Since all gas that remained from their birth environment was used to form stars at early times, star clusters have long been thought of as “infertile” stellar systems which cannot form new stars. Only collisions or mergers of stars can lead to rejuvenation of much older stars, making them look younger than most normal stars in much the same way as humans apply facelifts. Such stars are known as‘blue stragglers’, because they appear to ‘straggle’ behind the natural evolution of most stars in a star cluster: they still resemble extremely hot (and therefore blue) young stars. Using the Hubble Space Telescope, an international research team led by Dr Licai Deng from National Astronomical Observatories of Chinese Academy of Sciences (NAOC), has detected an unexpected population of blue straggler stars in a young ‘globular’ cluster, known by its catalogue number ‘NGC 2173’. This is surprising, because blue straggler stars in this cluster seem to have formed in a well-defined burst. The team’s research results were published in The Astrophysical Journal on 21 March.

“In principle, stellar collisions or binary mergers should not take place at the same time. They will happen randomly in star clusters and produce blue straggler stars that seem to have different ages”, explained Dr Chengyuan Li, who is the first author of this article and from Macquarie University.

Astronomers study stellar ages using a common diagram which relates stellar brightnesses to their temperatures at the stellar surface. Blue straggler stars formed randomly would produce a smattering of stars across the diagram. If, instead, they were born at the same time, they would clearly show a tight sequence. In NGC 2173, Dr Li and his colleagues detected two distinct sequences of blue straggler stars in the diagnostic diagram (see the Figure below).

Although this is not the first time astronomers have detected such clearly distinct blue straggler sequences in star clusters, previously such features were only found in old ‘globular’ clusters, with ages much greater than 10 billion years. This is the first time astronomers have found a similar pattern in a much younger cluster of only 1 to 2 billion years old.

“When the very cores of clusters collapse under the gravity of all the stars in that small volume of space, we witness one of the most extreme astronomical events. When this happens, the cluster becomes extremely dense and you can imagine that many stellar collisions could happen in the core region. As a result, lots of blue straggler stars could be produced. For this reason, the double sequence of blue stragglers can be expected in clusters only when they got old, at least older than 10 billion years” explained Dr Licai Deng.

“However, we did not find any evidence that supports the presence of a collapsed core in this cluster. In addition, the conditions in this cluster even disfavor the occurrence many stellar collisions,” said Chengyuan Li.

“This work certainly presents an unexpected, and therefore interesting observational results” comment the anonymous reviewer solicited by the editors of The Astrophysical Journal, “it challenges the generality of explanations put forward for other such blue straggler sequences.”

“This is contradictory to our expectation; Star clusters and their populations seem to keep providing surprises every time we look just a litter closer” commented Prof. Alison Sills from McMaster University in Canada in a News & Views commentary published in the prestigious professional journal Nature Astronomy.

The research consortium included the Macquarie Astronomy and Astrophysics research centre, National Astronomical Observatories of Chinese Academy of Sciences, Kavli Institute for Astronomy and Astrophysics at Peking University, Yunnan Observatories of Chinese Academy of Sciences, and the Department of Astronomy of China West Normal University.

Figure: Diagnostic diagram of blue straggler stars in the star cluster NGC 2173. Each point represents one star. Black points are normal stars. Blue and red points represent blue straggler stars belonging to two clear sequences. The blue solid and blue dashed lines are theoretical model predictions. The left sequence is composed of blue straggler stars formed roughly at the same time.