“galaxy”的概念、定义、翻译、参考文献-科学参考

galaxy

Physics

A vast collection of stars, dust, and gas held together by the gravitational attraction between its components. Galaxies are usually classified as elliptical, spiral, or irregular in shape. Elliptical galaxies appear like ellipsoidal clouds of stars, with very little internal structure apart from (in some cases) a denser nucleus. Spiral galaxies are flat disk-shaped collections of stars with prominent spiral arms. Irregular galaxies have no apparent structure or shape.

The sun belongs to a spiral galaxy known as the Galaxy (with a capital G) or the Milky Way System. There are some 10¹¹ stars in the system, which is about 30 000 parsecs across with a maximum thickness at the centre of about 4000 parsecs. The sun is about 10 000 parsecs from the centre of the Galaxy. The galaxies are separated from each other by enormous distances, the nearest large galaxy to our own (the Andromeda galaxy) being about 6.7×10⁵ parsecs away.

It is thought that there is a very massive black hole at the centre of each galaxy. It is not known whether the black hole or the galaxy came first, but it appears that black holes were present in the early evolution of galaxies.

Astronomy

A system of stars, often with interstellar gas and dust, bound together by gravity. Galaxies are the principal visible structures of the Universe. They range from dwarfs with less than one million stars to supergiants with over a million million stars, and in diameter from a few hundred to over 600 000 l.y. Galaxies may be isolated, or in small groups such as our Local Group, or in large clusters such as the Virgo Cluster.
Galaxies are usually classified according to appearance (see Hubble classification). They come in two main shapes: spiral (with arms), and elliptical (without arms). Elliptical galaxies (designated E) have a smooth, centrally concentrated distribution of stars, and very little interstellar gas or dust. Spiral galaxies are subdivided into ordinary spirals (S) and barred spirals (SB). Both types have disks containing interstellar material as well as stars. Lenticular galaxies (S0) show a clear disk, but with no spiral arms visible.
Irregular galaxies (Irr) have a rather amorphous, irregular structure, sometimes with evidence of a spiral arm or bar. A few galaxies look unlike any of the main types, and may be classified as peculiar (p or pec). Many of these are probably the results of the interaction or merger of pairs or small groups of galaxies. The most numerous type of galaxy may be the small, relatively faint dwarf spheroidal galaxies (dSph) which are approximately elliptical in shape.
Galaxies are believed to have formed by gravitational accumulation of dark matter and gas, some time after the recombination epoch. Gas clouds would have begun to form stars, perhaps as a result of mutual collisions. The type of galaxy created may have depended on the rate at which the gas was turned into stars, ellipticals forming where the gas turned rapidly into stars, and spirals forming if star formation was slow enough to allow a significant disk of gas to grow. Galaxies evolve as they progressively convert their remaining gas into stars. Simulations suggest that galaxies can change their morphology over time as a result of major and minor mergers and the inflow of cold gas from the intergalactic medium.
The relative numbers of galaxies of different types is closely related to their intrinsic brightness and the type of group or cluster to which they belong. In dense clusters, with hundreds to thousands of galaxies, a high proportion of the bright galaxies are ellipticals and lenticulars, with few spirals (5–10%). However, the proportion of spirals may have been higher in the past, spirals either having been stripped of gas so that they now resemble lenticulars, or undergone mergers with other spiral and irregular galaxies to become ellipticals. Outside clusters, most galaxies belong to groups containing from a few to several dozen members, and isolated galaxies are rare. Spirals account for 80% of bright galaxies in these low-density environments, with a correspondingly small proportion of ellipticals and lenticulars. At low luminosities there are no spirals: only irregulars, dwarf ellipticals, and dwarf spheroidals.
Some galaxies display unusual central activity, such as Seyfert galaxies or N galaxies. A radio galaxy is an unusually strong emitter of radio energy.

Space Exploration

A grouping of millions or billions of stars, held together by gravity. It is believed that there are billions of galaxies in the Universe. There are different types, including spiral, barred spiral, and elliptical galaxies. Our own galaxy, the Milky Way, is about 100 000 light years across (a light year is the distance light travels in a year, about 9.5 billion km), and contains at least 100 billion stars.
The galaxies are moving away from our own Galaxy in all directions. The Universe is thus expanding in all directions. The evidence for this comes from examining light from the galaxies by splitting the light into a spectrum. A feature known as the redshift appears, where the light is shifted towards the red end of the spectrum due to an increase in wavelength, owing to the source's velocity.
Spiral galaxies, such as the Milky Way, are flattened in shape, with a central bulge of old stars surrounded by a disc of younger stars, arranged in spiral arms like a Catherine wheel.
Barred spirals are spiral galaxies that have a straight bar of stars across their centre, from the ends of which the spiral arms emerge. The arms of spiral galaxies contain gas and dust from which new stars are still forming.
Elliptical galaxies contain old stars and very little gas. They include the most massive galaxies known, containing a trillion stars.
The Milky Way is a member of a small cluster, the Local Group. The Sun lies in one of its spiral arms, about 25 000 light years from the centre.

单词	galaxy
释义	galaxy Physics A vast collection of stars, dust, and gas held together by the gravitational attraction between its components. Galaxies are usually classified as elliptical, spiral, or irregular in shape. Elliptical galaxies appear like ellipsoidal clouds of stars, with very little internal structure apart from (in some cases) a denser nucleus. Spiral galaxies are flat disk-shaped collections of stars with prominent spiral arms. Irregular galaxies have no apparent structure or shape. The sun belongs to a spiral galaxy known as the Galaxy (with a capital G) or the Milky Way System. There are some 10¹¹ stars in the system, which is about 30 000 parsecs across with a maximum thickness at the centre of about 4000 parsecs. The sun is about 10 000 parsecs from the centre of the Galaxy. The galaxies are separated from each other by enormous distances, the nearest large galaxy to our own (the Andromeda galaxy) being about 6.7×10⁵ parsecs away. It is thought that there is a very massive black hole at the centre of each galaxy. It is not known whether the black hole or the galaxy came first, but it appears that black holes were present in the early evolution of galaxies. Astronomy A system of stars, often with interstellar gas and dust, bound together by gravity. Galaxies are the principal visible structures of the Universe. They range from dwarfs with less than one million stars to supergiants with over a million million stars, and in diameter from a few hundred to over 600 000 l.y. Galaxies may be isolated, or in small groups such as our Local Group, or in large clusters such as the Virgo Cluster. Galaxies are usually classified according to appearance (see Hubble classification). They come in two main shapes: spiral (with arms), and elliptical (without arms). Elliptical galaxies (designated E) have a smooth, centrally concentrated distribution of stars, and very little interstellar gas or dust. Spiral galaxies are subdivided into ordinary spirals (S) and barred spirals (SB). Both types have disks containing interstellar material as well as stars. Lenticular galaxies (S0) show a clear disk, but with no spiral arms visible. Irregular galaxies (Irr) have a rather amorphous, irregular structure, sometimes with evidence of a spiral arm or bar. A few galaxies look unlike any of the main types, and may be classified as peculiar (p or pec). Many of these are probably the results of the interaction or merger of pairs or small groups of galaxies. The most numerous type of galaxy may be the small, relatively faint dwarf spheroidal galaxies (dSph) which are approximately elliptical in shape. Galaxies are believed to have formed by gravitational accumulation of dark matter and gas, some time after the recombination epoch. Gas clouds would have begun to form stars, perhaps as a result of mutual collisions. The type of galaxy created may have depended on the rate at which the gas was turned into stars, ellipticals forming where the gas turned rapidly into stars, and spirals forming if star formation was slow enough to allow a significant disk of gas to grow. Galaxies evolve as they progressively convert their remaining gas into stars. Simulations suggest that galaxies can change their morphology over time as a result of major and minor mergers and the inflow of cold gas from the intergalactic medium. The relative numbers of galaxies of different types is closely related to their intrinsic brightness and the type of group or cluster to which they belong. In dense clusters, with hundreds to thousands of galaxies, a high proportion of the bright galaxies are ellipticals and lenticulars, with few spirals (5–10%). However, the proportion of spirals may have been higher in the past, spirals either having been stripped of gas so that they now resemble lenticulars, or undergone mergers with other spiral and irregular galaxies to become ellipticals. Outside clusters, most galaxies belong to groups containing from a few to several dozen members, and isolated galaxies are rare. Spirals account for 80% of bright galaxies in these low-density environments, with a correspondingly small proportion of ellipticals and lenticulars. At low luminosities there are no spirals: only irregulars, dwarf ellipticals, and dwarf spheroidals. Some galaxies display unusual central activity, such as Seyfert galaxies or N galaxies. A radio galaxy is an unusually strong emitter of radio energy. Space Exploration A grouping of millions or billions of stars, held together by gravity. It is believed that there are billions of galaxies in the Universe. There are different types, including spiral, barred spiral, and elliptical galaxies. Our own galaxy, the Milky Way, is about 100 000 light years across (a light year is the distance light travels in a year, about 9.5 billion km), and contains at least 100 billion stars. The galaxies are moving away from our own Galaxy in all directions. The Universe is thus expanding in all directions. The evidence for this comes from examining light from the galaxies by splitting the light into a spectrum. A feature known as the redshift appears, where the light is shifted towards the red end of the spectrum due to an increase in wavelength, owing to the source's velocity. Spiral galaxies, such as the Milky Way, are flattened in shape, with a central bulge of old stars surrounded by a disc of younger stars, arranged in spiral arms like a Catherine wheel. Barred spirals are spiral galaxies that have a straight bar of stars across their centre, from the ends of which the spiral arms emerge. The arms of spiral galaxies contain gas and dust from which new stars are still forming. Elliptical galaxies contain old stars and very little gas. They include the most massive galaxies known, containing a trillion stars. The Milky Way is a member of a small cluster, the Local Group. The Sun lies in one of its spiral arms, about 25 000 light years from the centre.
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