Ursa Major
© Universal History Archive/UIG via Getty Images
An eighteenth century engraving of Ursa Major. Somewhere within the bear lurks a star from outside the galaxy.
Researchers have identified a star in the Milky Way they believe originated from outside the galaxy.

In a paper published in the journal Nature Astronomy, a team led by Qian-Fan Xing from the National Astronomical Observatories at the Chinese Academy of Sciences in Beijing, China, describes a star dubbed J1124+4535, located in the Milky Way constellation known as Ursa Major, or the Big Dipper.

Interest in the star began following observations made through China's Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST), which indicated that the star had an unusually low abundance of magnesium.

Seeking more detail, the researchers switched to the High Dispersion Spectrograph (HDS) on the Subaru Telescope in Hawaii.

The second set of observations confirmed the paucity of magnesium, but also revealed comparatively high levels of another element, europium.

The ratio between the two elements had never previously been found in a Milky Way star, which through up questions about J1124+4535's origins.

Stars form from massive clouds of interstellar gas. It follows, then, that stars in any given neighbourhood, having formed from the same cloud, will have very similar chemical compositions. Thus, J1124+4535 must have a different origin.

Qian-Fan and colleagues note that stars with similarly anomalous element ratios have been observed in dwarf galaxies near the Milky Way.

They suggest that their target star is evidence that at some stage in the distant past such a dwarf galaxy merged with the Milky Way.

The chemical composition of the star, they add, indicates that the smaller galaxy was already fully mature when it met its effective end.