Astronomers continue to find stars that should be dead. Now we can finally find out why

The most massive stars in the Universe also have the shortest lifespans. The more mass a star has, the faster it burns up its fuel stores, resulting in lifespans of less than about 10 million years.

This fascinating fact leads us to a riddle. Most of these stars are relatively close to the regions where they were born. But a number of them have been found hidden in strange pockets of the Milky Way, far from the galactic disc where star formation occurs; in other words, their places of birth.

So far, in fact, that the travel time it would have taken to get there far exceeds the lifetimes of many of the stars.

“Astronomers find massive stars far from their place of origin, so far away, in fact, that it takes longer than the lifetime of the star to get there,” said astronomer Douglas Gies of Georgia State University. “How this could happen is a subject of active debate among scientists.”

This absolute idiot of a cosmic pickle, which has long puzzled astronomers, may now have an explanation thanks to new research.

The study focused on a star named HD 93521. It is an O-type star, the most massive star category in the main sequence. HD 93521 is also about 3,600 light-years from the galactic disk, located in a sparsely populated region called the galactic halo. It’s quite a distance, so Gies and his colleagues wanted to know if there was a reasonable way to get there.

They used data from the European Space Agency’s Gaia satellite. This is an ongoing project to map the Milky Way as accurately as possible, in three dimensions and including data on star motions and velocities. They also carefully analyzed the spectrum of light emitted by the star, to help determine its mass, age and rotation.

Data from Gaia revealed that HD 93521 is approximately 4,064 light-years from Earth and the aforementioned 3,600 light-years from the galactic disk.

The team also calculated that the star is around 17 times the mass of the Sun, with an average temperature of around 30,000 Kelvin. At this mass and temperature, the star should be about 5 million years old, with a margin of error of about 2 million years. Its maximum lifespan is about 8.3 million years.

However, migrating from its birthplace in the galactic disk to its current location would take a journey of approximately 39 million years.

It’s a real headache, but the star herself could hold the key to the mystery. The rate of rotation of our Sun is just under 2 kilometers (1.24 miles) per second. The HD 93521 rotates at an absolute speed of 435 kilometers (270 miles) per second.

There are several mechanisms that can increase a star’s rotational speed. One of the most important effects would be a stellar merger, which would not only combine the spins of the two stars, but also the angular momentum of their orbits.

That’s what the team thinks happened with HD 93521. It started life as a binary made up of two medium-mass stars, which merged to form the star as we see it today. in the relatively recent past.

These medium-mass stars would be long enough to survive the trip through the galactic halo, the researchers said.

They even found a binary that could validate their discovery. Another star system IT Librae is a binary composed of two B-type stars (one step smaller than O-type stars), one of which is more massive than the other.

This larger star also appears to have too short a lifespan for the travel time it would have taken to reach its current position. But in a paper currently in press, a team of researchers explain that the two stars are in close binary, and that the smaller one has already started to transfer mass to the larger one.

This means that the current mass of the largest is misleading; because it started out smaller, its lifespan is probably longer than it currently looks.

“The observed properties of HD 93521 all appear to be in line with expectations for a fusion product. The star appears to be too young relative to its flight time from the galactic disk as it has been rejuvenated through stellar fusion of binary components “, said the researchers wrote.

“Investigations of such systems will provide important clues to the properties of post-mass transfer and fusion systems that are essential to understanding their ultimate supernova progeny.”

The research has been published in The Astronomical Journal.