Hubble Solves Monster Star Dimming Mystery

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Astronomers were baffled last year when Betelguese, the bright red supergiant star in the constellation Orion, dramatically faded, but then recovered. The dimming lasted for weeks. Now, astronomers have turned their gaze to a monstrous star in the adjoining constellation Canis Major, the Great Dog.

The red hypergiant VY Canis Majoris, which is much larger, more massive, and more violent than Betelgeuse, experiences much longer and fainter periods lasting for years. New findings from NASA’s Hubble Space Telescope suggest that the same processes that occurred at Betelgeuse are happening in this hypergiant, but on a much larger scale.

“VY Canis Majoris is behaving very similarly to Betelgeuse on steroids,” explained study leader Roberta Humphreys of the University of Minnesota, Minneapolis, astrophysicist.

As with Betelgeuse, the Hubble data suggests the answer to why this larger star is dimming. For Betelgeuse, the dimming corresponded to a gaseous outflow that may have formed dust, briefly obstructing part of the Betelgeuse light from our view, creating the dimming effect.

“In VY Canis Majoris we see something similar, but on a much larger scale. Massive ejections of material corresponding to its very deep fading, which is probably due to dust temporarily blocking the star’s light,” Humphreys said.

The huge red hypergiant is 300,000 times brighter than our Sun. If it were to replace the Sun in our own solar system, the bloated monster would stretch hundreds of millions of miles between the orbits of Jupiter and Saturn.

“This star is absolutely amazing. It is one of the largest stars we know of, a highly evolved red supergiant. It has had multiple giant eruptions,” Humphreys explained.

Giant arcs of plasma surround the star at distances thousands of times farther than Earth from the Sun. These arcs resemble the solar prominences of our own Sun, only on a much larger scale. Also, they are not physically connected to the star, but appear to have been thrown away. Some of the other structures near the star are still relatively compact, looking like little knots and hazy features.

In earlier Hubble work, Humphreys and his team were able to determine when these large structures were ejected from the star. They found dates spanning the last hundreds of years, some as recently as the last 100 to 200 years.

Now, in new work with Hubble, the researchers resolved features much closer to the star that may be less than a century old. By using Hubble to determine the speeds and motions of nearby hot gas knots and other features, Humphreys and his team were able to date these eruptions more accurately. What they found was remarkable: Many of these knots are linked to multiple episodes in the 19th and 20th centuries when VY Canis Majoris faded to one sixth of its usual brightness.

Unlike Betelgeuse, VY Canis Majoris is now too weak to be seen with the naked eye. The star was once visible, but it has dimmed so much that it can now only be seen with telescopes.

The hypergiant sheds 100 times more mass than Betelgeuse. The mass at some of the nodes is more than twice the mass of Jupiter. “It’s amazing that the star can do it,” Humphreys said. “The origin of these high mass loss episodes in both VY Canis Majoris and Betelgeuse is probably due to large-scale surface activity, large convective cells like in the Sun. But in VY Canis Majoris, the cells can be as large like the Sun set or more “.

“This is probably more common in red supergiants than scientists thought and VY Canis Majoris is an extreme example,” Humphreys continued. “It may even be the main mechanism driving the mass loss, which has always been a mystery to red supergiants.”

Although other red supergiants are comparatively bright and eject a lot of dust, none of them are as complex as VY Canis Majoris. “So what’s so special about it? VY Canis Majoris may be in a unique evolutionary state that separates it from the other stars. It’s probably that active for a very short period, maybe only a few thousand years. We’re not going to see a lot of those around, “Humphreys said.

The star began life as a superhot, bright blue supergiant star, perhaps 35 to 40 times the mass of our Sun. After a few million years, as the burning rate of hydrogen fusion changed in its core, the star swelled into a red supergiant. Humphreys suspects that the star may have briefly returned to a warmer state and then swelled back to a red supergiant stage.

“Perhaps what makes VY Canis Majoris so special, so extreme, with this complex ejecta, could be that it’s a second-stage red supergiant,” Humphreys explained. VY Canis Majoris may have already lost half of its mass. Instead of exploding like a supernova, it could simply collapse directly into a black hole.

The team’s findings appear in the February 4, 2021 issue of The Astronomical Journal.

The Hubble Space Telescope is an international cooperation project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, DC

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