Since the limited amount of gas stored after the first large-scale process of star birth will be quickly crowded out over several million years, star clusters have long been considered “barren”. Only the processes of collision or fusion can cause the “rejuvenation” of ancient stars.
These stars look younger than their neighbors and are referred to as blue stragglers, because they still seem hot and young. With the help of the Hubble Space Telescope, scientists from the National Astronomical Observatory of the Chinese Academy of Sciences found an unexpected population of blue straggled stars in the young globular cluster NGC 2173. This is surprising, because the stars in this cluster were formed simultaneously in a specific burst.
Astronomers study the stellar ages with the help of a general diagram linking the stellar brightness with the surface temperature of an object. Blue stars, formed by chance, would create a glow in the diagram. But, if they appeared at the same time, they would demonstrate a condensed sequence. In NGC 2173, two different sequences of blue distant stars could be fixed to the diagnostic diagram.
Each dot represents a star: black - regular, blue and red - stars with this color. The blue solid and dashed lines are theoretical predictions of the model. The left sequence is represented by blue retired stars.
This is not the first case of such a detection, but earlier signs were found only in ancient globular clusters, whose age exceeded 10 billion years. This accumulation reaches only 1-2 billion years.
When the nuclei of clusters are destroyed by gravity, one of the most extreme astronomical events can be captured. At the same time, the cluster becomes incredibly dense. Many stellar collisions occur in the main area, because of which many blue distant stars may appear. However, no evidence was found in this cluster in favor of a collapsing nucleus. This is an amazing find, causing many questions that can be solved by further observation and finding similar cases in other clusters.