An international team of astronomers, led by Dr Francesca D’Antona from the INAF-Osservatorio Astronomico di Roma, Italy, has found that the so-called blue hook stars – extremely luminous and hot stars found in massive star clusters – throw off their cool outer layers late in life because they are rotating so rapidly, making them more luminous than usual. The stars of this rare type are known to exist in a few globular clusters: NGC 2808, Omega Centauri, NGC 6273, NGC 6388, and Messier 54.

“We have solved an old puzzle. These stars are only half the mass of our Sun yet we could not explain how they became so luminous. As the star was forming billions of years ago from a disc of gas in the congested center of the star cluster, another star or stars must have collided with the disc and destroyed it,” explained Dr Antonino Milone of Australian National University, who is a team member and a-coauthor of the paper published in the journal Nature.

The astronomers studied Omega Centauri, the only cluster visible to the naked eye, which contains around 10 million stars in close proximity to one another.

“The model shows the formation of stars in clusters do not all form at once,” said study co-author Dr Aaron Dotter, also from Australian National University.

“These blue stars must form in a second generation of star formation. Our new explanation is quite simple, and it hangs together really nicely.”

Usually the large disc of ionized gas around a newly-forming star locks its rotation through magnetic effects.

For the progenitors of blue hook stars, however, an early destruction of its disc allows the stars to spin up as the gas comes together to form a star.

Because its high rotation rate partially balances the inward force of gravity, the star consumes its hydrogen fuel more slowly and evolves differently throughout its life.

The blue hook phase of its life occurs after more than 10 billion years, when the star has consumed nearly all its hydrogen and begins burning the hotter fuel helium.

The different evolution processes leave it with a heavier core which burns brighter than typical helium-burning stars.

_____

Marco Tailo et al. Rapidly rotating second-generation progenitors for the ‘blue hook’ stars of ω Centauri. Nature, published online June 22, 2015; doi: 10.1038/nature14516