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Magnetic star baffles astronomers

Artist’s impression of the magnetar in Westerlund 1

An artist’s impression of the magnetar in the very rich and young star cluster Westerlund 1. Astronomers have demonstrated that this magnetar was formed from a star with at least 40 times as much mass as the Sun. A star as massive as this would have been expected to become a black hole, not a magnetar.

  • Explosion of a star 40 times the mass of the Sun
  • Formed a magnetar instead of a black hole

Astronomers have for the first time demonstrated that a magnetar—an unusual type of neutron star—was formed from the explosion of a star with at least 40 times as much mass as the Sun.

A magnetar is a type of neutron star with an incredibly strong magnetic field — a million billion times stronger than that of the Earth, formed when certain stars undergo supernova explosions.

The result presents great challenges to current ideas of how stars evolve, as a star as massive as this was expected to become a black hole, not a magnetar.

And it raises a fundamental question—just how massive does a star really have to be to become a black hole?

The astronomers studied the extraordinary star cluster Westerlund 1, located 16 000 light-years away, and the closest super star cluster known

Westerlund 1contains hundreds of very massive stars, some shining with a brilliance of almost one million Suns, and some 2,000 times the diameter of the Sun.

“If the Sun were located at the heart of this remarkable cluster, our night sky would be full of hundreds of stars as bright as the full Moon,” says Ben Ritchie, lead author of the paper reporting these results.

The stars all share one thing—they all have the same age, estimated at between 3.5 and 5 million years, as the cluster was formed in a single star-formation episode.

Star cluster Westerlund 1

The young star cluster Westerlund 1 contains hundreds of very massive stars, some shining with a brilliance of almost one million Suns.

Stellar lifespan the key

Westerlund 1 hosts one of the few magnetars known in the Milky Way. As all the stars in Westerlund 1 have the same age, the star that exploded and became the magnetar must have had a shorter lifespan than the surviving stars in the cluster.

“Because the lifespan of a star is directly linked to its mass—the heavier a star, the shorter its life—if we can measure the mass of any one surviving star, we know for sure that the shorter-lived star that became the magnetar must have been even more massive,” says co-author and team leader Simon Clark.

“This is of great significance since there is no accepted theory for how such extremely magnetic objects are formed.”

The astronomers therefore studied the stars that belong to the “eclipsing” double system W13 in Westerlund 1 using the fact that, in such a system, masses can be directly determined from the motions of the stars.

The work shows for the first time that magnetars can evolve from stars so massive they would normally be expect to form black holes.

The previous assumption was that stars with initial masses between about 10 and 25 solar masses would form neutron stars and those above 25 solar masses would produce black holes.

“These stars must get rid of more than nine tenths of their mass before exploding as a supernova, or they would otherwise have created a black hole instead,” says co-author Ignacio Negueruela.

Adapted from information issued by ESO / L. Calçada.

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