Backwards stars are the last refugees of destroyed galaxies

The universe can be a brutal place, as entire galaxies are ripped apart in cataclysmic collisions. No traces of the old galaxies are left behind, except for a few out-of-place stars, stubbornly orbiting the wrong way in their new galaxy.

All stars orbit around the center of their galaxy, and normally all of them should orbit in the same direction. But astronomers have noticed that in certain galaxies some of the stars near galactic center are orbiting in the opposite direction of those further out. An obvious way to explain this is that these stars are the remnants of a much smaller galaxy gobbled up long ago by the surviving galaxy, but there had been no proof for this either way.

Now researchers at Spain's La Laguna University think they've found just the evidence they need to support this idea. They examined the menagerie of stars that make up NGC 1700, an elliptical galaxy with a core that rotates in the wrong direction relative to the rest of it. Their observations revealed that the stars in the core are significantly younger than those further out, which makes no sense if all the stars formed in the same galaxy, but can easily be explained by the galactic cannibalism theory.

There's also another clue. In elliptical galaxies with "normal" cores, the central stars should be high in the heavy elements. The core stars of NGC 1700 have a tiny fraction of the heavy elements they should, which is more proof that they formed in some other position in some other galaxy, only to wind up in this strange new place after a gigantic cataclysm that boggles the imagination.

Via New Scientist. Image via.