Flavors of White Dwarfs

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Science  02 Aug 2002:
Vol. 297, Issue 5582, pp. 741
DOI: 10.1126/science.297.5582.741a

Globular clusters—clumps of stars that formed in a single starburst—are among the oldest objects in the universe. Most of the low-mass stars in clusters have gravitationally collapsed into white dwarfs, compact objects that cool and fade with time. The halos of spiral galaxies may contain the bulk of the missing or “dark” matter predicted by stellar velocity anomalies. Halo white dwarfs have been thought to account for some of this dark matter.

Richer et al. and Hansen et al. used 123 orbits of the Hubble Space Telescope to stare at the globular cluster Messier 4, located in the halo of the Milky Way. The reams of long-exposure images identified many white dwarfs and allowed the color-magnitude relations and mass and luminosity functions for the entire population to be derived more precisely. Messier 4 is 12.7 billion years old compared to the Milky Way, which is 7.3 billion years old. The time gap is consistent with the idea that a thick-disk phase of growth of a proto-Milky Way preceded the main normal-disk growth of the Galaxy. Astronomers looking for halo white dwarfs to which to attribute dark matter in our Galaxy thus need to exclude normal-disk and thick-disk white dwarfs … The “white dwarfs are dark matter” plot thickens. — LR

Astrophys. J.574, L151, L155 (2002).

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