Gas clouds in the Milky Way give birth to stars, and the heaviest among them are true giants. Previously, it was thought that such a star ignites almost instantly, like a light bulb. But new research shows that a star grows gradually, like a snowball rolling down a mountain: the longer it rolls, the bigger it gets.
The authors compared two catalogs: bright gas clouds around young stars and the stars themselves. Earlier, the mismatch between these catalogs was called the "lifetime problem"—as if the clouds disappeared faster than the stars could burn out. Now it turns out that the stars simply hadn't finished growing when they became visible. The formation time for a star with a mass of 60 Suns is about four million years. It's as if a baby took several years to develop instead of nine months, and we saw it already in kindergarten, not in the maternity ward.
Such slow growth explains why there are so many bright clouds around still-growing stars in our Galaxy. Previously, this seemed puzzling, but now it confirms the gradual formation model. A star doesn't switch on like a flashlight; it kindles like a campfire as you add wood.
A star with a mass of 60 Suns accretes material at a rate of roughly one Earth mass per year, as if an asteroid were falling onto it every second.