Astronomers challenge current theories on stellar evolution
Using the European Southern Observatory's (ESO) Very Large Telescope (VLT), astronomers have for the first time demonstrated that a magnetar — an unusual type of neutron star — formed from a star with at least 40 times as much mass as the Sun. The result presents great challenges to current theories of how stars evolve because a star as massive as this was expected to become a black hole, not a magnetar. This now raises a fundamental question: Just how massive does a star have to be to become a black hole?
To reach their conclusions, the astronomers looked in detail at the extraordinary star cluster Westerlund 1, located 16,000 light-years away in the southern constellation Ara the Altar. From previous studies, astronomers knew that Westerlund 1 was the closest super star cluster known, containing hundreds of massive stars, some shining with a brilliance of almost 1 million Suns and some 2 thousand times the diameter of the Sun — as large as the orbit of Saturn.
"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," said Ben Ritchie from The Open University in the United Kingdom.
Westerlund 1 is a fantastic stellar zoo with a diverse and exotic population of stars. The stars in the cluster share one thing: They all have the same age, estimated at between 3.5 and 5 million years, because the cluster was formed in a single star-formation event.
A magnetar is a type of neutron star with an incredibly strong magnetic field that is formed when certain stars undergo supernova explosions. The Westerlund 1 cluster hosts one of the few magnetars known in the Milky Way. Thanks to its home in the cluster, the astronomers were able to make the remarkable deduction that this magnetar must have formed from a star at least 40 times as massive as the Sun.
As all the stars in Westerlund 1 have the same age, the star that exploded and left a magnetar remnant must have had a shorter life 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," said Simon Clark from The Open University in the U.K. "This is of great significance since there is no accepted theory for how such extremely magnetic objects are formed."
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