The Andromeda Galaxy is bigger than previously thought. Perhaps as much as five times bigger. I know that you’re thinking – “Oh, that’s because most of it is dark matter, right?” No, this is not another dark matter story. In fact, there are many newly discovered stars from a recent study! The suburbs of the galaxy are much more extensive than previously identified. It’s rather good news, since the galaxy makes more sense than it did before, in the context of our understanding of how galaxies form and evolve. (Image above is of Andromeda in the infra-red, from NASA.)
The point here is that current theories of evolution for galaxies have the oldest stars out in the suburbs (or “halo” as is said in the trade) – not in the central bright bulge you’re familiar with from the typical galaxy shape. They form first, and so get oldest first. So in a typical galaxy of this sort (such as ours, the Milky Way, and Andromeda, one of our neighbours) you expect to find a certain amount of those older stars in the outer parts of the galaxy, containing a particular proportion of heavy metals. It was apparently always a bit of an anomaly for Andromeda. It did not seem to have as many of these sorts of stars in the right location. So a new close study of the galaxy was conducted to have a look at what was going on, and they found lots of red giant stars (i.e., old stars) that had not been taken into account before, with the right distribution of heavy metals that you would expect from galaxy formation theory. You can read more about this on the BBC site, and also at a site at UCSC here, from which I quote:
…collaborators at UCSC, UCLA, and the University of Virginia are conducting an ongoing study of Andromeda’s stellar halo, using observations at the Kitt Peak National Observatory in Arizona and the W. M. Keck Observatory in Hawaii. Their new findings are based on data gathered using the 4-meter Mayall Telescope at Kitt Peak and the DEIMOS spectrograph on the 10-meter Keck II Telescope in Hawaii.
The researchers detected a sparse population of red giant stars–bright, bloated stars in a late stage of stellar evolution–that appear to be smoothly distributed around the galaxy out to a distance of at least 500,000 light-years from the center. Even at that great distance, the stars are bound to the galaxy by gravity. These stars probably represent Andromeda’s stellar halo, a distinct structural component of the galaxy that has eluded astronomers for over 20 years…