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From what I understand of current models, the bulge of the galaxy formed first, and thus, would contain older population II like stars. Currently, however, the halo has a higher population of stars with lower metalicity than the bulge. Is this because the density of stars in the bulge is much higher? My thinking is that this is a densely populated region of massive hot stars that die quickly and create a newer region of next generation population I stars that are metal rich. The inverse would be true for the halo where stars are less densely packed. Is this correct?

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marked as duplicate by Sir Cumference, Hohmannfan, called2voyage May 11 '16 at 20:23

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Your intuition is largely correct: the key is that the proto-bulge region had a deep enough potential well so that the supernovas couldn't expel the remaining gas, and so new stars could form out of the gas (enriched by the supernova ejecta) in a continuing cycle. In the low-mass, isolated protogalactic clouds which probably contributed to the halo, the initial round of supernovas ejected most of the gas (including the original gas that hadn't yet formed stars) -- thus, little opportunity to form more stars (out of higher-metallicity gas) on a continuing basis.

It's really the total mass in a given region that matters. For example, the density of stars in the central regions of a globular cluster is pretty high, but the mass of the cluster as a whole isn't enough to keep most of the gas when its massive stars go supernova.

(Side note: the traditional terms "Population I" and "Population II" aren't used all that much any more, since age and metallicity can vary continuously and aren't always strongly associated.)

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  • $\begingroup$ That makes perfect sense. Are there any articles or readings that you know of that I could read for further understanding? $\endgroup$ – NotSoSN May 8 '16 at 13:53

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