Outbursts from a Milky Way hypergiant

A luminous, sprawling red star in the Milky Way, surrounded by gas and dust and prone to violent mass ejections, abruptly fades in brightness when viewed from Earth. That could well describe Betelgeuse, the famous red supergiant whose fluctuations in apparent magnitude since 2019 have captured the attention of sky watchers and stellar astronomers.

But that description also fits the profile of VY Canis Majoris, a red hypergiant—and one of the largest known stars in the galaxy by radius—that has fluctuated in brightness and shed mass frequently for at least a century and a half. After obtaining the highest precision measurements of VY CMa and its surroundings to date, Roberta Humphreys of the University of Minnesota Twin Cities and her colleagues now report a correlation in the timing of major episodes of mass loss with that of reported dimming of the star. The findings could help astronomers understand the interplay between surface activity, magnetic fields, and mass loss in the extreme red stars, including Betelgeuse, that sit atop the Hertzsprung–Russell diagram.

When Humphreys, who specializes in very massive stars in the Milky Way and its neighbors, started studying VY CMa, it was thought to be shrouded in a relatively uniform shell of previously expelled gas and dust. In 1999 Humphreys and her team found evidence that the red hypergiant is surrounded instead by knots, filaments, and other discrete clumps of material—presumably the remnants of multiple recent outbursts. The clumps’ movements in different directions suggest that localized instabilities on the stellar surface are responsible.

In their new study, Humphreys and colleagues examined several of the clumps nearest to the star by using observations taken in 2018 by the Hubble Space Telescope. An order-of-magnitude improvement in spatial resolution over previous measurements, combined with the strong potassium emission lines from the ejecta, allowed the researchers to pinpoint the positions and velocities of the clumps. Extrapolating from those measurements, the researchers determined that VY CMa underwent significant, localized mass-loss events about 250, 200, 120, and 70 years ago. A small knot of material closer to the star may have been expelled as recently as 1995, which indicates that the star remains highly active.

In a final step, Humphreys and colleagues compared the timing of VY CMa’s outbursts with measurements of the star’s apparent magnitude dating back to the early 19th century, when the star clocked in at about +6.5 magnitude. The events matched up with periods of variable and fading brightness, which have culminated in the star’s apparent magnitude dipping to around +8.5 today. The researchers attribute the star’s prolonged dimming to a dust-filled expulsion whose direction toward Earth has led to sustained obscuration. Other outbursts in different directions obscured our line of sight only temporarily.

Does VY CMa yield lessons for those examining Betelgeuse’s mysterious and precipitous drop in brightness? Humphreys and colleagues highlight evidence that Betelgeuse has recently ejected material of its own. Studying the outbursts and brightness changes of a remarkable star like VY CMa, the researchers say, may expose the slightly smaller-scale processes that are influencing the merely extraordinary Betelgeuse. (R. M. Humphreys et al., Astron. J. 161, 98, 2021 .)