Sky & Telescope - August 2024 - 39

BEATRIZ INGLESSIS / S&T
Supermassive stars
100,000?
tive abundances in early galaxies. These chemical patterns
can tell us about the mass of stars that produced them.
The chemical makeup of one of the most distant galaxies
ever imaged, GN-z11, may indicate gargantuan stars have
formed there. This galaxy, located only about 440 million
years after the Big Bang (at a redshift of about 11), has high
levels of nitrogen and low levels of oxygen - in fact, the ratio
of nitrogen to oxygen is four times what is seen in our Sun. A
group of astronomers led by Corinne Charbonnel (University
of Geneva, Switzerland) recently suggested that this overabundance
of nitrogen implies that GN-z11 hosts truly massive
stars - on the order of 5,000 to 10,000 solar masses!
However, Chiaki Kobayashi (University of Hertfordshire,
UK) and Andrea Ferrara (Scuola Normale Superiore, Pisa,
Italy) suggest that such an abundance pattern could instead
be created by starting, stopping, and then re-starting star
formation. In this scenario, hot, blue Wolf-Rayet stars on
the order of about 100 solar masses add abundant nitrogen
after the second burst of star formation. Another group led
by Roberto Maiolino (University of Cambridge, UK) suggests
that this abundance pattern could be created by an accreting
black hole of about 1 million solar masses.
One specific chemical abundance pattern is made by
what are called pair-instability supernovae. Theoretically, for a
Population III star between about 140 and 260 solar masses,
conditions in the star may be just right so that the gamma
rays created by nuclear fusion start converting into electronpositron
pairs. Unlike photons, these particles do not exert
the pressure necessary to hold the star up against the force of
gravity. The core collapses.
The collapse triggers more fusion, creating more gamma
rays, which in turn create more electron-positron pairs.
The core suddenly ignites in a runaway reaction, causing an
explosion that completely demolishes the star: No neutron
star or black hole is left behind, and all of the elements produced
during the star's life and death are thrown into space,
enriching the surroundings.
A pair-instability supernova should leave behind a very
particular chemical abundance pattern, one that many
astronomers are searching for. If detected, it would mean
that Population III stars can reach at least 130 solar masses.
Astronomers have found a star with hints of this pattern, but
it has yet to be confirmed.
True Stellar Monsters
There's one final reason to suspect that early stars could have
been large - very large. If some stars were incredibly massive,
then that could help explain one of the biggest
mysteries in astrophysics: how the first big black
holes formed.
In almost every large galaxy lurks a
supermassive black hole, weighing on the
order of millions to billions of times
the mass of our Sun. Observations
show that massive black holes
Rigel
21
Antares
12
Sun
1
were in place within galaxies when the
universe was only a few hundred million
years old (S&T: May 2024, p. 20).
If these massive objects started their
lives when a Population III star collapsed
into a black hole, packing all of
its roughly 100 solar masses inside, then
it's difficult to explain how the black
holes grew by a factor of 1,000 or more
so quickly.
Some astronomers thus claim that
supermassive stars might have been
the seeds of these monstrous black
holes instead. These gigantic stars,
with tens to hundreds of thousands
of solar masses, would have collapsed
directly into black holes. To form one
of these massive stars, a huge amount
of gas would need to collapse, without
fragmenting into smaller chunks. This
requires a careful balance of conditions
that is difficult to achieve but might be
possible in the early universe.
" In principle, there is gas, and if
you're able to find a way to condense all
Population
III
200-1,000
R136a1
~250
Earendel
20-200
Largest
Arches
stars
130
Stellar masses, in Suns
JUST HOW MASSIVE? Familiar stars such as Antares and Rigel are
significantly more massive than the Sun, but even these are puny when
compared with the behemoths found in intense star-forming regions in
the Milky Way and other galaxies. Astronomers speculate that in the
earliest days of the universe, stars may have been thousands of solar
masses or more.
sk yand tele scope .o r g * AUGUST 2024 39
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