Mystery Of Dimming Hypergiant Star Dubbed "Betelgeuse On Steroids" Solved

Artist impression of what VY Canis Majoris looks like up close. Image Credit: NASA, ESA, and R. Humphreys (University of Minnesota), and J. Olmsted (STScI)

Betelgeuse’s mysterious dimming across 2019 and 2020 was a big puzzle for astronomers. It turned out the cause was a mixture of the star's regular changes and the release of material that when cooling down formed dust that obscured the light of the notorious star. Astronomers have now found a similar scenario around another star, VY Canis Majoris, but this one is even more dramatic than Betelgeuse.

VY Canis Majoris is a red hypergiant star. It is larger, more massive, and a lot more active than Betelgeuse, and about 300,000 times brighter than our Sun. It is so large that if it was placed where the Sun is, it would extend past the orbit of Jupiter.

When Betelgeuse experienced its strange dimming, it faded so much it dropped out of the top 20 brightest stars in the night sky. The situation for VY Canis Majoris is much worse. It was a faint but visible star up to 200 years ago but since then we can no longer see it with the naked eye.  A new study published in The Astronomical Journal shows the reason for this is also massive ejections of material. 

"VY Canis Majoris is behaving a lot like Betelgeuse on steroids," lead author Dr Roberta Humphreys from the University of Minnesota, Minneapolis said in a statement.

"In VY Canis Majoris we see something similar, but on a much larger scale. Massive ejections of material which correspond to its very deep fading, which is probably due to dust that temporarily blocks light from the star."

VY Canis Majoris
Hubble observations of VY Canis Majoris (left, middle) showing the centuries of eruptions and an artist's impression of what the star might look up close (right). Image Credit: NASA, ESA, and R. Humphreys (University of Minnesota), and J. Olmsted (STScI)

Previous work using Hubble showed eruptions from the star dating between 100 and 200 years ago. The new observations were able to look much closer at the star, at the more recent eruptions, and measure their size and velocity.

The team was able to date these events more precisely to create a more accurate picture of what’s going on around the star. The eruption takes the form of large knots, similar to solar protuberances, extending for hundreds of billions of kilometers from the star. Some of these structures weigh twice the mass of Jupiter.

"It's amazing the star can do it. The origin of these high mass-loss episodes in both 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 may be as large as the whole Sun or larger," Humphreys said. 

"This is probably more common in red supergiants than scientists thought and VY Canis Majoris is an extreme example. It may even be the main mechanism that's driving the mass loss, which has always been a bit of a mystery for red supergiants."

This extreme activity has not been seen in other stars like this, which suggests it only happens for a brief period of maybe a few thousand years. So we are lucky to be catching VY Canis Majoris in this incredible phase.

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