Free Astronomy Magazine January-February 2021

42 ASTRO PUBLISHING A stronomers used the Atacama Large Millime- ter/submillimeter Array (ALMA) to observe a set of stellar winds around aging stars and present an explanation for the mesmerizing shapes of planetary nebulae. Contrary to common consen- sus, the team found that stellar winds are not spher- ical but have a form similar to that of planetary nebulae. The team concludes that interaction with an accompanying star or exoplanet shapes both the stellar winds and planetary nebulae. Dying stars swell and cool to eventually become red giants. They produce stellar winds, flows of particles that the star expels, which causes them to lose mass. Because detailed observations were lacking, as- tronomers have always assumed that these winds were spherical, like the stars they surround. As the star evolves further, it heats up again, and the stellar radiation causes the expanding ejected layers of stel- lar material to glow, forming a planetary nebula. For centuries, astronomers were in the dark about the extraordinary variety of colorful shapes observed in planetary nebulas. The nebulae all seem to have a certain symmetry but are rarely round. “The Sun – which will ultimately become a red giant – is as round as a billiard ball, so we wondered: how can such a star produce all these different shapes?” says corresponding author Leen Decin (KU Leuven). by ALMA Observatory Astronomers capture stellar winds in unprecedented detail T his image gallery of stellar winds around cool age- ing stars shows a variety of morphologies, includ- ing disks, cones, and spirals. The blue color represents material that is coming towards you; red is material that is moving away from you. [L. Decin, ESO/ALMA, ATOMIUM Large Program]