Astronomy 2021A/B Lecture Notes - Lecture 13: Milky Way, Orbital Speed, Galactic Habitable Zone
4 Oct 2017
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Chapter 13: extrasolar planets: their nature and potential habitability. All stars born from gravitational collapse of interstellar clouds of gas and dust. The composition of stars at birth reflect the composition of interstellar clouds. These clouds have approximately the same composition throughout the universe: Three-quarters hydrogen and one-quarter helium (by mass) Other elements represent only a very small portion of the total mass. Early universe was made of 75% hydrogen and 25% helium. All other ele(cid:373)e(cid:374)ts are (cid:862)star stuff(cid:863) that (cid:449)as produ(cid:272)ed later (cid:271)(cid:455) stars. Very old stars born when the universe was much younger. Means there had been few prior generations of stars to produce elements besides hydrogen and helium. These stars were born with very small proportions of these other elements. More recent stars were born from gas clouds that had as much as 2% of their mass in. Less than 0. 1% of their mass the form of elements heavier than helium.
