21 June 2018

A team of international astrophysicists from institutes in seven countries, including Dr Martin Krause at the University of Hertfordshire, propose a solution to a problem that has perplexed scientists for more than 50 years: why are the stars in globular clusters made of material different to other stars found in the Milky Way?

In a study published by Monthly Notices of the Royal Astronomical Society the team introduce a new actor to the equation that could solve the problem – a supermassive star.

The Milky Way galaxy

The Milky Way galaxy hosts over 150 old globular clusters, each containing hundreds of thousands of stars densely packed together and held by gravity. These stars are almost as old as the Universe. Since the 1960s, it has been known that most stars in these clusters contain chemical elements in different proportions than all other stars in the Milky Way. These could not have been produced in the stars themselves because the required temperatures are about 10 times higher than the temperatures of the stars in the globular clusters.

The scientists argue that a supermassive star, with a mass that is tens of thousands times the mass of the Sun, formed at the same time as the globular clusters. At that time, globular clusters were filled with dense gas out of which the stars were forming. As the stars collect more and more gas, they get so close to each other that they could physically collide and form a supermassive star in a runaway collision process. The supermassive star was hot enough to produce all the observed elements and “pollute” the other stars in the cluster with the peculiar elements we observe today.

'This makes the new model extremely viable'

Co-author, Dr Martin Krause from the University of Hertfordshire, said: “Many models have been suggested to solve this problem. They have more or less all been ruled out by observations. A supermassive star gets the proportions of the elements right relatively easily, which is the crucial observation that needs to be explained. It is also straight forward to understand why the effect occurs in massive star clusters, as the smaller ones just don’t have enough density to produce a supermassive star. In addition, small star clusters don’t have the mass to bind gas so ejecta from massive stars are easily lost from the cluster. This makes the new model extremely viable."

The team proposes various ways to test this new model of globular clusters and supermassive star formation with existing and upcoming telescopes, which can peer deep into the regions where the globular clusters formed, when the Universe was very young.

Image credit: NASA, ESA. Hubble Space Telescope image of the young massive star cluster R136 in the 30 Doradus star forming region in the Large Magellanic Cloud. The core of this cluster contains several very massive stars with masses of several 100 times the mass of the Sun, which could have formed by stellar collisions.