Georgia State University astronomers have found an explanation for the strange occurrence of massive stars. These stars are located far from their birthplace in the disk of our Milky Way Galaxy.
Stars more massive than the sun have very hot cores. This drives nuclear energy generation at very high rates. They are the brightest objects in our galaxy. They burn through their hydrogen fuel so quickly. Their lifetimes are relatively short. It is 10 million years compared to 10 billion years for the sun.
There is little time for them to stray too far from their birthplace. Most massive stars are found in the flat disk part of our galaxy. Here gas clouds are dense enough to promote star birth. Here astronomers find young clusters of massive stars.
When a massive star is found far away from the galaxy’s disk, how did it get there?
This is the problem presented by the massive star known as HD93521. It lies about 3,600 light years above the galaxy’s disk. But scientists have found a profound discrepancy. The flight time to reach this location exceeds the predicted age of this massive star.
Astronomers used a new distance estimate from the European Space Agency’s Gaia spacecraft. They have also used an investigation of the star’s spectrum. They wanted to determine the star’s mass and age as well as its motion through space. They found out HD93521 has a mass about 17 times larger than the sun. This leads to a predicted age of about 5 million years. The motion of the star suggests that its journey from the disk has taken much longer which is nearly 39 million years.
Astronomers explained this strange difference between the star’s lifetime and travel time. They suggested that HD93521 left the disk as two lower-mass and longer-lived stars. The study has been published in The Astronomical Journal.
HD93521 is one of the fastest rotating stars in the galaxy. Stars can spin up through stellar mergers. There two close orbiting stars can grow over time and collide to form one star.
Intermediate mass stars live long enough to match the long flight time of HD93521.
HD93521 is not the only massive star found so far away from its birthplace. Scientists investigated an example of a distant massive binary pair. It is probably representative of the stage just before a merger. This star is known as IT Librae. It has an orientation which creates mutual eclipses as the two stars pass in front of each other.