A new series of observations suggests that we have underestimated the number of large stars that form in starburst events. If this finding is more than just an exception to the rule, there could be consequences for many astronomical theories.

As reported in Science, an international group of astronomers has studied the stars within 30 Doradus, also known as the Tarantula Nebula, a starburst region in the Large Magellanic Cloud. The team managed to characterize the properties of 452 stars in 30 Doradus and, out of all of them, 247 were more massive than 15 times our Sun. There are about 25 to 50 more heavy stars than theoretical predictions, known as the initial mass function (IMF), would expect.

The IMF describes the distribution of masses for any population of stars when it formed. It’s an empirical distribution and is very important. The mass of stars determines their evolution and how they’re going to end their life. For example, more massive stars mean more supernovae, which leads to more black holes and neutron stars. It also influences the evolution of the stars’ host galaxies as a whole. And since galaxies have up to 100 billion stars, the IMF is very useful for providing statistics.

Nevertheless, this doesn’t mean that the IMF is perfect. Since its proposal in 1955 by Edwin Salpeter, the IMF has been tweaked to better characterize the low-mass end of star mass distribution. It turns out that there are a lot more small stars than predicted, and the new study suggests that some tweaking might be necessary for certain environments, even at the high end of mass distribution.

The study raises several questions that will require more observation. Is the excess of massive stars connected to advantageous conditions in the gas clouds? Is it common during starburst events? Are there other mechanisms at work?

What remains interesting is the presence of some of the most massive stars ever observed, with some weighing over 200 times the mass of the Sun. The researchers estimate that bigger stars might still exist in the core of the nebular, which was not resolved.

The Tarantula Nebula is the most active and largest (over 600 light-years) starburst region in the local group of galaxies. Supernova 1987A, the closest supernova observed since the invention of the telescope, occurred on the outskirts of this nebula.