New study reveals mechanism behind extreme mass loss in hypergiant dying stars

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Shot of VY Canis Majoris as captured by the SPHERE instrument mounted aboard the ESO's Very Large Telescope (VLT)(Credit: ESO)

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Through observations of one of the largest stars known to exist in the Milky Way, a red hypergiant known as VY Canis Majoris, astronomers have been able to unravel the mystery as to how enormous stars shed vast quantities of mass prior to meeting their end in a cataclysmic supernova explosion.

Located in the constellation Canis Major, VY Canis Majoris is a true monster, even when compared to other stellar bodies. With a mass of between 30 - 40 times that of our Sun (and 300,000 times as bright), if VY Canis Majoris were to be switched with our Sun, its bulk would fill our solar system beyond the orbit of Jupiter.

We know from previous observations that VY Canis Majoris is coming to the end of its life. As the hypergiant progressed through its geriatric years, it expanded exponentially, expelling around 30 times the mass of Earth each year in the process.

It is not expected that the ancient star will meet its end for hundreds of thousands of years (a relatively short period in the life of a red hypergiant), and the resultant radiation will pose no threat to life here on Earth. From our vantage point, the tumultuous supernova will likely appear as bright as a second full Moon in the sky.

Under normal circumstances, it is next to impossible to observe objects in close proximity to a star due to the high levels of interference thrown off by bodies such as VY Canis Majoris. However, by using the advanced adaptive optics boasted by the SPHERE instrument mounted aboard the ESO's Very Large Telescope (VLT) located at the Paranal Observatory, Chile, the team were able to cut through the distortions and observe the clouds of stellar material thrown off by the host in its twilight years.

Starmap displaying the location of VY Canis Majoris as a small dotted circle within the constellation Canis Major(Credit: ESO, IAU and Sky & Telescope)

Based on an analysis of the polarisation of the starlight as it was scattered by the surrounding cloud of stellar material, it was discovered that the particles expelled from VY Canis Majoris were roughly 50 times the size of the standard particles known to exist in interstellar space.

Whilst we are still talking about relatively minuscule particles, spanning only 0.5 micrometers across, the comparatively large size of the gas and dust thrown off by VY Canis Majoris is the key to how the star sheds vast quantities of mass as it prepares to go supernova.

Prior to recent observations it had been theorized that radiation pressure, the force exerted by starlight, was the driving force behind how enormous stars shed and push away stellar material from the upper atmosphere, but it is the size of the newly discovered particles that makes the theory workable.

Had the particles been much lighter, the starlight would have essentially passed through, having little effect, much larger, and they would have been too heavy for the radiation pressure to shift. The particles detected by SPHERE thread the needle, being just the right size to boast the surface area needed for the starlight to propel the dust and gas away from VY Canis Majoris prior to its cataclysmic death.

It is further theorized that the large size of the particles will grant them a better chance of surviving the death of its host, allowing larger quantities of surviving gas and dust to be used by nearby stars to create the next generation of planets, thus continuing the complicated cycle of destruction and formation that allows our galaxy to thrive.

Source: ESO

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