Since the limited amount of gas survived from the first bulk star forming process will be quickly expelled within several million years, star clusters have long been thought of as "infertile" stellar systems that 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. DENG Licai from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) 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 findings were published in The Astrophysical Journal.

"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," said Dr. LI Chengyuan of Australia's Macquarie University, the first author of this work.

Astronomers study stellar ages using a common diagram that relates stellar brightness to temperature at the stellar surface. Blue straggler stars formed randomly would produce a smattering of stars across the diagram. If they were born at the same time, however, they would clearly show a tight sequence. In NGC 2173, Dr. DENG and his colleagues detected two distinct sequences of blue straggler stars in the diagnostic diagram.

Although this is not the first time astronomers have detected such clearly distinct blue straggler sequences in star clusters, such features were previously only found in old "globular" clusters, with ages more 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 get old, older than 10 billion years," said Dr. 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 of many stellar collisions," said Dr. LI.

"This work certainly presents unexpected, and therefore interesting observational results," commented 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," said Prof. Alison Sills from McMaster University in Canada in a News and Views commentary published in the prestigious professional journal Nature Astronomy.

Research paper

Related Links
Chinese Academy of Sciences Headquarters
Stellar Chemistry, The Universe And All Within It

Tweet

Thanks for being here; We need your help. The SpaceDaily news network continues to grow but revenues have never been harder to maintain. With the rise of Ad Blockers, and Facebook - our traditional revenue sources via quality network advertising continues to decline. And unlike so many other news sites, we don't have a paywall - with those annoying usernames and passwords. Our news coverage takes time and effort to publish 365 days a year. If you find our news sites informative and useful then please consider becoming a regular supporter or for now make a one off contribution.
SpaceDaily Contributor $5 Billed Once credit card or paypal		SpaceDaily Monthly Supporter $5 Billed Monthly paypal only

NGC 6231: Stellar Family Portrait in X-rays
Boston MA (SPX) May 03, 2018
In some ways, star clusters are like giant families with thousands of stellar siblings. These stars come from the same origins - a common cloud of gas and dust - and are bound to one another by gravity. Astronomers think that our Sun was born in a star cluster about 4.6 billion years ago that quickly dispersed. By studying young star clusters, astronomers hope to learn more about how stars - including our Sun - are born. NGC 6231, located about 5,200 light years from Earth, is an ideal testbed for ... read more