Watching the Death of a Rare Giant Star

NASA / ESA / Hubble

Artist’s impression of the red hypergiant star VY Canis Majoris.

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A team of astronomers led by the University of Arizona has created a detailed, three-dimensional image of a dying hypergiant star.

The team, led by researchers Ambesh Singh and Lucy Ziurys at the University of Arizona, traced the distribution, directions and velocities of a variety of molecules around a red hypergiant star known as VY Canis Majoris.

Their findings, which they presented June 13 at the 240th Meeting of the American Astronomical Society in Pasadena, California, offer insights on an unprecedented scale into the processes that accompany the death of giant stars.

The work was done with collaborators Robert Humphreys at the University of Minnesota and Anita Richards at the University of Manchester, UK.

As extreme supergiant starsalso known as hypergiants, are very rare, with only a few known to exist in the Milky Way.

Examples include Betelgeuse, the second brightest star in the constellation Orion, and NML Cygni, also known as V1489 Cygni, in the direction of the constellation Swan.

Unlike lower-mass stars — which are more likely to swell as they enter the red giant phase, but generally maintain a spherical shape — hypergiants tend to undergo substantial mass loss events that form structures complex and highly irregular composed of arcs, clusters and nodes.

Located about 3009 light-years from Earth, VY Canis Majoris — or VY CMa for short — is a pulsating variable star in the constellation Canis Major.

Spanning between 10,000 and 15,000 astronomical units (1 astronomical unit, or AU, is the average distance between the Earth and the Sun, about 150 million kilometers), VY CMa is possibly the most massive star in the Milky Wayaccording to Ziurys.

“Think of it as Betelgeuse on steroids,” said Ziurys, Regent Professor in the Department of Chemistry and Biochemistry at the University of Arizona and the Steward Observatory. “It’s much bigger, much more massive, and it erupts massively every 200 years or so.”

The team chose to study VY CMa because it is considered one of the best examples of these types of stars.

“We’re particularly interested in what hypergiant stars do at the end of their lives,” said Singh, in his 4th year of PhD and a member of Ziurys’ lab. “People used to think that these massive stars just evolved into supernovaebut we are no longer sure.”

“If that were the case, we should see many more supernova explosions across the sky,” added Ziurys. “We now think they can quietly collapse into black holes, but we don’t know which ones end up like this, or why that happens and how.”

Previous images of VY CMa with NASA’s Hubble Space Telescope and spectroscopy have shown the presence of distinct arcs and other clusters and nodes, many extending thousands of AU from the central star.

To uncover more details of the processes by which hypergiant stars end their lives, the team began tracing certain molecules around the hypergiant and mapping them to pre-existing images of the dust taken by the Hubble Space Telescope.

“No one has been able to get a complete picture of this star,” said Ziurys, explaining that his team set out to understand the mechanisms by which the star’s mass is released, which appear to be different from those of smaller stars entering the star. your red giant stage at the end of their lives.

“You don’t see this nice symmetrical loss of mass, but convection cells that ‘blow’ through the star’s photosphere like giant bullets and eject mass in different directions,” said Ziurys. “These are analogous to the coronal arcs seen on the Sun, but a billion times larger.”

The team used the Atacama Large Millimeter Array (ALMA) in Chile to track a variety of molecules in material ejected from the stellar surface.

While some observations are still ongoing, preliminary maps of sulfur oxide, sulfur dioxide, silicon oxide, phosphorus oxide and sodium chloride have been obtained. From these data, the group constructed an image of the structure of the global molecular flux of VY CMa at scales that encompassed all of the material ejected from the star.

“As molecules trace the arcs on the casingwhich tells us that the molecules and dust are well mixed together,” Singh said.

“What’s good about emission from molecules at radio wavelengths is that they give us velocity information, as opposed to emission from dust, which is static,” he added.

By moving ALMA’s 48 antennas to different configurations, the researchers were able to obtain information about the directions and velocities of the molecules and map them through the different regions of the hypergiant’s shell in considerable detail, even correlating them with different mass ejection events. over time.

Processing the data required some “heavy lifting” in terms of computing power, Singh said.

“So far, we have processed almost a terabyte of ALMA and we still get data that we have to analyze to get the best possible resolution,” he said.

“The calibration and cleaning of the data alone requires up to 20,000 iterations, which takes a day or two for each molecule”, he adds.

“With these observations, we can now map them in the sky,” added Ziurys. “Until now, only small portions of this huge structure have been studied, but you can’t understand the mass loss and how these big stars die unless you look at the entire region. That’s why we wanted to create a complete picture.”

With funding from the NSF (National Science Foundation), the team plans to publish their findings in a series of scientific articles.