A Dying Hypergiant Star Is Losing "30 Earths" Of Mass Every Year

Close-up of the VY Canis Majoris Nebular, with the star itself is hidden behind an obscuring disc. The crosses are artefacts due to features in the instrument. By ESO.

One of the largest stars in the Milky Way, VY Canis Majoris, has been caught undergoing a dramatic weight loss, an event that heralds the beginning of the end for the star.

Astronomers from the European Southern Observatory (ESO) have used the Very Large Telescope to look at the area around VY Canis Majoris and discovered that every year it sheds a mass equivalent to 30 Earths. They used the SPHERE instrument, which has the primary goal of directly imaging planets, to get detailed information about the material ejected by the stars.

VY Canis Majoris is a red hypergiant about 30 times the mass of the Sun. It has a radius over 1,400 times that of the Sun, and if it were at the center of the Solar System it would extend to the orbit of Jupiter. There is controversy on the precise size of this star: Red giants are variable, evolving systems, and our understanding of them is somewhat limited.

Giant stars tend to lose a significant fraction of material in the form of dust and gas. These clouds form an envelope around an aging star until the unavoidable supernova explosion tears the nebula apart. Scientists have been curious on how these old stars expel material so efficiently.

The explanation that has been put forward is that the starlight pushes the dust out, in the same way that the wind pushes a sail. This mechanism is called radiation pressure and it is very weak. Akin to wind, only the larger grains of dust get enough of a push to escape the star's gravity.

“Massive stars live short lives,” said Peter Scicluna, lead author of the paper to be published in the journal Astronomy & Astrophysics, in a statement. “When they near their final days, they lose a lot of mass. In the past, we could only theorise about how this happened. But now, with the new SPHERE data, we have found large grains of dust around this hypergiant. These are big enough to be pushed away by the star’s intense radiation pressure, which explains the star’s rapid mass loss.”

The dust ejected by stars like VY Canis Majoris plays a crucial role in the enrichment of the Milky Way. It will spread within the interstellar medium, becoming part of the next generation of planets and stars. 


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