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.
.jpg)
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.
Reference:
Bochanski et al. (2014), “The Most Distant Stars in the
Milky Way”, arXiv:1407.2610 [astro-ph.SR]