Starless cloud cores reveal why some stars are bigger than others

For the first time, astronomers have observed a massive starless cloud, providing the answer to a long held question: How do some stars grow to be behemoths when the vast majority are much smaller?

In the new study, published in the Astrophysical Journal , astronomers used the ALMA telescope in Chile, South America, to survey the cores of some of the darkest, coldest, and densest clouds in our Galaxy to search for the telltale signs of star formation.

Professor Paola Caselli, from the School of Physics and Astronomy at the University of Leeds and co-author of the research paper, said: “There are many questions surrounding massive star formation because, until now, nobody has ever observed the earliest stages of their formation.”

“These stars form very fast, and are mostly hidden from view. But our knowledge of the chemical processes in the interstellar medium and the unique capabilities of ALMA have allowed us to pierce the cold clouds and show us what is happening with unprecedented resolution.”

Average stars like our Sun begin life as dense, but relatively low-mass concentrations of hydrogen, helium, and other trace elements inside large molecular clouds. After the initial kernel emerges from the surrounding gas, material collapses under gravity into the central region in a relatively ordered fashion via a swirling accretion disk, where eventually planets can form. After enough mass accumulates, nuclear fusion begins at the core and a star is born.

While this model of star formation can account for the vast majority of stars in our Galaxy, something extra is needed to explain the formation of more massive stars – stars at least 8 times as massive as our Sun.

“Some additional force is needed to balance out the normal process of collapse, otherwise our Galaxy would have a fairly uniform stellar population,” said Professor Jonathan Tan, from the University of Florida, USA, and lead author of the new study. “Alternatively, there has been speculation that two separate models of star formation are needed: one for Sun-like stars and one for these massive stars.”

The key to teasing out the answer is to find examples of massive starless cores – to witness the very beginnings of massive star birth.

The astronomers were looking for a particular molecule that contains deuterium to essentially take the temperatures of these clouds to see if stars had formed. Deuterium is important because it tends to bond with certain molecules in cold conditions. Once stars turn on and heat the surrounding gas, the deuterium is quickly lost through chemical reactions.

The ALMA observations detected copious amounts of deuterium within a massive cloud, suggesting that it is cold and starless. This would indicate that some counter force is forestalling core collapse and buying enough time to form a massive star.

“We think that magnetic fields can act to slow down the collapse, allowing enough time for large amounts of material to accumulate into cloud cores before the central star is formed. As a result, star formation can be viewed as a universal process – one model for both Sun-like and massive stars – with the stellar mass determined by the mass of the parent cloud core,” said Professor Caselli.