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Earth and Space Sciences · 术语表

什么是 Hubble sequence; elliptical, lenticular, spiral and irregular galaxies？

又称： Hubble sequence · elliptical galaxy · spiral galaxy · lenticular galaxy · irregular galaxy

Definition 29.11 University Earth and Space Sciences — Year 3 · 第 29 章 — The Milky Way and Galaxies

The Hubble sequence classifies galaxies by their appearance. An elliptical galaxy (type E) is a smooth system of stars, with brightness falling steadily from the centre and no disc; it is labelled E $n$ , with $n = 10(1 - b/a)$ and $b/a$ the apparent axis ratio, from E0 (round) to E7. A spiral galaxy has a disc with spiral arms around a central bulge; it is labelled Sa, Sb, Sc in order of a decreasing bulge, more open arms and arms more broken into star-forming knots, and SBa, SBb, SBc if a bar crosses its centre. A lenticular galaxy (S0) has a disc and a bulge but no arms. An irregular galaxy (Irr, Im) has no organized structure.

The Hubble sequence drawn as a tuning fork, schematically. Ellipticals are labelled by their apparent flattening; the fork branches at the lenticulars into ordinary and barred spirals, ordered from large bulges and tightly wound arms (a) to small bulges and open arms (c); irregulars close the sequence.

The spiral galaxy M101 seen face-on by the Hubble Space Telescope, a mosaic of 51 exposures, cropped at its edges: a small, bright, yellowish bulge; open arms, bluer and knotted with clusters of young stars; brown lanes of dust along the arms. Image ESA and NASA (Hubble, heic0602), CC BY 4.0.

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