Hubble solves the mystery of the monster star’s gradation

Last year, astronomers were puzzled when Betelguese, the bright red supergiant star in the constellation Orion, dramatically faded, but then recovered. The graduation lasted for weeks. Now, astronomers have turned to a monster star in the nearby constellation Canis Major, the Canis Major.

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

“VY Canis Majoris behaves a lot like Betelgeuse on steroids,” said study leader astrophysicist Roberta Humphreys of the University of Minnesota, Minneapolis.

As with Betelgeuse, data from Hubble suggests the answer to why this larger star is darkening. For Betelgeuse, the gradation corresponded to a gas flow that may have formed dust, which briefly obstructed part of the Betelgeuse light from our view, creating the gradation effect.

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

The huge red hypergiant is 300,000 times brighter than our Sun. If it replaced the Sun in our own solar system, the swollen monster would stretch for hundreds of millions of kilometers, between the orbits of Jupiter and Saturn.

“This star is absolutely amazing. It’s one of the biggest stars we know of – a very evolved red super giant. It’s had multiple giant eruptions,” Humphreys explained.

Giant arcs of plasma surround the star at distances from it that are thousands of times farther than the Earth is from the Sun. These arcs resemble the solar prominences of our own Sun, but on a much larger scale. Also, they are not physically connected to the star, but rather appear to have been thrown and move away. Some of the other structures close to the star are still relatively compact, resembling small knots and cloudy lines.

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

Now, in new work with Hubble, researchers have solved features much closer to the star that may be less than a century old. By using Hubble to determine the speeds and motions of close knots of hot gas and other features, Humphreys and his team were able to date these eruptions more accurately. What they found was remarkable: many of these knots relate to several episodes in the 19th and 20th centuries when VY Canis Majoris went down to 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 has faded so much that it can only be seen with telescopes.

The hypergiant throws 100 times more mass than Betelgeuse. The mass of some nodes is more than double the mass of Jupiter. “It’s amazing the star can do this,” Humphreys said. “The origin of these episodes of high mass loss in VY Canis Majoris and Betelgeuse is probably caused by large-scale surface activity, large convective cells like on the Sun. But on VY Canis Majoris, the cells can also be larger than the Sun set or larger. “

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

While other red supergiants are comparatively bright and throw a lot of dust, none of them are as complex as VY Canis Majoris. “So what’s special? VY Canis Majoris may be in a unique evolutionary state that separates him from other stars. He’s probably also active for a very short time, maybe only a few thousand years old. We’re not going to see many … of those around, “Humphreys said.

The star started life as a super hot, bright blue super giant star, perhaps up to 35 to 40 times the mass of our Sun. After a few million years, as the rate of combustion of the fusion of hydrogen in its nucleus changed, the star swelled until it became a red supergiant. Humphreys suspects that the star may have briefly returned to a warmer state, then ascended to a stage of a red supergiant.

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

The team’s results appear in the February 4, 2021 edition of The Astronomical Journal.