Chinese and Japanese astronomers have found a star in the Milky Way that appears to be different from all others found in the galaxy. It is so peculiar that they believe it didn’t form there at all and instead was probably stolen from one of the companions of the Milky Way.
The star, known as J112456.61+453531.3 (J1124+4535 for short), was spotted by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey for its unusual chemistry. Preliminary analysis showed that the star has a low abundance of certain elements such as magnesium, which is uncommon for stars in the Milky Way.
The team performed follow-up observations on the object using the High Dispersion Spectrograph on the Subaru Telescope. By looking at the light from the star in detail, researchers can figure out its chemical composition. The team was able to confirm that this star is indeed low in magnesium, but it is also surprisingly high in europium, one of the least abundant elements in the universe. It is so rare that for every metric ton of matter in the universe, there are only 500 micrograms of europium.
This specific ratio of elements has never been observed before in a star in the Milky Way. It is also rare in cosmic terms – such stars have only been found in dwarf galaxies around the Milky Way, such as the Ursa Minor dwarf and the Sculptor dwarf. The researchers of the study, published in Nature Astronomy, suggest it might have formed in a now-vanished dwarf galaxy cannibalized by the Milky Way a long time ago.
Over the many billions of years that our galaxy has existed, it has absorbed several neighboring galaxies. We can still see some leftovers from an ancient dwarf galaxy in the so-called Sagittarius Stream, a ribbon of stars that wrap around the Milky Way.
This star belongs to the halo of the Milky Way, the diffuse spherical region that surrounds the disk of the galaxy. J1124+4535 comes as close as 11,000 light-years from the core of the Milky Way in its orbit and as far as 42,000 light-years.