The outer reaches of the Milky Way galaxy contain the oldest known stars. Nonetheless, the birthplace of these stars, and the time when they became members of the Milky Way, remain the subject of controversy. The simplest view is that the stars formed more or less where they are now, and are the long-lived fossil remnants of the earliest stage of galaxy formation. On the other hand, the most successful models of structure formation in the Universe, those involving ‘cold dark matter’ (CDM), have stars forming in small dwarf galaxies, which are only much later accreted into larger galaxies — old stars in a new home. Yet, although CDM models are rather successful on large scales, they are poor at describing individual galaxies. The models should work best in the outer parts of galaxies, where one expects the fragile, dwarf galaxies predicted by CDM models to have left behind some of their stars as they are tidally devoured by the Milky Way over many billions of years. A study by Alex Stephens1 in the Astronomical Journal looks at the relative abundances of chemical elements in stars that are currently near the Sun, but which orbit far into the outer Galaxy. This work has greatly improved the available evidence, but at the same time created more problems for CDM models.
High-mass stars live short lives, during which they create new chemical elements. Stars less massive than the Sun live for longer, and retain in their atmospheres the original distribution of chemical elements in the gas from which they formed. Modern spectroscopic analyses of low-mass stars with a range of ages can therefore tell us the evolutionary history of these chemical elements. Stars outside the disk of the Milky Way, but within an area known as the Galactic halo, are some of the oldest, and are thought to be part of the original Milky Way, before much of it collapsed to a disk. Others argue that these halo stars may have been captured from satellite galaxies, but the evidence is inconclusive.
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