“Hubble classification”的概念、定义、翻译、参考文献-科学参考

Hubble classification

Astronomy

A widely used system for classifying galaxies according to their visual appearance, illustrated on the tuning-fork diagram. The sequence is based on three criteria: the relative sizes of the central bulge of stars and the flattened disk; the existence and character of spiral arms; and the resolution of the spiral arms and/or disk into stars and H II regions. The system was originated by E. P. Hubble.
The sequence starts with round elliptical galaxies (E0) showing no disks. Increasing flattening of a galaxy is indicated by a number which is calculated from 10 (a − b)/a, where a and b are the major and minor axes as measured on the sky. No elliptical is known that is flatter than E7. Beyond this a clear disk is apparent in the lenticular or S0 galaxies. The classification then splits into two parallel sequences of disk galaxies showing spiral structure: ordinary spirals, S, and barred spirals, SB. The spiral types are subdivided into Sa, Sb, Sc, Sd (SBa, SBb, SBc, SBd for barred spirals). With each successive subdivision the arms become less tightly wound (but more easily resolvable into stars and H II regions), and the central bulge becomes less dominant. Two types of irregular galaxy are defined. Irr I galaxies show rather amorphous, irregular structure with perhaps a hint of a spiral arm or bar, and can be placed at the far end of the spiral sequence. Irr II galaxies are sufficiently unusual to defy assignment to any of the other types, although this category encompasses only about 2% of bright or moderately bright galaxies in the nearby Universe. The original, erroneous idea that the sequence might be an evolutionary one led to the ellipticals being referred to as early-type galaxies, and the spirals and Irr I irregulars as late-type galaxies.
Colour and amount of interstellar material vary systematically along the Hubble sequence: ellipticals are red and contain little interstellar gas or dust, whereas late spirals and Irr I galaxies are blue, with significant amounts of interstellar material. The relatively faint dwarf spheroidal galaxies (see dwarf galaxy) were not recognized as a separate type in the Hubble classification. Some variants of the Hubble classification use plus and minus signs to subdivide classes, so that Sa⁺ is later than Sa, but earlier than Sb⁻.

Space Exploration

A scheme for classifying galaxies according to their shapes, originally devised by US astronomer Edwin Hubble in the 1920s.
Elliptical galaxies are classed from type E0 to type E7, where the figure denotes the degree of ellipticity. An E0 galaxy appears circular to an observer, while an E7 is highly elliptical (this is based on the apparent shape; the true shape, distorted by foreshortening, may be quite different).
Spiral galaxies are classed as type Sa, Sb, or Sc: Sa is a tightly wound spiral with a large central bulge, Sc is loosely wound with a small bulge, and Sb is in between. Intermediate types are denoted by Sab or Sbc.
Barred spiral galaxies, which have a prominent bar across their centres, are similarly classed as type SBa, SBb, or SBc, with intermediates SBab or SBbc.
Lenticular galaxies, which have no spiral arms, are classed as type S0.
Irregular galaxies, type Irr, can be subdivided into Irr I, which resemble poorly formed spirals, and Irr II which cannot be classified because of disturbance.
The Hubble classification was once believed to reveal an evolutionary sequence (from ellipticals to spirals) but this is now known not to be the case. Our own Milky Way is a spiral galaxy, classified as type Sb or Sc, but may have a bar.

单词	Hubble classification
释义	Hubble classification Astronomy A widely used system for classifying galaxies according to their visual appearance, illustrated on the tuning-fork diagram. The sequence is based on three criteria: the relative sizes of the central bulge of stars and the flattened disk; the existence and character of spiral arms; and the resolution of the spiral arms and/or disk into stars and H II regions. The system was originated by E. P. Hubble. The sequence starts with round elliptical galaxies (E0) showing no disks. Increasing flattening of a galaxy is indicated by a number which is calculated from 10 (a − b)/a, where a and b are the major and minor axes as measured on the sky. No elliptical is known that is flatter than E7. Beyond this a clear disk is apparent in the lenticular or S0 galaxies. The classification then splits into two parallel sequences of disk galaxies showing spiral structure: ordinary spirals, S, and barred spirals, SB. The spiral types are subdivided into Sa, Sb, Sc, Sd (SBa, SBb, SBc, SBd for barred spirals). With each successive subdivision the arms become less tightly wound (but more easily resolvable into stars and H II regions), and the central bulge becomes less dominant. Two types of irregular galaxy are defined. Irr I galaxies show rather amorphous, irregular structure with perhaps a hint of a spiral arm or bar, and can be placed at the far end of the spiral sequence. Irr II galaxies are sufficiently unusual to defy assignment to any of the other types, although this category encompasses only about 2% of bright or moderately bright galaxies in the nearby Universe. The original, erroneous idea that the sequence might be an evolutionary one led to the ellipticals being referred to as early-type galaxies, and the spirals and Irr I irregulars as late-type galaxies. Colour and amount of interstellar material vary systematically along the Hubble sequence: ellipticals are red and contain little interstellar gas or dust, whereas late spirals and Irr I galaxies are blue, with significant amounts of interstellar material. The relatively faint dwarf spheroidal galaxies (see dwarf galaxy) were not recognized as a separate type in the Hubble classification. Some variants of the Hubble classification use plus and minus signs to subdivide classes, so that Sa⁺ is later than Sa, but earlier than Sb⁻. Space Exploration A scheme for classifying galaxies according to their shapes, originally devised by US astronomer Edwin Hubble in the 1920s. Elliptical galaxies are classed from type E0 to type E7, where the figure denotes the degree of ellipticity. An E0 galaxy appears circular to an observer, while an E7 is highly elliptical (this is based on the apparent shape; the true shape, distorted by foreshortening, may be quite different). Spiral galaxies are classed as type Sa, Sb, or Sc: Sa is a tightly wound spiral with a large central bulge, Sc is loosely wound with a small bulge, and Sb is in between. Intermediate types are denoted by Sab or Sbc. Barred spiral galaxies, which have a prominent bar across their centres, are similarly classed as type SBa, SBb, or SBc, with intermediates SBab or SBbc. Lenticular galaxies, which have no spiral arms, are classed as type S0. Irregular galaxies, type Irr, can be subdivided into Irr I, which resemble poorly formed spirals, and Irr II which cannot be classified because of disturbance. The Hubble classification was once believed to reveal an evolutionary sequence (from ellipticals to spirals) but this is now known not to be the case. Our own Milky Way is a spiral galaxy, classified as type Sb or Sc, but may have a bar.
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