The Large Magellanic Cloud is a Bit of a Bully
The Large Magellanic Cloud (LMC) is one of two neighboring galaxies to our own Milky Way, actually visible to the naked eye if you happen to be in the southern hemisphere. And to quote (gratuitously) Dave Bowman in 2001: A Space Odyssey, “My god, it's full of stars!" — including lots of baby stars-in-the-making.
But now it's looking like the LMC may have poached a few of those stars from its smaller sister galaxy, the Small Magellanic Cloud (SMC), according to a new analysis of the spectra of the LMC's giant and supergiant stars by astronomers at the National Optical Astronomy Observatory (NOAO) in Tucson, Arizona, and their collaborators at Maryland's Space Telescope Science Institute.
It's an especially robust conclusion because there are two separate lines of evidence relating to the stars' motion, on the one hand, and their composition on the other.
First, let's tackle the motion question. Most of the stars in the LHC rotate in roughly the same direction, since they all were formed in the same turning cloud of collapsing gas.
But the NOAO astronomers discovered that around 5% of the stars they surveyed had peculiar orbits: they were rotating in a counter direction to the majority of the other LMC stars. This makes it highly unlikely those stars formed in the LMC. They were pulled over from the SMC by the larger galaxy's stronger gravitational field.
Then the astronomers took a closer look at the chemical composition of the various stars being surveyed, and found there were fewer heavy elements present — things like iron and calcium — than other stars elsewhere in the LMC. Rather, the composition was markedly similar to stars typically found in the SMC.
Taken together, the data indicated that at least some of those stars now in the LMC likely originated in the SMC. The next step: combine that analysis of data from ground-based telescopes with infrared data collected by NASA's Spitzer Space Telescope. This gave the astronomers a complete map of all the stars in the LMC (see image, right), as well as a more detailed picture of how galaxies interact and evolve over time.
The data also fits another unusual feature of the LMC: an unexpectedly high rate of new star formation in a region colloquially known as the Tarantula Nebula (a.k.a. the less mellifluous 30 Doradus). The astronomers surmise that this region, in the southwest tip of the LMC, is the point of collision between the LMC and its smaller sibling. Gas from the SMC is being pulled into the bigger galaxy along with the stolen stars, causing a shock wave that pressurizes and concentrates the gas — perfect conditions for making new stars.
So the LMC is basically the schoolyard bully shaking down its smaller sibling to make off with the SMC's lunch money (assuming the cosmic currency is stars).
The LMC also appears to be suffering from a spot of indigestion, in the form of a “superbubble" (see top image): a large glowing shell of gas and dust carved out by the combination of stellar winds and the shock waves created by supernova explosions.
Located in an LMC nebula known as LHA 120-N44, it was spotted by an Argentine amateur astronomer named Manu Mejias, who was participating in a “Hidden Treasures" competition sponsored by the European Southern Observatory (ESO). The objective was to let folks like Mejias pore over the ESO's extensive archives of data in hopes of uncovered something relevant that had been missed — hence the moniker “Hidden Treasures."
Mejias used those datasets to compose an image of the nebula surrounding the NGC 1929 star cluster, showcasing the “superbubble" around it in eye-popping detail. (That data was collected by the ESO's Very Large Telescope.)
While the superbubble might be formed from the death throes of stars, it's all part of the universe's cycle of life. The hot gas around its edges is highly compressed, leading to a new generation of stars.
Image credits: ESO