Nebula

Japanese: 星雲 - せいうん（英語表記）nebula

Interstellar matter in the Milky Way, consisting of gas and dust, can appear to glow or darken due to various reasons, absorbing light. When viewed through binoculars or a small telescope, they appear dimly spread out, unlike point sources of light like stars. For historical reasons, they have simply been called "nebulae," but because of the ambiguity, names given to each classification that reflects the properties of each type of nebula are often used today. When referring to the whole group, they are sometimes called gas nebulae.

[Okamura Sadanori]

Observation history

In the days when the performance of telescopes was poor, any faintly visible celestial object other than a point source of light that could be clearly identified as a star was called a nebula. A group of stars was called a star cluster, but a star cluster that was only faintly visible was sometimes called a nebula. Such faint "nebulae" were easily confused with comets, and were a problem for comet hunters at the time. Between 1771 and 1784, French comet hunter C. Messier published the Messier Catalogue, which included 103 such celestial objects that could be easily confused with comets. The celestial objects in this catalogue are called Messier objects, and the catalogue numbers are prefixed with the symbol M, such as M31.

Later, in 1856, J.F.W. Herschel, son of F.W. Herschel, published the General Catalogue, which included about 5,000 nebulae and clusters. Dryer revised and expanded this, and compiled the New General Catalogue of Nebulae and Clusters of Stars, which included 7,840 objects, in 1888. He also published two Index Catalogues, the IC Catalogue, which included 5,836 objects, in 1895 and 1905. The objects in these catalogs are called NGC and IC, respectively, at the beginning of their catalog numbers, such as NGC224 and IC10.

Later, it was discovered that the objects classified as nebulae in the NGC and IC catalogs were of two types with very different distributions on the celestial sphere. One was distributed along the Milky Way, while the other was distributed away from it. When viewed through a high-performance telescope, some of the latter had a spiral pattern, and they came to be called spiral nebulae.

Even at the beginning of the 20th century, the size of the universe and the position of the sun within it were still not well understood, and the true nature of spiral nebulae was unknown. Astronomers had two main theories. They all agreed that the Milky Way is a group of many stars, but one theory was that the sun is at the edge of that group, and that spiral nebulae are the appearance of stars being born from gas within that group. The other theory was that the sun is near the center of the group, and that outside of that there are many large groups of stars of similar size (island universes), and that spiral nebulae are the appearance of these.

In 1920, Shapley and Heber Doust Curtis (1872-1942), representatives of these two schools of thought, held a public debate at the National Academy of Sciences of the United States of America, pitting their respective views against each other. This later came to be called the Great Debate. The debate was not resolved here, but in 1923, Hubble discovered a variable star called a Cepheid in the spiral nebula (M31), which settled the debate. Once the Cepheid was found, it was possible to measure the distance, and as a result, it was discovered that M31 is an island universe located far outside the Milky Way stellar mass.

Island universes were once called extragalactic nebulae, but are now called galaxies. The Milky Way star cluster is also a galaxy, but it is called the Milky Way to distinguish it from the Milky Way because it is the galaxy we live in. In contrast, the nebulae distributed along the Milky Way are shining interstellar matter within the Milky Way. These were once called intragalactic nebulae in contrast to extragalactic nebulae, but this term is not used much these days.

[Okamura Sadanori]

Nebula Classification

Diffuse or gas nebulae include reflection nebulae and emission nebulae. Reflection nebulae are lit by dust particles mixed with the gas reflecting and scattering light from nearby stars. Emission nebulae are also called H II regions (or hydrogen ionized regions) and are lit by hydrogen gas heated by ultraviolet light emitted from hot stars. Dense areas of low temperature gas appear darker (blacker) than the surroundings in the sky because the dense dust mixed with the gas blocks the light coming from behind. These are called dark nebulae. Reflection nebulae, emission nebulae (H II regions), and dark nebulae are names given to interstellar matter that appears differently in visible light depending on the distribution of the interstellar matter and its relationship to the stars that illuminate it, so in star-forming regions, they often coexist in a complex intertwined manner.

Planetary nebulae are gases ejected from low-mass stars in the final stage of their evolution, and glow when they are illuminated by ultraviolet light from the hot star at the center. Gases ejected in supernova explosions that occur at the end of the evolution of massive stars are called supernova remnants.

For a more detailed description of each nebula, please refer to the respective article.

[Okamura Sadanori]

Veil Nebula
A diffuse nebula in Cygnus. It is thought to be the remains of a supernova explosion that occurred about 10,000 years ago. The belt-like NGC6992 (left) and NGC6960 (right) and nearby nebulae form a huge loop with an actual diameter of several tens of light years. Photographed by the Ultraviolet Space Telescope © NASA / JPL-Caltech ">

Veil Nebula

Dumbbell Nebula
A planetary nebula in the constellation Vulpecula. NGC6853 (M27). The central part is constricted, making it look like an iron dumbbell. Infrared image taken by the Spitzer Space Telescope ©NASA/JPL-Caltech/Harvard-Smithsonian CfA ">

Dumbbell Nebula

S-shaped Nebula
A dark nebula in the constellation Ophiuchus. B72. A small S-shape can be seen at the top center of the image. ©ESO/Y.Beletsky ">

S-shaped Nebula

Omega Nebula
A diffuse nebula (emission nebula) in Sagittarius. NGC6618 (M17). It is also called the Swan Nebula. Infrared image taken by the Spitzer Space Telescope ©NASA/JPL-Caltech/University of Wisconsin ">

Omega Nebula

Orion Nebula
A diffuse nebula (emission nebula) in Orion. NGC1976 (M42). It looks like a bird with its wings spread. The bird's head is NGC1982 (M43). ©National Astronomical Observatory of Japan ">

Orion Nebula

Crab Nebula
A supernova remnant in Taurus. NGC1952 (M1). It was born from a supernova explosion in 1054. Numerous filament-like structures can be seen. Photographed by the Hubble Space Telescope © NASA/ESA/JPL/Arizona State Univ. ">

Crab Nebula

California Nebula
A diffuse nebula in the constellation Perseus. NGC1499. Its viewing angle spans 140 minutes. Photographed by the Wide-field Infrared Survey Explorer (WISE) © NASA/JPL-Caltech/UCLA ">

California Nebula

Ring Nebula
A planetary nebula in the constellation Lyra. NGC6720 (M57). It is also called the Ring Nebula. Photographed by the Hubble Space Telescope © NASA/JPL-Caltech/ESA, the Hubble Heritage Team STScI/AURA ">

Ring Nebula

North America Nebula
A diffuse nebula in Cygnus (the large blue area on the left of the photo). NGC7000. An emission nebula near Deneb, the part that corresponds to the Gulf of Mexico is a dark nebula. The pale blue area on the right of the photo is the Pelican Nebula. A composite of infrared and visible light images taken by the Spitzer Space Telescope © NASA/JPL-Caltech ">

North America Nebula

Trifid Nebula
A diffuse nebula in Sagittarius. NGC6514 (M20). A black band (dark nebula) splits the nebula into three parts. Photographed with the Murikabushi Telescope at Ishigakijima Astronomical Observatory © National Astronomical Observatory of Japan ">

Trifid Nebula

Horsehead Nebula
A dark nebula near Zeta, southeast of the three stars of Orion © Gunma Astronomical Observatory

Horsehead Nebula

Rose Nebula
A diffuse nebula in Monoceros. NGC2237-38-39-46. It is an emission nebula shaped like a rose. At its center is the open cluster NGC2244. Photographed by the Wide-field Infrared Survey Explorer (WISE) © NASA/JPL-Caltech/UCLA ">

Rose Nebula

Lagoon Nebula
A diffuse emission nebula in Sagittarius. NGC6523 (M8) ©National Astronomical Observatory of Japan ">

Lagoon Nebula

Owl Nebula
A planetary nebula in the constellation Ursa Major. NGC3587 (M97) ©National Astronomical Observatory of Japan">

Owl Nebula

Spiral Nebula
Planetary nebula in the constellation Aquarius. NGC7293. Composite image taken by the Hubble Space Telescope and Kitt Peak National Observatory's 90 cm telescope ©NASA/STScI ">

Spiral Nebula

Eagle Nebula
NGC6611 (M16), an open cluster in the constellation Serpens, and IC4703, an emissive nebula. The three small protrusions extending from the right side of the picture toward the center are the "Pillars of Creation." Infrared image taken by the Spitzer Space Telescope ©NASA/JPL-Caltech/Institut dAstrophysique Spatiale ">

Eagle Nebula

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

銀河系内にあるガスとダスト（塵(ちり)）からなる星間物質が、さまざまな原因で光って見えたり、逆に光を吸収して暗く見えたりするもの。双眼鏡や小型望遠鏡で見ると、星のような点光源とは違ってぼんやりと広がって見える。歴史的な経緯から、単に「星雲」とよばれてきたが、あいまいさが残るので、現在では、各種の星雲の性質を反映した分類ごとにつけられた名称が用いられることが多い。全体を総称する場合には、ガス星雲ということもある。

［岡村定矩］

観測の歴史

天体望遠鏡の性能が悪かった時代には、はっきり星とわかる点光源以外の、ぼんやりと淡く見える天体をすべて星雲とよんでいた。星の集まりは星団とよばれていたが、実際には星団であってもぼんやりとしか見えなかったものは星雲とされることもあった。このようなぼんやりとした「星雲」は、彗星(すいせい)（ほうき星）と紛らわしいので、当時のコメットハンターには困りものだった。フランスのコメットハンターであったC・メシエは、1771年から1784年にかけて、彗星と紛らわしいこのような天体103個を収録した『メシエ・カタログ』を出版した。このカタログに掲載された天体はメシエ天体とよばれ、カタログ番号の頭に記号Mをつけて、M31などとよばれる。

　この後1856年には、F・W・ハーシェルの息子のJ・F・W・ハーシェルが、約5000個の星雲と星団を含むGeneral Catalogue『一般カタログ』を出版した。ドライヤーはこれを増補改訂して、1888年に7840天体を含むNew General Catalogue of Nebulae and Clusters of Stars『NGCカタログ』を編纂(へんさん)し、さらに、5836天体を含む二つのIndex Catalogue『ICカタログ』を1895年と1905年に出版した。これらのカタログにある天体は、カタログ番号の頭にそれぞれNGCとICをつけて、NGC224やIC10などのようによばれる。

　その後、NGCカタログやICカタログで星雲と分類された天体には、天球上の分布に大きな違いのある二種類のものがあることがわかった。一つは天の川に沿って分布し、もう一つは天の川を避けるように分布していた。性能のよい望遠鏡で見ると、後者の中には渦巻き模様が見られるものがあり、それらは渦巻星雲とよばれるようになった。

　20世紀初頭になっても、宇宙の大きさとその中での太陽の位置はまだよくわかっておらず、渦巻星雲の正体もわかっていなかった。天文学者の考えには大きく二つのものがあった。天の川は多数の恒星の集団であることは共通していたが、一つの考えでは、太陽はその集団の端にあり、渦巻星雲はその集団の中で星がガスから誕生しているようすであるとした。もう一つの考えでは、太陽は集団の中心近くにあり、その外にも同じような規模の恒星の大集団（島宇宙）が多数あって、渦巻星雲はそれらの姿であるとした。

　1920年に、これら二つの考え方の代表者であるシャプリーとカーチスHeber Doust Curtis（1872―1942）が、アメリカ国立科学院でそれぞれの主張を戦わせる公開討論会が行われた。これは後に、The Great Debate（大論争）とよばれるようになった。ここでは決着がつかなかったが、1923年にハッブルが渦巻星雲（M31）にセファイドとよばれる変光星を発見して論争の決着がついた。セファイドが見つかると距離を測ることができ、その結果M31は天の川恒星集団のはるか外にある島宇宙であることがわかった。

　島宇宙は銀河系外星雲とよばれた時期もあるが現在では銀河（galaxy）とよばれている。天の川恒星集団も一つの銀河であるが、われわれの住む銀河ということで、銀河系とよんで区別している。これに対して、天の川に沿って分布する星雲は、銀河系内の星間物質が光るものである。これらは銀河系外星雲に対して銀河系内星雲とよばれた時期もあるが、現在ではあまり使われない。

［岡村定矩］

星雲の分類

散光星雲あるいはガス星雲とよばれるものには、反射星雲と発光星雲がある。反射星雲は、ガスに混じっているダストが近くの星からの光を反射・散乱して光っているものである。発光星雲はHⅡ領域（あるいは電離水素領域）ともよばれ、水素ガスが高温度星から放たれる紫外線で熱せられて発光しているものである。低温度のガスが濃く集まっているところは、ガスに混じっている濃いダストが背後から来る光を遮るので、空の上で周辺より暗く（黒く）見える。これは暗黒星雲とよばれる。反射星雲、発光星雲（HⅡ領域）、および暗黒星雲は、星間物質の分布状態とそれを照らす星との関係によって、星間物質が可視光で異なった見え方をするものに対してつけられた名前であるので、星生成領域ではそれらが複雑に絡み合って共存しているようすがよく見られる。

　惑星状星雲は、低質量星の進化の最終段階で星から噴き出されたガスが中心の高温度星からの紫外線に照らされて光るものである。また、大質量星の進化の最後に起こる超新星爆発で吹き飛ばされたガスが光っているものは超新星残骸とよばれる。

　それぞれの星雲についてのより詳しい説明は、当該項目を参照されたい。

［岡村定矩］