Birth of a monster star

NEW observations using the Atacama Large Millimeter/submillimeter Array (ALMA) have given astronomers the best view yet of a monster star in the process of forming within a dark cloud. The most massive and the brightest stars in the galaxy form within cool and dark clouds, but the process remains not just shrouded in dust but also in mystery. An international team of astronomers led by Nicolas Peretto of the CEA/AIM Paris-Saclay, France, and Cardiff University, U.K., has used ALMA to perform a microwave prenatal scan to get a clearer look at the formation of one such star that is located around 11,000 light years away, in a cloud known as the Spitzer Dark Cloud (SDC) 335.579-0.292.

There are two theories on the formation of the most massive stars. One suggests that the parental dark cloud fragments, creating several small cores that collapse on their own and eventually form stars. The other is more dramatic: the entire cloud begins to collapse inwards, with material racing towards the cloud’s centre to form one or more massive behemoths there. SDC335.579-0.292 was first revealed as dark, dense filaments of gas and dust through observations with NASA’s Spitzer Space Telescope and the European Space Agency’s Herschel Space Observatory. Now the team used the unique sensitivity of ALMA to look at the cloud in detail.

“We wanted to see how monster stars form and grow, and we certainly achieved our aim!” says Peretto. “One of the sources we have found is an absolute giant—the largest protostellar core ever spotted in the Milky Way.” This core has over 500 times the mass of the sun swirling around within it. And the ALMA observations show that much more material is still flowing inwards and increasing the mass further. This material will eventually collapse to form a young star up to 100 times as massive as our home star—a very rare beast. “Even though we already believed that the region was a good candidate for being a massive star-forming cloud, we were not expecting to find such a massive embryonic star at its centre,” says Peretto. “This object is expected to form a star that is up to 100 times more massive than the sun. Only about one in ten thousand of all the stars in the Milky Way reach that kind of mass!”

A research paper on this is to appear in the journal Astronomy & Astrophysics .