The Milky Way's oldest stars reside in its halo, which surrounds the spiral-sculpted disk harboring the sun. Because halo stars formed when the galaxy did, they hold clues to its origin and early evolution. Now, astronomers report in Astronomy & Astrophysics that they have used an interferometer—an array of telescopes that yields crisp views of the heavens—to measure precise diameters of two halo stars so bright you can see them through binoculars. Groombridge 1830 (left inset), a main-sequence star like the sun, lies just 30 light-years from Earth; the new observations reveal that it has a diameter two-thirds that of the sun and a surface temperature of 4820 K, nearly a thousand degrees cooler. The other halo star (right inset), a giant named HD 122563, is 24 times as wide as the sun with a surface temperature of 4600 K. The measurements should help astronomers infer properties of much farther, fainter halo objects and thus gain insight into the ancient Milky Way.
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