Sky & Telescope - August 2024 - 36

Stellar Science
The Perfect Sample: Arches
In 2005, astronomer Donald Figer (now at Rochester Institute
of Technology) found a perfect place to obtain this statistical
sample: the Arches cluster. Arches is a beautiful open cluster
in the constellation Sagittarius. As the densest cluster in the
Milky Way, it contains 11,000 solar masses' worth of stars. If
the area around our Sun were as populated as Arches is, there
would be more than 100,000 stars between us and our nearest
star, Proxima Centauri.
Another reason why Arches is a perfect place to search for
massive stars is that it is young. Massive stars live fast and die
young, burning through their nuclear fuel within a few million
years. As far as clusters go, Arches is a baby at 2½ million
years old. To put this in perspective, our hominid ancestors
were walking on Earth before Arches formed. Figer calculated
that Arches was young enough that if stars up to 500 times
the mass of our Sun had formed there, they would still be
shining today.
Observing Arches is not easy, however. It's 25,000 lightyears
from Earth, and its stars are packed so tightly that they
are difficult to resolve individually.
Yet, even with these difficulties, Figer found stars with up
to 130 times the mass of our Sun. But he didn't find any stars
even close to 500 solar masses. Figer concluded that stars of
150 solar masses or more simply couldn't form.
The results were convincing. After that, astronomers used
150 solar masses as the limit for how large a star could be.
But the Tarantula told a different story.
Massive Nebula, Massive Stars
The Tarantula Nebula, located in the Large Magellanic Cloud
some 160,000 light-years from us, is one of the most massive
nebulae known. Light takes more than 500 years to traverse
its extremes. It is so huge that if it were located where the
Orion Nebula is (some 1,500 light-years from Earth), it would
have an angular size of about 20°, about the diagonal size of
the Great Square of Pegasus.
Such a giant nebula creates an opportunity for more
massive stars to form, explains Paul Crowther (University of
Sheffield, UK). Only a small percentage of gas in a nebula will
end up in stars, so the more massive a nebula is, the more raw
materials it has to work with. If there are stars larger than
150 times the mass of our Sun out there, then the Tarantula
Nebula is an ideal place to look for them.
When the Gemini South telescope in Chile set its sights
on the middle of the Tarantula Nebula in 2021, it was able to
resolve stars in the massive cluster at its center, R136. A star
within this cluster, R136a1, is the most massive star yet discovered.
Various studies peg it at anywhere from 200 to more
than 300 times the mass of our Sun.
R136a1 is not alone. There are hundreds of massive stars
lurking near the center of this nebula, ranging from a few
tens to hundreds of solar masses.
These stars may have even started out more massive than
we see them today, says Erin Higgins (Armagh Observatory
and Planetarium, Northern Ireland). That's because very
massive stars often have strong winds, which propel gas out
and away, whittling the star down over time. These winds can
be so strong that the star can lose a fifth of its mass within a
million years.
Observations from the Tarantula Nebula indicate that
perhaps the limit to stellar mass is around 300 Suns - that is,
if the stars there didn't begin life larger.
But again, is this the whole story? Is there any way to build
an even bigger star? To answer this question, astronomers
turn to a second approach besides observations: the theory of
how stars form.
A Galactic Cookbook: Stellar Edition
In order to build a massive star, you need a lot of gas. Pressure
from the motions of warm gas counteracts the cloud's own
gravity and keeps it from collapsing. In this way, a nebula can
remain stable . . . for a while.
Compression can come along and upset this balance. A
nearby supernova could compress the gas, or a shock wave
from a collision with another nebula or galaxy could ripple
through a cloud to initiate collapse. Density waves within a
galaxy can also squeeze the gas - we see the results of these
waves in the starbursting spiral arms of many galaxies (S&T:
Mar. 2023, p. 14). But compression from the outside isn't
always necessary: Perhaps the gas is simply clumped just
enough that its own gravity wins.
ARCHES CLUSTER This cluster lies some 25,000 light-years from
Earth, in the galactic center, and is the densest known star cluster in the
Milky Way. Dusty clouds obscure the cluster from view in visible light;
this is an infrared image.
36 AUGUST 2024 * SKY & TELESCOPE
NASA / ESA
Sky & Telescope - August 2024

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