White dwarf: Armagh astronomers say the scar indicates stars are 'eating' planets

  • Written by Rebecca Wilson
  • BBC News NI

Image source, ISO/L. Calada

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An artist's impression of the magnetic white dwarf WD 0816-310 with a metallic scar

Astronomers in Armagh have discovered a sign of a star feeding on nearby planets and asteroids.

Astronomer John Landstreet said that a metallic scar found on the surface of the dead star had never been seen before.

It was discovered at the Armagh Observatory's Astronomical Research Center and Planetarium.

The scar, thought to be 500 kilometers (310 miles) long, was on the remains of an Earth-sized star from a nearby solar system.

The system was no longer producing energy at its core, so the star died.

The white dwarf star has been named WD 0816-310 by research astronomers.

Research found that the scar developed after the white dwarf swallowed the surrounding planets and asteroids that were born at the same time as it, causing a concentration of minerals that scarred the surface of the star.

Dr. Landstreet, co-author of the research paper, is a member of the team that discovered the first magnetic white dwarf in 1970.

“Surprisingly, the matter was not evenly mixed on the surface of the star, as theory predicted,” he said.

“Instead, this scar is a concentrated patch of planetary material, held in place by the same magnetic field that directed the falling fragments. Nothing like this has ever been seen before.”

What is a white dwarf?

A white dwarf is the burned-out remains of a dead star, and is what our Sun will become in about five billion years.

When a star dies, gravity means that heavier elements move towards the center while lighter elements such as hydrogen or helium rise to the outer layer.

At their hottest, stars will have a hydrogen atmosphere, but as stars cool over time, they tend to have a helium atmosphere.

But typical white dwarfs do not have one side of the star devoted to one element and the other side dominated by another element.

The scar is 500 kilometers long, and is the same size as the scar of Vesta, the second largest asteroid in our solar system.

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The research used the European Southern Observatory's Very Large Telescope (VLT) in Chile.

The astronomers said the power of the metal detection coincided with noticeable changes in the star's magnetic field, leading the team to determine that the metallic scar was present on one of the star's magnetic poles.

This showed that metals were drawn into the star by its magnetic field, creating the scar.

Stefano Pagnolo, an astronomer at Armagh Observatory and Planetarium, said it was known that some white dwarfs were eating parts of their planetary systems.

“Now we have discovered that the white dwarf’s magnetic field plays a major role in this process, leading to a scar on its surface,” he said.

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