VY Canis Majoris, one of the largest known stars in the Milky Way and 300,000 times brighter than our Sun, “is behaving a lot like Betelgeuse on steroids,” said astrophysicist Roberta Humphreys at the University of Minnesota. VY Canis Majoris is a seething bright red supergiant star in the constellation Orion that dramatically dimmed for weeks, but has since recovered.
Unlike Betelgeuse –a star so huge it could someday collapse into a black hole or neutron star, which would make it the closest black hole to Earth some 725 light-years distant– VY Canis Majoris is even larger and more massive. If it replaced the Sun in our own solar system, the bloated monster would extend out for hundreds of millions of miles, between the orbits of Jupiter and Saturn. VY Canis Majoris is also much more volatile, experiencing much longer, dimmer periods that last for years.
A new study released in February, 2021 by an international team of scientists concluded that Betelgeuse is in the early core helium-burning phase (more than 100,000 years before a supernova event) and has smaller mass and radius – and is closer to Earth – than previously thought.
Multiple Eruptions of Material Correspond to Deep Fading
New findings from NASA’s Hubble Space Telescope suggest the same processes that occurred on Betelgeuse are happening in this hypergiant, which lives only a few hundred thousand to a million years. “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,” said Humphreys about Hubble data that suggests the answer for why this bigger star is dimming. For Betelgeuse, Humphreys explains, “the dimming corresponded to a gaseous outflow that may have formed dust, which briefly obstructed some of Betelgeuse’s light from our view, creating the dimming effect.”
“This star is absolutely amazing. It’s one of the largest stars that we know of—a very evolved, red supergiant. It has had multiple, giant eruptions,” explained Humphreys.
The NASA Image –A Nebula a Trillion Miles Across
This NASA zoom into VY Canis Majoris is a combination of Hubble imaging and an artist’s impression. The left panel is a multicolor Hubble image of the huge nebula of material cast off by the hypergiant star. This nebula is approximately a trillion miles across. The middle panel is a close-up Hubble view of the region around the star. This image reveals close-in knots, arcs, and filaments of material ejected from the star as it goes through its violent process of casting off material into space. VY Canis Majoris is not seen in this view, but the tiny red square marks the location of the hypergiant, and represents the diameter of the solar system out to the orbit of Neptune, which is 5.5 billion miles across. The final panel is an artist’s impression of the hypergiant star with vast convection cells and undergoing violent ejections. VY Canis Majoris is so large that if it replaced the Sun, the star would extend for hundreds of millions of miles, to between the orbits of Jupiter and Saturn. (NASA, ESA, and R. Humphreys (University of Minnesota), and J. Olmsted, STScI)
Giant Arcs of Detached Plasma
Giant arcs of plasma surround VY Canis Majoris at distances from it that are thousands of times farther away than the Earth is from the Sun. These arcs, reports Humphreys, look like the solar prominences from our own Sun, only on a much grander scale. Also, they’re not physically connected to the star, but rather, appear to have been detached, thrown out and are moving away. Some of the other structures close to the star are still relatively compact, looking like little knots and nebulous features. In previous Hubble work, Humphreys and her team were able to determine when these large structures were ejected from the star. They found dates ranging over the past several hundred years, some as recently as the past 100 to 200 years.
In new work with Hubble, researchers resolved features much closer to the star that may be less than a century old. By using Hubble to determine the velocities and motions of the close-in knots of hot gas and other features, Humphreys and her team were able to date these eruptions more accurately. What they found was remarkable: many of these knots link to multiple episodes in the 19th and 20th centuries when VY Canis Majoris faded to one-sixth its usual brightness.
Sheds Knots of Plasma Twice the Mass of Jupiter
The hypergiant sheds 100 times as much mass as Betelgeuse. The mass in some of the knots is more than twice the mass of Jupiter. “It’s amazing the star can do it,” Humphreys said. “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.”
“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 that’s driving the mass loss, which has always been a bit of a mystery for red supergiants.”
A Unique Evolutionary State
Though other red supergiants are comparably bright and eject a lot of dust, none of them is as complex as VY Canis Majoris. “So what’s special about it? VY Canis Majoris may be in a unique evolutionary state that separates it from the other stars. It’s probably this active over a very short period, maybe only a few thousand years. We’re not going to see many of those around,” said Humphreys.
Morphing Into a Black Hole?
The star began life as a super-hot, brilliant, blue supergiant star perhaps as much as 35 to 40 times our Sun’s mass. After a few million years, as the hydrogen fusion burning rate in its core changed, the star swelled up to a red supergiant. Humphreys suspects that the star may have briefly returned to a hotter state and then swelled back up to a red supergiant stage.
“Maybe what makes VY Canis Majoris so special, so extreme, with this very complex ejecta, might be that it’s a second-stage red supergiant,” explained Humphreys. VY Canis Majoris may have already shed half of its mass. Rather than exploding as a supernova, it might simply collapse directly to a black hole.