Newborn stars can grow to be 10 to a 100 times more massive than the Sun if they form in stellar nurseries surrounded by lots of older stars, scientists say.
The discovery, reported in the Astrophysical Journal, and on the pre-press website ArXiv.org, helps explain why stars over eight times the mass of the Sun exist, even though astronomical models indicate they shouldn't.
"This observation may lift the veil on the formation of the most massive stars which remains, so far, poorly understood," said Alana Rivera-Ingraham, who led the study while a graduate student at the University of Toronto.
All stars are born in giant clouds of molecular gas and dust.
Once young stars are massive enough to trigger the nuclear fusion process that makes them shine, they begin generating stellar winds.
These winds blow away any remaining gas and dust, preventing the star from accumulating any more mass and so limiting its size.
"The radiation during the birth of high-mass stars is so intense that it tends to destroy and push away the material from which they need to feed for further growth," said Rivera-Ingraham, who is now at the Institut de Recherche en Astrophysique et Planétologie in France.
Rivera-Ingraham and colleagues used high-resolution images from the European Space Agency's Herschel Space Telescope to study high-mass star formation in a giant cloud of gas and dust called Westerhout 3, located 6500 light-years away.
They examined the cloud at microwave and far-infrared wavelengths, mapping dust densities and temperatures, looking for the densest regions where the dust is both cold and shielded from radiation. These are areas where new stars are about to be born.
Rivera-Ingraham and colleagues noticed that the densest region of the cloud was surrounded by a constellation of old high-mass stars.
They believe stellar winds from these stars prevented gas and dust from the newborn stars from being blown away, and allowing the star to accumulate more mass in a process known as convergent constructive feedback.
This corralling effect could also be provide the initial compression of the molecular gas and dust cloud needed to ignite the star formation process in the first place, say the researchers.
They suggest each generation of these stars may have created the right conditions for the next generation of stars to grow comparably or even more massive in its midst, ultimately leading to the formation of a rare cluster of high-mass stars.
Astronomer Maria Cunningham from the University of New South Wales said the research is "startling".
"We've all been looking for the youngest stars to be on the outskirts of these clusters, not in the centre," says Cunningham.
"But this is good evidence that the age of star formation is decreasing as you get closer to the centre of a cluster."
Cunningham says the new findings tick many boxes.
"It explains why we see high mass stars grouped together, and why there are so few high mass stars compared to other stars," she says.
"If the cluster is not only needed for the triggering of star birth, but also for the formation of the massive star, then this all starts to make a lot of sense."