Our Milky Way galaxy is a barred spiral galaxy, a flattened disk of stars, gas and dust about 100,000 light-years in diameter containing several hundred billion stars. About 0.5% of them are red giants, stars that have consumed most of their hydrogen fuel and whose outer layers have swollen and cooled. Although they are rare, red giants are large and luminous, as much as 10,000 times brighter than their smaller and more numerous stellar cousins. Our sun will evolve into a red giant star in another eight billion years or so, when its radius will have expanded to beyond the current orbit of Mercury.
The galactic disk is surrounded by a large spherical, diffuse halo roughly about 500,000 light-years in diameter (although its ill-defined edges could extend to much larger distances) and also contains red giant stars. Because red giant stars are big and luminous, they can be spotted at greater distances than many other stars, and so offer an important tool in the study of the remote regions of the galactic halo. CfA astronomers Nelson Caldwell and Warren Brown and five colleagues have just announced the discovery of the most distant known star in the Milky Way, the red giant ULAS J001535.72+015549.6 located about 900,000 light-years away. For comparison, the Large Magellanic Cloud, a small neighbor galaxy to the Milky Way, is five times closer. Before the discovery of this star (and a second slightly less distant red giant in the study) the seven most distant stars known were about 500,000 light-years away.
Where did it come from? There are currently three theories. The first is that it is not in fact a part of the Milky Way but rather a member of a very faint, undetected dwarf galaxy nearby. The second is that it was was ejected from the Milky Way disk, perhaps when a binary star system or a black hole-star system was disrupted. The third hypothesis is that it was stripped away from an as yet-undetected nearby dwarf galaxy. The astronomers used the MMT telescope on Mt. Hopkins to obtain spectra of the star to try to solve this mystery, but the results so far are ambiguous. Further observations are planned, but meanwhile the discovery enables astronomers to test theories of the formation and development of the Milky Way, its small neighboring galaxies, and the galactic environment.
"The Most Distant Stars in the Milky Way," John J. Bochanski, Beth Willman, Nelson Caldwell, Robyn Sanderson, Andrew A. West, Jay Strader, and Warren Brown, ApJ Letters 790, L5, 2014.