Forming truly massive stars has been really hard to explain. As young stars form, they light up, and their light can push away infalling material, shutting off their growth. The bigger a star, the more brightly it will shine, and it seems like truly massive stars should shut off their own formation with their light. Since we see really massive stars, they must somehow find a way to grow. Now, in a new paper in Nature, a team of astronomers led by Xi Chen offers up a set of observations that seem to explain what is going on. 

While observing the twelve solar mass proto-star G358-MM1, they were able to see light from knots in the protostellar disc that shined as masers [Ed. note: microwave amplification by the stimulated emission of radiation]. These naturally occurring microwave-colored lasers only shine when conditions – temperature, density, and the like – are exactly right, and the maser in G358-MM1 appeared to turn on and off in ways that indicated the material in this particular protostellar disc is clumped up into spiral arms, and the clumps are falling into the star in ways that cause it to periodically flare up in brightness. 

According to Chen: G358-MM1 is thus the first example of a massive protostar whose sudden increase in brightness clearly coincides with the formation of a spiral, a structure that suggests an unstable, massive disk.

As they explain it, in these systems, the disk of material around this star is so massive that it becomes unstable. Its gravitational pull is such that it is pulling material from the surroundings onto itself, and as that material tries to flow toward the star, gravity from denser regions will pull more strongly, making those regions even denser, clumping the material up. The spiral structure that is seen is something that recurs throughout astronomy at different scales and wasn’t surprising. That said, seeing something you knew might be out there is always exciting because it means we actually do understand how parts of the universe we weren’t able to see work. 

More Information

Max Planck Institute for Astronomy news release 

“New Maser Species Tracing Spiral-Arm Accretion Flows in a High-Mass Young Stellar Object,” Xi Chen et al., 2020 July 13, Nature Astronomy (Preprint)