The most distant stars in our galaxy

The Milky Way, the Sombrero (shown here), and other spiral galaxies are enveloped by huge spherical halos of dark matter, low-density gas, and sparsely distributed stars. John Bochanski of Haverford College and his collaborators are searching for our galaxy’s outermost halo stars, whose properties carry clues about the galaxy’s past. The search focuses on red giants. Besides being luminous—and therefore detectable at large distances—red giants have IR spectra that set them apart in photometric surveys from other stars except red dwarfs. But red dwarfs, being less luminous, are found at shorter distances; their apparent motions across the sky tend to be quicker than those of distant red giants. From two photometric surveys and one kinematic survey, Bochanski’s team has identified 404 red giant candidates. Fifteen were subjected to the first round of spectroscopic observations. One candidate was unidentifiable, nine were red dwarfs, and three were not-too-distant red giants. But two candidates turned out to be red giants whose distances, 238 kiloparsecs (8.09 × 10⁵ light-years) and 274 kiloparsecs (8.94 × 10⁵ light-years), are roughly seven times larger than the diameter of our galaxy’s disk. At those record-breaking distances, the ambient gas is too rare to feed star formation; the two stars were born elsewhere. Two origins are plausible: The stars could be leftovers from a satellite galaxy that the Milky Way consumed, or they could have been flung into the halo from our galaxy’s disk by a violent event. (J. J. Bochanski et al., Astrophys. J. Lett. 790, L5, 2014, doi:10.1088/2041-8205/790/1/L5 .)