[en] [de] [it] [fr]

梅西爾天體 Nr.2

寶瓶座 的 球狀星團 M2 (NGC 7089)，類型 II

[m2.jpg]
赤經座標 21 : 33.5 (時:分)
赤緯座標 -00 : 49 (角:分)
距離 37.5 (千光年)
目視星等 6.5 (等)
天體範圍 16.0 (角分)

瓊•多明尼克•馬拉第於西元1746年發現。

球狀星團M2是由馬拉第於西元1746年9月11日所發現，查理斯•梅西爾則正好於14年後的西元1760年9月11日在未知情況下獨自發現，並將它納入自己的天體目錄，不過這個天體目錄對它的描述卻是「不含恆星的瀰漫星雲」，威廉•赫歇爾則是首位將這個球狀星團的個別恆星解析出來的天文學家。

這個球狀星團的直徑約為175光年，內部是由大約150,000顆恆星所聚集而成，它算是其中一個恆星聚集程度相當高的球狀星團，因此天文學家根據集中度分類法將它歸類在類型II，它的外觀呈橢圓球形(其橢圓率為9或E1形式)，從本頁照片可以看出星團內部的恆星在方位角135度方向往外延伸。相對於地球而言，這個球狀星團位於銀河系中心的另外一端，距離我們約37,500光年的位置(根據W•E•哈里斯的天體資料庫)，其目視星等為6.5等、在天球上的直徑約為6~8角分，明亮且恆星較密集的中心區域則佔了5角分。從一般天體照片來看，這個球狀星團的最外圍恆星可以延伸至12.9角分的範圍，若以深空天體照片來看，其範圍甚至可以達到16.0角分。

就如同大部分的球狀星團，M2中心部分的恆星密集度相當高，其核心直徑只有0.34角分或20角秒，以距離換算約為3.7光年、其直量半寬度則為0.93角分(56角秒或10光年)；換句話說，這個球狀星團的潮汐半徑約為21.45角分或233光年，位於此範圍之外的恆星可能會受到銀河系的潮汐力而從球狀星團逃逸。

M2內部最明亮的恆星多為亮度13.1等的紅、黃巨星，其水平支系星亮度則為16.1等。這個球狀星團的整體光譜類型為F0、色指數為-0.06；而近代測量的光譜類型為F4、色指數為0.66。

赫爾頓•亞帕(1962)根據星色星等圖估計M2的年齡大約是1,300萬年，這個年齡和球狀星團M3和M5相當。

在這個球狀星團內部已知的21顆變星當中，最早被發現的是貝利於1895年所發現。 Of its 21 known variables, the first two have been discovered by Bailey in 1895 (Pickering and Bailley 1895), and a total of 8 until 1897. Most of them are so-called "cluster variables" of RR Lyrae type, with short periods of less than a day. Three of them, however, are "classical" Cepheids of type II (W Virginis stars) with periods of 15.57, 17.55 and 19.30 days respectively, and an apparent visual beightness of about 13th magnitude. These stars have been studied by H.C. Arp (1955) and G. Wallerstein (1970). One variable is a RV Tauri star whose apparent magnitude varies between 12.5 and 14.0 with a 69.09 day period; this star has alternating deep and shallow minima, and was discovered in 1897 by the French amateur A. Chèvremont. It lies at the eastern edge of the cluster, slightly toward the North.

M2 is approaching us at the low velocity of 5.3 km/sec. Situated in the Galactic halo, it has been classified as an "H2" halo globular by Woltjer (1975) and Nincovic (1983); the latter estimating its orbital excentricity at 0.60. To determine its orbit within the Galaxy, the position with respect to the Galactic center, and the space velocity of the cluster must be known. As its radial velocity is considerably well determined, an appropriately acurate determination of the globular cluster's proper motion is crucial to estimate its space velocity. An estimate from its motion with respect to background galaxies has been undertaken at Lick Observatory (Cudworth and Hanson 1993), as 0.41 and 0.22 arc seconds per century, or 4.1 and 2.2 mas/yr in RA and Dec, respectively. Dauphole et.al. (1996) have assumed these values to be +5.5 and -4.2 mas/yr, and calculated an orbit with excentricity e=0.67, an apogalacitc distance of 91,000 light-years, and a maximal elevation above and below the galactic plane of 78,000 light-years. Geffert et.al. (1997) have obtained improved values for its space motion from data obtained by ESA's astrometric satellite Hipparcos, and Brosche et.al. (1997) have calculated an orbit for this and other globular clusters. According to these calculations, M2 is moving on a highly excentric (e=0.76) orbit of box type, with a perigalactic distance of 23,500 light-years, which carries it out to an enormous apogalactic distance of 171,000 light-years, and up to 165,000 light-years above and below the Galactic plane.

M2 is found rather easily from Alpha and Beta Aquarii, as well as Epsilon Pegasi. It is 5 degrees north of Beta Aquarii, on the same declination as Alpha Aquarii.

With its visual magnitude of 6.5 mag, M2 is a difficult object for naked-eye observing (just not visible under "average" conditions), but an easy target for the slightest optical aids like binoculars or opera glasses, in particular as it is situated in a star-poor field. A four-inch obstruction-free telescope (refractor or schiefspiegler) doesn't resolve this cluster, but only shows some of the brightest member stars spread over the mottled nebulous background image caused by the unresolved stars. Observing with his 4-inch refractor, John Mallas reports a dark curving lane feature crossing the north-eastern corner of the cluster, which can also be found in photographs. With an 8-inch, this globular cluster is partly resolved into stars, well into the center under good viewing conditions. Larger scopes, 10-inch up, are required to fully resolve this cluster. A peculiar dark lane crosses the north-east edge of the cluster, suggestions of which are visible in our image; larger telescopes (16-inch up) show several other, less prominent darker features or regions.

  • M2的歷年觀測記錄及描述
  • M2的GALEX紫外線頻譜影像
  • 更多M2的影像
  • M2的業餘拍攝影像

  • Marco Castellani's data for M 2
  • Christine Clement's Catalog of Variable Stars in M2
  • SIMBAD Data of M2
  • NED Data of M2
  • Publications on M2 (NASA ADS)
  • Observing Reports for M2 (IAAC Netastrocatalog)
  • NGC Online data for M2

    參考資料：


    Hartmut Frommert
    Christine Kronberg
    [聯絡作者]

    [SEDS] [MAA] [Home] [M 1] [M 3] [Next Cluster] [Next Nebula] [Next Galaxy] [Image Browser] [DSSM] [Indexes] [PNG Image]

    最後修改：2007 年 7 月 9 日

    翻譯者：鄭龍凱／2007 年 8 月 ? 日