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Earth and Space Sciences · 术语表

什么是 Molecular cloud, dense core？

又称： molecular cloud · dense core

Definition 1.2 University Earth and Space Sciences — Year 2 · 第 1 章 — The Interstellar Medium and Star Formation

A molecular cloud is a region of the interstellar medium dense and opaque enough for its hydrogen to be mostly in molecules, $\ce{H2}$ , shielded by dust from the ultraviolet light of the stars that would break them; its temperature is about $10\,\mathrm{K}$ to $20\,\mathrm{K}$ . A dense core is a compact condensation inside a molecular cloud, a few tenths of a parsec across or less, with densities of $10^4$ to $10^5$ particles per cubic centimetre, from which a single star or a small group of stars forms.

	dark clouds	clumps	dense cores
mass (solar masses)	$10^3$ – $10^4$	50–500	0.5–5
size (pc)	2–15	0.3–3	0.03–0.2
mean density ( $\mathrm{cm}^{-3}$ )	50–500	$10^3$ – $10^4$	$10^4$ – $10^5$
gas temperature (K)	about 10	10–20	8–12
examples	Taurus, Musca	—	L1544, B68

Table 1.2. The hierarchy of a dark molecular cloud. Densities count all particles. Data: Bergin and Tafalla (2007).

The “Pillars of Creation” in the Eagle Nebula, imaged by the Hubble Space Telescope in visible light. The columns are cold, dense molecular gas and dust, light-years long, eroded and lit from above by the ultraviolet light of massive young stars outside the picture; the dust makes them opaque. Small condensations at their tips are forming stars. NASA, ESA and the Hubble Heritage Team (STScI/AURA), public domain.

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