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Astronomers Keep Finding Stars That
Should Be Dead. Now, We May Finally Know
Why
MICHELLE STARR
5 FEBRUARY 2022
The most massive stars in the Universe are also the shortest-lived. The more
mass a star has, the more quickly it burns through its fuel reserves, resulting
in lifespans that are less than around 10 million years.
This fascinating fact leads us to a puzzle. Most of these stars are found
relatively close to the regions where they were born. But a number 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 far, in fact, that the travel time it would have taken to get there vastly
exceeds the lifespans of several of the stars.
"Astronomers are finding massive stars far away from their place of origin,
so far, in fact, that it takes longer than the star's lifetime to get there,"
said
astronomer Douglas Gies
of Georgia State University. "How this could
happen is a topic of active debate among scientists."
This absolute dilly of a cosmic pickle, having long perplexed 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, sitting in a sparsely populated
region called the galactic halo. That's quite a distance, so Gies and his
colleagues wanted to find out if there's 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 precision possible, in
three dimensions and including data on the motions and velocities of the
stars. They also carefully analyzed the spectrum of light the star is emitting,
to help determine its mass, age, and spin.
The Gaia data revealed that HD 93521 is about 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 about 17 times the mass of the Sun,
with an average temperature of roughly 30,000 Kelvin. At that mass and
temperature, the star should be around 5 million years old, with a margin for
error of about 2 million years. Its maximum lifespan is approximately 8.3
million years.
To migrate from its birthplace in the galactic disk to its current position,
however, would take a journey of roughly 39 million years.
This is a real head-scratcher, but the star itself could hold the key to the
mystery. Our Sun's rotation rate is just under 2 kilometers (1.24 miles) per
second. HD 93521 rotates at an absolute breakneck velocity of 435
kilometers (270 miles) per second.
There are several mechanisms that can increase the rotation speed of a star.
One of the biggest effects would be through a stellar merger, which would
combine not just the spins of the two stars, but also the angular momentum
of their orbit.
This is what the team thinks happened with HD 93521. It started its life as a
binary consisting of two middling-mass stars, which merged together to form
the star as we see it today in the relatively recent past.
These middling-mass stars would have long enough lifespans to survive the
journey into the galactic halo, the researchers said.
They've even found a binary that could validate their finding. Another star
system IT Librae is a binary consisting of two B-type stars (a step smaller
than O-type stars), one of which is more massive than the other.
That larger star, too, seems to be 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 explains that the two stars are in a close binary, and the
smaller one has already started transferring mass to the larger.
That means that the larger one's current mass is deceptive; because it
started out smaller, its lifespan is likely longer than it currently appears.
"The observed properties of HD 93521 all appear to agree with expectations
for a merger product. The star appears to be too young compared to its time
of flight from the Galactic disk because it was rejuvenated through the stellar
merger of the binary components,"
the researchers wrote
.
"Investigations of such systems will provide important clues about the
properties of post-mass-transfer and merger systems that are key to
understanding their final supernova progeny."
The research has been published in
The Astronomical Journal
.
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