When astronomers pointed the Atacama Large Millimeter/submillimeter array (ALMA) at the Spitzer dark cloud SDC335.579-0.292, hoping to study partially developed proto-stars, they were not expecting to discover a record-breaking 500 stellar mass monster. The unborn star, situated some 11000 light years away, is the largest ever seen in the Milky Way, and will likely evolve into a supergiant star some 100 times the mass of the Sun.

Stars form in vast clouds of cold gas and dust, which coalesce into dense knots of material that have enough gravity to draw in more and more material. The clouds collapse inwards, piling more and more matter into the protostar until its core temperature and pressure are high enough to begin nuclear fusion. As the protostar blossoms, it’s intense radiation and stellar wind blow away the remains of the cloud, revealing a newborn star to the universe. This happens in hundreds of places within the cloud, and explains why new stars usually form in clusters which gradually disperse over billions of years. But when it comes to the formation of rare supergiants like the one discovered in SDC335, astronomers haven’t been able to agree on how this process can produce such huge stars.
One of the two leading theories suggests that the cloud fragments as it collapses into discrete smaller cores which are then drawn in to the growing protostar over time. The other states that the entire molecular cloud collapses inwards, with material falling in towards one or two massive cores, and this appears to be the case at SDC335.
“The remarkable observations from ALMA allowed us to get the first really in-depth look at what was going on within this cloud,” says Nicolas Peretto, leader of the team that made this discovery. “We wanted to see how monster stars form and grow, and we certainly achieved our aim! One of the sources we have found is an absolute giant — the largest protostellar core ever spotted in the Milky Way.”
“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!”
The protostar is still at a very early stage in its development, but already has a volume of gas equivalent to more than 500 times the mass of the Sun swirling around its core and observations have shown that more gas and dust continue to flow inwards. The nature of the collapse supports the globular collapse theory, in which the entire cloud collapses directly into the protostar.
“Not only are these stars rare, but their birth is extremely rapid and their childhood is short, so finding such a massive object so early in its evolution is a spectacular result,” adds team member Gary Fuller from the University of Manchester, UK.