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Earth and Space Sciences · 术语表

什么是 Galactic disc, thick disc, bulge and stellar halo？

又称： Galactic disc · thick disc · Galactic bulge · stellar halo

Definition 29.1 University Earth and Space Sciences — Year 3 · 第 29 章 — The Milky Way and Galaxies

The Galactic disc is the flattened component of the Galaxy, made of stars, gas and dust in nearly circular orbits about its centre; its density falls off roughly exponentially with height above its mid-plane and with distance from its centre, over a scale height and a scale length. The thick disc is a second, older disc component of larger scale height and smaller mass. The Galactic bulge is the central concentration of stars that swells out of the disc. The stellar halo is the sparse, roughly spheroidal component of old stars and globular clusters that surrounds the disc and the bulge.

A sketch of the Galaxy, to scale for its main dimensions. (a) Edge-on: the thin disc (dark band), the thick disc (light band), the box-shaped bulge, and the stellar halo with its globular clusters (orange dots); the Sun lies in the disc, about 8.2\, kpc from the centre and about 25\, pc above the mid-plane (too small to show). (b) Seen from the north Galactic pole: the bar, about 5\, kpc in half-length, tilted by 27 to the line from the Sun to the centre (dotted); the solar circle (dashed); spiral arms drawn schematically; the Galaxy turns clockwise in this view.
A sketch of the Galaxy, to scale for its main dimensions. (a) Edge-on: the thin disc (dark band), the thick disc (light band), the box-shaped bulge, and the stellar halo with its globular clusters (orange dots); the Sun lies in the disc, about 8.2 kpc8.2\,\mathrm{kpc} from the centre and about 25 pc25\,\mathrm{pc} above the mid-plane (too small to show). (b) Seen from the north Galactic pole: the bar, about 5 kpc5\,\mathrm{kpc} in half-length, tilted by 27∘27{}^{\circ} to the line from the Sun to the centre (dotted); the solar circle (dashed); spiral arms drawn schematically; the Galaxy turns clockwise in this view.
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