Luminous blue variables erupt in stellar geysers


Modelling explains rare but spectacular astronomical phenomenon. Ben Lewis reports.


A simulation of the turbulent gas that develops in the outer layer of a luminous blue variable star, eventually erupting into space. Lighter colours represent denser areas.

A simulation of the turbulent gas that develops in the outer layer of a luminous blue variable star, eventually erupting into space. Lighter colours represent denser areas.

JOSEPH INSLEY/ARGONNE LEADERSHIP COMPUTING FACILITY

Some of the biggest, brightest and rarest stars in the universe are also some of the moodiest, periodically erupting in dazzling “stellar geysers”. Now, astrophysicists may have an explanation for their violent outbursts, finding that their outer layers form dense clumps of stellar material, causing an instability that ejects them out into space.

The research into the stars, called luminous blue variables, was carried out by Matteo Cantiello from the Flatiron Institute in New York, US, and answers one of the enduring mysteries of star behaviour.

In 3D simulations published in the journal Nature, Cantiello and his collaborators found that turbulence in the outer layers of a massive star causes material to agglomerate into piles that can rival the size of a planet. After forming, these lumps catch the star’s light, and behave like a solar sail, setting off in into space.

“This finding represents an important step forward in understanding the life and death of the biggest stars in the universe,” says Cantiello.

“These massive stars, despite their small number, largely determine the evolution of galaxies through their stellar winds and supernova explosions. And when they die, they leave behind black holes.”

While large blue variables are extremely rare, with only about 12 having been seen in the Milky Way, the researchers believe every extremely massive star probably spends at least part of its life in the form.

The outbursts occur when balance shifts in the tug of war between extreme gravity pulling material in and extreme luminosity pushing it out.

In a normal state, the outer layers of the star aren’t sufficiently dense to catch enough light to overcome gravity. However, simulating how matter, heat and light flow and interact within the outer layers, the researchers found that convection and mixing results in some regions being more dense than others.

Some of these regions were sufficiently dense to absorb enough photons from the star to be pushed outwards, overcoming the gravitational pull. An eruption can take between days and weeks to complete.

After the mass has been jettisoned, the star calms down again until the outer layers reform and the process recommences.

Over the course of a year, the eruptions were calculated to eject around 10 billion trillion metric tons of material from a typic luminous blue variable – around twice the mass of Earth.

JOSEPH INSLEY/ARGONNE LEADERSHIP COMPUTING FACILITY
  1. https://www.nature.com/articles/s41586-018-0525-0
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