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NGC 6715 (Globular Cluster, M54)


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NGC 6715, M54
Globular Cluster (球状星団)
Gb III


別名 (Other names)
GCL 104, ESO 458-SC8, Messier 54

明るさ (Brightness)
7.7等級

星座 (Constellation)
いて座 (Sgr) (Sagittarius)

距離 (Distance)
90,000光年 (90,000 light-years)

M54 (NGC6715) はいて座にある球状星団。1778年メシエが発見した。「非常に明るい星団。中心が明るく星はない。口径3.5フィートの色消し望遠鏡でよくみえる」と記した。メシエは星に分離して見ることはできなかったが、ウィリアム・ハーシェルも周辺部の星を少し分離しただけで、球状星団であるとは認識できなかった。ジョン・ハーシェルは「球状星団。非常に明るくまるく中心部が周りから急に明るくなっている。星によく分かれ、主に15等級の星からできている。外周2.5'のところに14等星がある」と記している。

双眼鏡では恒星状にしか見えない。ジョーンズは小さくちょっとみたところ、惑星状星雲のようだとした。口径20cm程度の望遠鏡でも周辺部の星が見えてくるという人もいる。だが、口径40cmでも星を分離するのは困難で、ざらざらとした印象に見え、ほとんど星は分離できない。明るい部分は東西に延びた楕円に見える。

1994年この星団までの詳細な距離が測定された。その結果、この星団は我々がいる銀河系に属しているのではなく、天の川を通して見える外の銀河・SagDEG(いて座矮小楕円銀河)に属する星団であることがはっきりとした。銀河系外で発見された最初の星団。

NGC 6715 (M54) is a globular cluster located 90,000 light years from the Earth in the constellation Sagittarius.

NGC 6715 (Globular Cluster, M54) : Picture

NGC 6715 (Globular Cluster, M54)
NGC 6715 (Globular Cluster, M54)
(C) NASA/ESA/Hubble
The object shown in this beautiful Hubble image, dubbed Messier 54, could be just another globular cluster, but this dense and faint group of stars was in fact the first globular cluster found that is outside our galaxy. Discovered by the famous astronomer Charles Messier in 1778, Messier 54 belongs to a satellite of the Milky Way called the Sagittarius Dwarf Elliptical Galaxy.

Messier had no idea of the significance of his discovery at the time, and it wasn’t until over two centuries later, in 1994, that astronomers found Messier 54 to be part of the miniature galaxy and not our own. Current estimates indicate that the Sagittarius dwarf, and hence the cluster, is situated almost 90,000 light-years away — more than three times as far from the centre of our galaxy than the Solar System.

Ironically, even though this globular cluster is now understood to lie outside the Milky Way, it will actually become part of it in the future. The strong gravitational pull of our galaxy is slowly engulfing the Sagittarius dwarf, which will eventually merge with the Milky Way creating one much larger galaxy.

Messier 54
The globular star cluster Messier 54
(C) ESO
This image from the VLT Survey Telescope at ESO’s Paranal Observatory in northern Chile shows the globular cluster Messier 54. This cluster looks very similar to many others, but it has a secret. Messier 54 doesn’t belong to the Milky Way, but actually is part of a small satellite galaxy, the Sagittarius Dwarf Galaxy. This unusual parentage has allowed astronomers to use the Very Large Telescope (VLT) to test whether unexpectedly low levels of the element lithium in stars are also found in  stars outside the Milky Way.

Messier 54
Wide-field view of the sky around the globular star cluster Messier 54
(C) ESO and Digitized Sky Survey 2
This visible-light wide-field image of the region around the globular star cluster Messier 54 was created from photographs forming part of the Digitized Sky Survey 2. The globular star cluster Messier 54 cluster appears at the centre.

Messier 54
The globular star cluster Messier 54 in the constellation of Sagittarius
(C) ESO, IAU and Sky & Telescope
This chart shows the location of the globular star cluster Messier 54 in the constellation of Sagittarius. This map shows most of the stars visible to the unaided eye under good conditions and the cluster itself is marked with a red circle. This globular cluster can be easily seen with a very small telescope or binoculars, but it is distant, and the individual stars are very hard to discern.

NGC 6715 (Globular Cluster, M54) Messier 54
NGC 6715 (Globular Cluster, M54)
(C) NASA/ESA/Hubble
Messier 54
(C) ESO
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「ウィキペディア(wikipedia):フリー百科事典」より文章引用。
M54. (2013, March 23). In Wikipedia.
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