Astronomers keep finding stars that should be dead. Now we may finally know why – Archyde

The most massive stars in the universe are also the shortest lived. The more massive a star is, the faster it burns up its fuel reserves, resulting in a lifespan of less than about 10 million years.

This fascinating fact leads us to a conundrum. Most of these stars are relatively close to the regions where they were born. But some of them have been found lurking in strange pockets of the Milky Way, far from the galactic disk where star formation takes place; in other words, their birthplaces.

So much so that the travel time it would have taken to get there far exceeds the lifespan of some stars.

“Astronomers find massive stars far from their point of origin, so far away, in fact, that it takes longer than the star’s lifetime to get there.” said the astronomer Douglas Gies from Georgia State University. “How this happened is a topic that is actively debated among scientists.”

This absolutely stupid cosmic cucumber that has long puzzled astronomers may now have an explanation thanks to new research.

The focus of the study was a star named HD 93521. This is an O-type star, the most massive category of stars on the main sequence. HD 93521 is also about 3,600 light-years from the galactic disk and is located in a sparsely populated region called the galactic halo. That’s quite a distance, so Gies and his colleagues wanted to see if there was a reasonable way it could have gotten there.

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

The Gaia data showed that HD 93521 is about 4,064 light-years from Earth and the 3,600 light-years from the galactic disk mentioned above.

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

However, to migrate from its birthplace in the galactic disk to its current position would require a journey of approximately 39 million years.

This is a real head-scratcher, but the star itself may hold the key to the mystery. The rotation speed of our sun is almost 2 kilometers per second. HD 93521 is spinning at an absolute breakneck speed of 435 kilometers (270 miles) per second.

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

The team believes this is what happened with HD 93521. It began life as a binary composed of two intermediate-mass stars that merged relatively recently to form the star we see today.

These intermediate-mass stars would have long lifespans to survive the journey into the galactic halo, the researchers said.

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

Even this larger star seems too short-lived 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 a close binary system and the smaller one has already started transferring mass to the larger one.

This means that the current bulk of the larger is deceptive; Since it started out smaller, its lifespan is likely longer than it currently appears.

“The observed properties of HD 93521 all appear to be consistent with expectations for a fusion product. The star appears too young compared to its flight time from the galactic disk because it has been rejuvenated by the stellar merger of the binary components.” the researchers wrote.

“Studying such systems will provide important clues to the properties of post-mass transfer and fusion systems, which are key to understanding their ultimate supernova progeny.”

The research was published in The Astronomical Journal.

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