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Compact Cosmos

16 17 The stellar inhabitants of galaxies like ours are divided into two camps,old population II and younger population I stars. Born in the earliest epoch of the galaxy, and estimated to have been around for 15 billion years,the older stars are mainly cool red giants, found either orbiting in the central bulge or grouped together in wandering globular clusters opposite top. With eccentric paths taking them far from the galactic plane, globular clusters hold tens of thousands to several million stars. Held together by gravity, and possibly centred on mediumsized black holes, these mature congregations measure from sixty to three hundred lightyears across. Their leisurely circuits last millions of years,swooping up to 300,000 lightyears out into the halo before plunging back through the disk lower opposite. Meanwhile,the young population I stars are found either alone or mingling in loose open clusters in the gassy,dusty nursery regions of the spiral arms. These stars have a higher heavy element content than their population II cousins, having been seeded from the fusionfactory remains of older stars.With nearcircular orbits about the galactic hub, young stars are happy to remain within the 300 lightyear thickness of the disk. An earlier generation of live fast,die young population III stars may have existed. Fueled solely on hydrogen and helium,their short lives would have ended in huge supernova explosions that flung freshly forged elements across the youthful galaxy. GLOBULARCLUSTERS old stars and young stars
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