Star cluster

Japanese: 星団 - せいだん（英語表記）star cluster

A star cluster is a group of stars that form a single dynamic group through gravitational interactions. There are two types of star clusters found in our galaxy: globular clusters, which are spherical clusters of tens of thousands to millions of stars, and open clusters, which are somewhat irregularly scattered clusters of tens to thousands of stars. Globular clusters contain only a few to a hundredth of the amount of heavy elements such as iron in the sun, and are called Population II objects. They are 10 to 14 billion years old, which is as old as the age of the Milky Way. They are thought to have been formed in the primordial galactic cloud, which was probably still contracting. On the other hand, open clusters are second generation (Population I) objects formed from interstellar material containing a lot of heavy elements scattered by Population II objects. Their ages vary from 0 to 5 billion years, and they are still being formed today. Representative examples of globular clusters are M13 in Hercules and the Omega Centauri cluster in the southern sky, while examples of open clusters include the Pleiades (M45) in Taurus and the double cluster of NGC869 (h Per) and NGC884 (χ Per) in Perseus. Star clusters exist not only in our Milky Way Galaxy, but also universally in other galaxies.

[Masanori Yoshizawa and Asahi Fujii]

Distribution and movement of star clusters

There are about 130 known globular clusters in the Milky Way. Of these, more than 80% are located in the southern sky. This is because globular clusters are distributed spherically around the center of the Milky Way, and the center of the Milky Way is in the southern direction of Sagittarius. The total extent of globular clusters is comparable in size to the Milky Way, and the average distance from the center of the Milky Way is about 30,000 light years.

The distribution of open star clusters on the celestial sphere is quite different from that of globular clusters, and they are concentrated in the Milky Way. For this reason, they are also called galactic clusters. Open clusters are found all over the galactic plane (the flat, disk-like surface of the galaxy), and it is estimated that there are tens of thousands of them in the galaxy as a whole, but only about 1,000 have been the subject of detailed research. Most of them are located within 10,000 light years of the Sun's vicinity, and among these, open clusters that are younger than 20 million years are particularly prominent in the three spiral arms (spirals of the galaxy) of the Perseus Arm, the Orion-Cygnus Arm, and the Carina Arm. This suggests that open clusters are born in the spiral arms. On the other hand, the distribution of open clusters perpendicular to the galactic plane is very thin, with an average height of about 200 light years from the galactic plane. If the galaxy were reduced to a disk with a radius of 10 centimeters, most open clusters would fit within a thickness of 1 millimeter. Open clusters are also representative examples of the disk components of the Milky Way.

The spatial motion of open clusters, like other Population I objects, shows strong galactic rotation (motion around the center of the galaxy). In the vicinity of the Sun, the rotation period is about 200 million years. However, each cluster also has a deviation from the circular motion called the characteristic motion, which causes young open clusters concentrated in the spiral arms to disperse into the surrounding space over the course of many years.

To know the spatial motion of a star cluster, it is necessary to know both the radial velocity and the motion perpendicular to the line of sight, or proper motion. Most globular clusters are very far away, so it is difficult to determine their proper motion from observations. For this reason, their spatial motion has been estimated under several assumptions, but according to these, globular clusters move mainly in the radial direction, and the galactic rotation is weak. Therefore, when viewed from the Sun, which is part of the galactic rotation, they show a very large relative velocity of over 100 kilometers per second. This property has also been observed in old stars known as high-velocity stars, and is thought to reflect the contraction motion of the Galaxy in its early stages of formation.

[Masanori Yoshizawa and Asahi Fujii]

Evolution of star clusters

In young star clusters, stars with masses ranging from tens of times that of the Sun to only a fraction of that. However, the lifespan of a star is finite, and it shortens rapidly as the mass increases. Therefore, the composition of stars in a star cluster changes as the cluster ages. Today's globular clusters have been in existence for over 10 billion years, and contain almost no stars heavier than the Sun. On the other hand, open clusters are in various stages of evolution due to their diverse ages, and they differ greatly from cluster to cluster. This evolution is visualized in the HR diagram (or color-magnitude diagram) of a star cluster. In general, the younger the cluster, the more bluish-white the stars are, and the older the cluster, the more reddish the stars are.

Apart from the evolution of individual stars in a cluster, the cluster as a whole also undergoes dynamic evolution and changes its structure. This is for the following reasons. Stars in a cluster move through the cluster along a predetermined orbit, but at this time, they are subjected to gravitational disturbances from other stars that happen to come close to them. As this disturbance accumulates, the energy eventually changes significantly, and some stars escape from the cluster. This mechanism is called "relaxation," and the time it takes for the relaxation to occur is called the relaxation time. The dynamic lifespan of a cluster is said to be about 10 times the relaxation time. The relaxation time is proportional to the total number of stars in the cluster and inversely proportional to the square root of the average mass density. For this reason, the relaxation time of a globular cluster is long, at several billion years, and its dynamic structure does not change significantly over a time scale of about 10 billion years. In contrast, the relaxation time of an open cluster is short, at several tens of millions of years, and open clusters are not as stable from a dynamic perspective as globular clusters. In addition, open clusters are susceptible to dynamical disturbances from interstellar clouds. Thus, the average lifespan of an open cluster is estimated to be several hundred million years. In other words, half of the open clusters will be dynamically disintegrated in about one Galactic rotation. However, the dynamical lifespan of each cluster is determined by a combination of various conditions. There are several known open clusters that are more than several billion years old, but these are probably due to favorable conditions. Examples are M67 (Cancer) and NGC188 (Cepheus). Both clusters are located quite far from the galactic plane, and are therefore less susceptible to the effects of interstellar clouds.

[Masanori Yoshizawa and Asahi Fujii]

"Arimoto Nobuo, Nebula and Cluster Series: Globular Clusters" (1982, Chijin Shokan)" ▽ "Furuta Toshimasa, Galactic Nebulae and Clusters - Open Clusters, Globular Clusters, Planetary Nebulae, Diffuse Nebulae, Dark Nebulae" (1989, Seibundo Shinkosha)" ▽ "Hata Hidetoshi and Tarusawa Kenichi, To the Far reaches of the Universe, Full Color Edition - Space Portraits" (1995, Nikkei BP Publishing Center)" ▽ "Fukui Yasuo, The Birth of the Great Universe - The Beginning and End of the Universe as Seen in the 'Egg of Stars'" (1998, Kobunsha)" ▽ "Okano Kunihiko, Digital Eye - Deep Universe Captured by a Cooled CCD" (1998, Chijin Shokan)" ▽ "Searching for Nebulae and Star Clusters" by Hideo Asada, photographed by Masao Tanigawa, supervised by Junichi Watanabe, and edited by the Skywatcher Editorial Department (1999, Rippushobo)" ▽ "The Latest Fujii Akira's Nebula and Star Cluster Class" by Akira Fujii (2004, Seibundo Shinkosha)"

M13 globular cluster
A globular cluster in the constellation Hercules. NGC6205. Also known as the Great Globular Cluster of Hercules. ©National Astronomical Observatory of Japan ">

M13 globular cluster

Omega Centauri (ω Cluster)
A globular cluster in the constellation Centaurus. NGC5139. Photographed using the Murikabushi Telescope at Ishigakijima Astronomical Observatory © National Astronomical Observatory of Japan ">

Omega Centauri (ω Cluster)

Pleiades (Subaru)
M45 is an open star cluster in Taurus. It is surrounded by a group of reflection nebulae. ©National Astronomical Observatory of Japan ">

Pleiades (Subaru)

Double Cluster
An open cluster between Perseus and Cassiopeia. Near the center of the photo, NGC869 (h Per) is on the right and NGC884 (χ Per) is on the left. ©Shogakukan Photo by Hideyuki Asakura ">

Double Cluster

Ptolemy Cluster
An open cluster in the constellation Scorpio. NGC6475 (M7) ©National Astronomical Observatory of Japan ">

Ptolemy Cluster

Beehive Star Cluster
An open star cluster in the constellation Cancer. NGC2632 (M44) ©National Astronomical Observatory of Japan ">

Beehive Star Cluster

M2 globular cluster
A globular cluster in the constellation Aquarius. NGC7089. Photographed using the Murikabushi Telescope at Ishigakijima Astronomical Observatory © National Astronomical Observatory of Japan ">

M2 globular cluster

M15 globular cluster
A globular cluster in the constellation Pegasus. NGC7078. Photographed using the Murikabushi Telescope at Ishigakijima Astronomical Observatory ©National Astronomical Observatory of Japan ">

M15 globular cluster

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

多数の恒星が、重力的な相互作用を通して、まとまった一つの力学集団を形成しているものを星団とよぶ。われわれの銀河系で発見されている星団には、球状星団とよばれる数万～数百万個の恒星が球状に密集したものと、数十～数千個程度の恒星がやや不規則に散在する散開星団とがある。このうち球状星団は、鉄などの重元素含有率が太陽の数分の1から100分の1程度と少なく、種族Ⅱの天体とよばれている。その年齢は100億～140億年であり、銀河系の年齢と同じ程度に古い。おそらくまだ収縮を続けていた原始銀河雲の中で生成されたものと考えられている。一方散開星団は、種族Ⅱの天体がまき散らした重元素を多く含む星間物質から生成された、第2世代（種族Ⅰ）の天体である。その年齢は0～50億年までさまざまであり、現在でもまだ生成が続いている。ヘルクレス座のM13や南天のオメガ・ケンタウリ星団が球状星団の代表例であり、おうし座のプレヤデス（M45）やペルセウス座NGC869（h Per）とNGC884（χ(カイ) Per）の二重星団などが散開星団の例である。星団はわれわれの銀河系だけではなく、他の銀河でも普遍的に存在している。

［吉澤正則・藤井　旭］

星団の分布と運動

銀河系に属する球状星団は約130個知られている。このうち80％以上は天球上で南天に位置している。これは、球状星団が銀河系の中心の周りに球状に分布しており、かつ、銀河系の中心が南天のいて座の方向にあるからである。球状星団全体の広がりは銀河系に匹敵する規模をもち、銀河系中心からの距離の平均は約3万光年である。

　散開星団の天球上における分布は、球状星団と著しい相違を示し、天の川に集中している。このため銀河星団ともよばれている。散開星団は銀河面（平たい円盤状の銀河系の面）上のいたる所にあり、銀河系全体では数万個以上あると推定されているが、詳しい研究の対象となっているのは1000個程度である。そのほとんどは太陽の近傍1万光年以内のものであるが、このうちとくに年齢が2000万年よりも若い散開星団は、ペルセウス腕、オリオン‐はくちょう腕、および、りゅうこつ‐いて腕の3本の渦状腕（銀河系の渦巻）に顕著である。このことは、散開星団が渦状腕で誕生することを示唆している。一方散開星団の銀河面に垂直方向の分布は非常に薄く、銀河面からの平均的な高さは約200光年となっている。もし銀河系を半径10センチメートルの円盤に縮小するとすれば、ほとんどの散開星団は厚さ1ミリメートルの範囲に収まってしまう。散開星団は銀河系の円盤成分の代表例でもある。

　散開星団の空間運動は、他の種族Ⅰの天体と同様に、強い銀河回転（銀河系中心をめぐる運動）を示す。太陽近傍でその回転周期は約2億年である。しかし個々の星団にはこのほかに特有運動とよばれる円運動からのずれがあり、このために渦状腕に集中していた若い散開星団も長年月の間には周囲の空間に拡散してしまう。

　星団の空間運動を知るためには、視線速度と視線方向に垂直な動き、固有運動の両方を知る必要がある。球状星団のほとんどは非常な遠方にあるため、その固有運動を観測から決めることはむずかしい。このためいくつかの仮定のもとで空間運動が推定されているが、それによれば、球状星団は主として動径方向に偏った運動をしており、銀河回転は弱い。したがって銀河回転に乗っている太陽から見ると、秒速100キロメートルを超す非常に大きな相対速度を示す。このような性質は高速度星と称される古い星でも観測されており、誕生初期における銀河系の収縮運動を反映したものと考えられる。

［吉澤正則・藤井　旭］

星団の進化

誕生直後の若い星団では、太陽の数十倍の質量をもつ星から、わずか数分の1のものまで、さまざまな星が混じっている。しかし恒星の寿命は有限であり、しかも質量の増大とともに急速に短くなる。したがって星団の年齢が古くなるにしたがってその中の星の構成は変化していく。今日の球状星団は誕生以来100億余年を経て、太陽より重い星はほとんど含まれていない。一方、散開星団ではその年齢の多様さのゆえに進化の段階もさまざまであり、星団ごとに大きく異なっている。このような進化のようすは星団のHR図（または色‐等級図）に視覚化されている。一般には若い星団ほど青白い星が多く、年齢が古くなるにつれて赤みがかった星が多くなってくる。

　星団の中の個々の星の進化とは別に、星団全体も力学的な進化を受けて、構造を変えていく。それは次のような理由による。星団の中の星はあらかじめ決められた軌道に沿って星団の中を動くが、このとき、偶然に接近した他の星から重力的な擾乱(じょうらん)を受ける。この擾乱が蓄積されていくと、やがてはエネルギーが大きく変化し、星団から脱出してしまうものも出てくる。このような機構を「緩和」（リラクセーション）とよび、緩和に至るまでの時間を緩和時間という。緩和時間の10倍程度が星団の力学的寿命といわれている。緩和時間は星団の中の星の総個数に比例して、また平均質量密度の平方根に反比例して、長くなる。このため球状星団の緩和時間は数十億年と長く、100億年程度の時間尺度ではその力学的構造はそれほど大きくは変化しない。これに対して散開星団の緩和時間は数千万年と短く、力学的にみた散開星団は球状星団ほどには安定でない。加えて、散開星団は星間雲などからの力学的擾乱も受けやすい。このようにして散開星団の平均的寿命は数億年と推定されている。すなわち、約1銀河回転の間に半数の散開星団が力学的に崩壊してしまう。ただし個々の星団がどの程度の力学的寿命を有するかは、種々の条件の組合せで決まる。年齢が数十億年を超える散開星団もいくつか知られているが、これらは諸条件が有利に働いたためであろう。たとえばM67（かに座）やNGC188（ケフェウス座）がある。両星団とも銀河面よりかなり離れたところにあり、星間雲などの影響を受けにくいのが原因と思われる。

［吉澤正則・藤井　旭］

『有本信雄著『星雲星団シリーズ　球状星団』（1982・地人書館）』▽『古田俊正著『銀河系の星雲・星団――散開星団・球状星団・惑星状星雲・散光星雲・暗黒星雲』（1989・誠文堂新光社）』▽『畑英利・樽沢賢一著『遥かなる宇宙へ　オールカラー版――スペース・ポートレート』（1995・日経BP出版センター）』▽『福井康雄著『大宇宙の誕生――「星のたまご」に見る宇宙の始まりと終わり』（1998・光文社）』▽『岡野邦彦著『デジタル・アイ――冷却CCDでとらえた深宇宙』（1998・地人書館）』▽『浅田英夫著、谷川正夫写真、渡部潤一監修、スカイウオッチャー編集部編『星雲星団を探す』（1999・立風書房）』▽『藤井旭著『最新　藤井旭の星雲・星団教室』（2004・誠文堂新光社）』