How Massive can a Star be?

New light on just how massive a star can become has been shed by a paper published this week in the Monthly Notices of the Royal Astronomical Society...

Stars come with a huge range of masses, ranging from the smallest brown dwarf stars, which have less than a tenth as much gas as our own Sun, to giant stars like Betelgeuse, with masses of twenty times that of the Sun.

But theorists who work on modelling the processes by which gas clouds can collapse down to form stars believe that there are hard limits to the lowest and highest masses that a star can have.

If a star has less than a twentieth the mass of the Sun, its core never becomes hot enough for to start nuclear fusion, the process which powers stars. What is formed resembles a gas giant planet like Jupiter, rather than a star that produces its own light.

At the opposite end of the mass spectrum, if a gas cloud has more than 150 times the mass of the Sun, it cannot gravitationally collapse to form a star. So much gravitational energy would be released in the process that the protostar would begin pulsating violently enough to rip the whole cloud apart.

However, a study in 2010 cast these theoretical models into some doubt. A group at the University of Bonn led by Sambaran Banerjee studied a star-forming region called the Tarantula Nebula, in a nearby dwarf galaxy called the Large Magellanic Cloud. Inside the region, they spotted four stars which they calculated to have masses of 400 times that of the Sun. This is over twice as large as the theoretical studies say it is possible to form.

But a follow-up paper, published by the same team in the Monthly Notices of the Royal Astronomical Society last week, finds that these stars may be easier to explain than previously thought. They built a computational model of the evolution of the Tarantula Nebula over time, modelling the way in which its stars move over time in each other's gravitational fields.

Because this is one of the most active star-forming regions in our Local Group of galaxies, Sambaran's team found that it was quite likely that some of its stars collide from time to time, subsequently merging to form more massive stars. Thus it is quite possible that, even if stars cannot form with masses over 150 solar masses, high mass stars can merge together later in life to form even more massive objects.

This is good news to the theorists who try to model star formation, who are no longer faced with the puzzle of explaining how to form stars that their models say should destroy themselves before they have time to form.