Sky & Telescope - August 2024 - 40

Stellar Science
Core
Core
Radiation
pressure from
infalling hot
gas
Gravity
Radiation
pressure from
infalling hot
gas
Gravity
BLACK HOLE SEEDS In the early universe, some massive stars created black holes when they went supernova. But it's also possible that other
black holes formed inside humongous, quasi-star objects, replacing what would normally be a core powered by nuclear fusion.
of this [gas] . . . into a bound object, then you're done, " says
Ferrara, who studies star formation in the distant universe.
" They would live a very short time, and then they would collapse
into a black hole. "
It's unclear how truly starlike these objects would be.
Hydrogen fusion might briefly ignite inside a supermassive
star. But it's also possible that the supermassive star might
skip the fusion phase altogether. In that case, the core would
be so dense that it would collapse into a black hole without
throwing off the immense weight of the surrounding layers.
What's left is a " star " with a black hole at its center, which
eats the object from the inside out. Material falling into the
black hole would heat up as it falls inward, and this heat
would make the quasi-star glow. These objects aren't stars in
the traditional sense but are a new type of object altogether.
Perhaps, then, we can define the maximum size of a star
as the greatest mass at which a supermassive star will sustain
fusion instead of collapsing directly. Unfortunately, we don't
know exactly what that mass is. Both the calculations and
the observations are complex, and the maximum mass varies
depending on the effects of general relativity on a supermassive
star's stability and how quickly gas accretes onto the
star. This process could lead to stars on the order of tens of
thousands of solar masses.
But that, of course, assumes supermassive stars ever
existed, and that we have gotten all of the physics right.
Could Some of the First Stars
Still Exist Today?
While massive stars have short lifetimes, small
stars can have very long lifetimes - longer than
the current age of the universe. If some very small
Population III stars managed to form in addition
to the big ones, then those small, pristine stars
may still exist in the modern universe. To find out,
astronomers have turned to Gaia, Europe's space
telescope tasked with mapping billions of stars in
the galaxy. Gaia has been unable to find a single
star lacking elements heavier than helium. While
it's possible that small Population III stars may be
hiding where we haven't found them yet, it's also
possible that they weren't small at all.
The Search Continues
So how massive can a star be? We've come a long way in the
past two decades, but we still don't have an exact answer. Star
formation is a complex process, and there are many factors
that can influence how large a star is when it's born. In our
local universe, it seems that stars can reach hundreds of solar
masses, while in the early universe, they may have been thousands
or even tens of thousands of solar masses.
More definitive answers will not come easily. Some astronomers,
like Higgins, are working to find a way to estimate
stars' birth masses based on how the stars look now. Others
are introducing more complexity into simulations of the
messy physics of star formation. And explorations of the early
universe - both in observation and theory - continue.
For now, the safest answer is the simplest one: We don't
know.
¢ ELIZABETH FERNANDEZ is a science writer with a PhD in
astronomy. She writes on the hard sciences as well as science
and society and has read S&T since she was young.
40 AUGUST 2024 * SKY & TELESCOPE
EXPLOSION: BEATRIZ INGLESSIS / S&T; BLACK HOLE: SUDAKARN VIVATVANICHKUL / SHUTTERSTOCK.COM

http://www.SHUTTERSTOCK.COM

Sky & Telescope - August 2024

Table of Contents for the Digital Edition of Sky & Telescope - August 2024