The Milky Way contains a population of a few hundred billion stars. Like any galaxy, the Milky Way does not have a well defined “edge”. As one moves away from the galaxy, the number of stars per given volume of space simply becomes ever so vanishingly small. Bochanski et al. (2014) report on the discovery of two of the most distant Milky Way stars known to date. The two stars are given the identifiers - ULAS J0015+01 and ULAS J0744+25. Both stars were bright enough to be detectable because they have entered their final stages of stellar evolution and have swelled into red giants, resulting in a considerable increase in their luminosities.
ULAS J0015+01 and ULAS J0744+25 have large estimated distances of 274 ± 74 kpc and 238 ± 64 kpc, respectively, making them the first two Milky Way stars found beyond 200 kpc. For comparison, the disk of the Milky Way, which contains the majority of the galaxy’s stars, is approximately 30 kpc, or 100,000 light-years in diameter. ULAS J0015+01 and ULAS J0744+25 are also moving away from the Milky Way’s centre at 52 ± 10 km/s and 24 ± 10 km/s, respectively.
A number of possible scenarios have been considered to explain the existence of ULAS J0015+01 and ULAS J0744+25. Both stars could not have formed in-situ because the gas density at their location is far too low for star formation. The most likely explanation for the origin of these stars is they are part of a population of stars that have been tidally-stripped from passing dwarf galaxies by the Milky Way’s gravity. Alternative scenarios include hypervelocity ejection (~600 km/s) from the Milky Way, or membership in an undetected dwarf galaxy of extremely low surface brightness.
Bochanski et al. (2014), “The Most Distant Stars in the Milky Way”, arXiv:1407.2610 [astro-ph.SR]