Image reveals a newborn star in a spiral-armed embrace

On a desert plateau in northern Chile, nearly five kilometers above sea level and unhindered by the dense, moist atmosphere below, the Atacama Large Millimeter/Submillimeter Array (ALMA) has a prime view of the night sky. So it’s little wonder that when ALMA’s 66 antennas turned their eyes toward the nascent star Elias 2-27, they picked up on a detail that other radio telescopes had missed. The star’s circumstellar disk—a blob in previous images—harbors a pair of massive spiral arms reminiscent of those commonly seen in galactic disks. (See the accompanying image.) The observation, reported by Laura Pérez (Max Planck Institute for Radio Astronomy ) and her colleagues, provides the strongest support to date for a theoretical prediction that certain massive circumstellar disks will collapse under their own gravity to form spiral density waves. Judging from the star’s accretion rate (10⁻⁷ solar masses per year), the disk should be just about the right temperature for the predicted gravitational instability to occur. On the other hand, Pérez and her coworkers estimate the disk to be just 0.15 solar masses, which is in theory almost a factor of two too small to produce the two-armed configuration in the image. Future observations of Elias 2-27 and other young stars may help the astronomers discern if the spiral arms are truly the by-products of a gravitational instability, or if an alternative mechanism—say, the gravitational pull of an as-yet-undiscovered planet—is at work. (L. M. Pérez et al., Science 353, 1519, 2016 .)