When a star that's 40 times larger than our own sun collapses, scientists expect it to form a black hole. But in the Westerlund 1 star cluster, located 16,000 light years away in the constellation Ara, a star that size collapsed and formed what is known as a magnetar, a type of neutron star with a magnetic pull millions of times more powerful than the strongest magnets on Earth. In fact, magnetars are considered the most powerful magnets known in the universe.
So why did the super star morph into a magnetar and not a black hole? It's suspected that perhaps another star was involved, so scientists at the European Southern Observatory in Germany trained their Very Large Telescope (VLT) on Westerlund 1 to look for a runaway star -- a sun that would have been rocketing out of the cluster as a result of its companion's supernova explosion prior to its collapse.
They found one -- Westerlund 1-5.
"Not only does this star have the high velocity expected if it is recoiling from a supernova explosion, but the combination of its low mass, high luminosity, and carbon-rich composition appear impossible to replicate in a single star -- a smoking gun that shows it must have originally formed with a binary companion," Ben Ritchie from the UK's Open University said in a statement. Ritchie is a co-author of a paper about the discovery that's soon to be published in the journal, Astronomy and Astrophysics.
Once they knew the magnetar had a companion star revolving around it in a binary system, the astronomers were able to piece together the tale of how one caused the other to form.
They say that as the bigger star of the two began to run out of fuel, it transferred some of its mass to the other star -- the one that would become the magnetar. This made the smaller star rotate faster, which helped contribute to its super-strong magnetic field. But eventually that smaller star gained so much mass from its companion that it started throwing off the excess -- some into the universe and some back to the other star. This allowed it to slim down enough to collapse into one of the Milky Way's 48 known magnetars instead of forming a black hole.
"It is this process of swapping material that has imparted the unique chemical signature to Westerlund 1-5 and allowed the mass of its companion to shrink to low enough levels that a magnetar was born instead of a black hole," said team member Francisco Najarro of Spain's Centro de Astrobiología in a statement. He added that it was "a game of stellar pass-the-parcel with cosmic consequences."