The stars of the Phoenix Stream moving around the Milky Way turned out to be extremely poor in heavy elements – apparently, they represent the remains of one of the most ancient globular clusters destroyed by our Galaxy.
Our home Milky Way is a fairly large and well-structured Galaxy. Like other such objects, narrow and long streams of stars swirl around its disk – associations of coherently moving stars that were once a dwarf galaxy or a cluster destroyed by the gravity of the Milky Way itself.
About two dozen such “currents” are known around the Galaxy, including the Phoenix Stream, discovered several years ago. Visible in the southern sky, it spans 8,000 light-years and is composed of old stars. The authors of a new article, published in the journal Nature, managed to find out the origin of the Phoenix Stream and restore the picture of the death of the globular cluster from which it was formed.
The ancient globular cluster came close to the Milky Way about two billion years ago and began to collapse under the influence of tidal forces. Astronomers have found that the stars of this stream are distinguished by an unusually low metallicity – the content of elements heavier than hydrogen and helium. Apparently, they appeared even when there were much fewer such elements in the Universe.
Indeed, all heavy elements were formed in the course of thermonuclear reactions in the interiors of stars. Replacing each other generation after generation, they filled space with material from which new stars were formed, and the content of “metals” in them increased each time. However, the metallicity of the Phoenix Stream stars was found to be only 0.3-0.4 percent of that of the Sun; until now, scientists have not seen such a cluster.
Apparently, the forerunner of the Phoenix Stream was a highly unusual and ancient globular cluster, unlike the predecessors of other stellar streams in the Milky Way. It is assumed that it was formed extremely long ago and, by some accident, managed to survive until relatively recently, providing astronomers with a unique opportunity to study the remains of the oldest clusters of the Universe.