Read: The mysterious origin of our galaxy’s gold
Some stars have a signature that’s entirely distinct from their neighbors’, and there are a few of them in our very own galaxy, including one identified recently by a group of scientists based in Japan and another by an international team. The chemical compositions of these stars, their ratios of one element to another—those markers make them unlike any other star in the Milky Way, which is home to some tens of billions.
The stars in the Milky Way have similar chemical makeups because they emerged from the same clouds of gas, infused over time with a range of elements from the stellar explosions we call supernovae. “Stars are formed from gas, and whatever spilled into the gas prior to the formation ends up being in the star,” says Anna Frebel, an astronomer at MIT who has detected and studied one of these rogue stars. “It’s like genes that are being passed on.”
The chemical signatures of the interlopers suggest that they originated in environments without too many stellar explosions. For astronomers, this is a clear indication that the stars flickered on somewhere else.
How does this happen? The Milky Way, like many galaxies, is surrounded by other, smaller galaxies. “Just like the Earth has satellites, artificial and natural—man-made satellites and the moon—our galaxy also has satellites,” says Marion Dierickx, a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics. “These occasionally fall in.”
The Milky Way has little trouble absorbing these galaxies and their contents when gravitational forces draw them near. “Our galaxy was built up over time as smaller galaxies collided and merged with each other,” says Douglas Boubert, a junior research fellow at Magdalen College at Oxford. “The oldest stars we see flying round the Milky Way today were all born in precursor galaxies.”
When galaxies merge, stars are jostled and settle into new orbits. So do planets and moons. The process is so slow, unfolding over millions of years, that any inhabitants of these planets, if they could fathom such things, wouldn’t know about the cosmic merger until millions of years after it happened. “We always think things are static in the cosmos, but they really are not,” Frebel says.
Astronomers have used spectroscopy to detect rogue stars in the satellite galaxies around our own. In 2011, they discovered that the composition of more than 5 percent of the stars inside the Large Magellanic Cloud didn’t match that of its other stellar residents. Those rogues resembled stars in the Small Magellanic Cloud, a nearby galaxy, instead. At some point, the larger cloud had stolen them away.
Astronomers say many more stars of this nature are in the Milky Way, but they are tricky to find. They orbit at the very edges of the galaxy; by the time their light reaches telescopes on Earth, it’s incredibly faint. “You can’t mount, at this point with our technology, a systematic campaign to identify these,” Dierickx says. “You find one candidate, you do thorough follow-up observations, and you come up with a detailed characterization—doing this kind of study for many stars would take a very long time.”