十亿年要年轻。疏散星团也称为银河星团，他们的成员星比球状星团年轻，数量也较少。另一项关于球状星团的特点是，疏散星团通常位于银河系的盘面上。
图文来源：http://www.phys.ncku.edu.tw/~astrolab/mirrors/apod/ap040331.html
示意图来源：http://oleshko.net.ru/astro/M39-NGC7000.gif
另附一份比较详细的英文说明：http://www.seds.org/messier/m/m039.html
感觉除了最后一段有不少简写不大明白之外其他的不是很难
M39 is a very large but very loose open cluster, situated some 9 degrees east and a bit north of Deneb (Alpha Cygni). Its distance is only about 800 light years, and it is of intermediate age (estimates between 230 and 300 million years). 30 stars are proven members and contained in a volume of about 7 light years diameter. Its apparent visual brightness of 4.6 magnitudes (e.g., Sky Catalogue 2000.0, Uranometria 2000) corresponds to an absolute magnitude of -2.5, or an intrinsic luminosity of 830 suns. Kenneth Glyn Jones gives its apparent visual brightness as 5.2 mag only, while Don Machholz has estimated it at mag 5.4, in agreement with estimates quoted by Mallas/Kreimer, who also mention D.F. Gray's estimate of a total visual brightness of 6.0 magnitudes. M39's brightest star is of magnitude 6.83 visually, and of spectral type A0. All stars were found to be main sequence stars in the Color-Magnitude Diagram (CMD), or Hertzsprung-Russell Diagram (HRD), with the brightest stars apparently just before the point of evolution toward the red giant phase. The Sky Catalogue 2000.0 gives an estimated age of 270 million years for this cluster - this is between the two determinations cited by Kenneth Glyn Jones of 300 million years by Lohmann and 230 million years by Van Hoerner. M39 is approaching us at 28 km/s; its proper motion was given as 0.024" per year in the direction of position angle 222 deg (by E.G. Ebbighausen 1940, according to Burnham). Woldemar Götz gives the Trumpler type of this cluster as III,2,m: Detached from the surrounding field but not concentrated toward its center, the stars are in a moderate range in brightness, and it is moderately rich (50--100 members). The Sky Catalogue 2000.0 has III,2,p (i.e., "poor", less than 50 member stars). While Kenneth Glyn Jones counts M39 as one of Charles Messier's original discoveries, cataloged by him on October 24, 1764, Burnham claims that its discovery "is often credited to Le Gentil in 1750, but P. Doig (1925) quotes a statement made by J.E. Gore that this cluster was noted by Aristotle as a cometary appearing object about 325 BC." Kenneth Glyn Jones rates the identification of Le Gentil's observation (done by Bigourdan) as "extremely doubtful". This cluster is best observed with lowest powers because of its considerable angular size of 32 arc minutes, more than the Moon. Under good conditions, it can just be glimpsed with the unaided eye. It is well seen in opera glasses and smaller binoculars as a nebulous object, resolved in a 7x50, great at small powers, where its shape stands out: An equilateral triangle with a bright star at each corner, the southerly side aligned about East-West: a 9th mag star at its Northern corner, and one of 7th mag each at the SE and SW corners. About 25 fainter stars within. Many of its stars are grouped in pairs. The cluster is impressive and despites its looseness, well defined in and detached from a rich Milky Way star field. At higher magnifications it fills many fields and gets less impressive. M39 is not difficult to find: From Deneb (Alpha Cygni) first locate Rho Cygni, a star of 4th magnitude which is about 9 deg East, possibly via intermediate Zeta Cygni. M39 is 3 deg N and 1/4 deg W. It is also 2 1/2 deg W and 1 deg S of 4.5-mag Pi2 Cygni. |
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