Sky & Telescope - June 2020 - 61

Zubeneschamali

ME THUSEL A H STAR: AKIR A FUJII; G A L A X Y
DIAG R A M: G REGG DINDER M A N / S&T

1 13 million

that of
suggested the star's iron fraction is at most /
the Sun. Spectroscopy also revealed weak absorption lines of
carbon, magnesium, and calcium in this ancient halo star.
In 2019, Frebel's team found another iron-poor star, SMSS
1605-1443. It has the lowest measured iron abundance, at
1.6 million times less than solar. This star also has 100,000
times less calcium and magnesium compared to the Sun.
While these stars have just traces of iron, they feature
a surprising overabundance of carbon. In the case of SMSS
1605-1443, for example, carbon is nearly 10,000 times more
abundant than iron compared to the same ratio in our
Sun. More recent observations of the iron-poor halo star
J0815+4729 reveal a similar overabundance of oxygen compared to iron. These results surprised astronomers, because
massive Pop III stars should die in spectacular supernovae
explosions that eject large amounts of iron as well as lighter
elements such as carbon, oxygen, magnesium, and calcium,
all of which should ﬁnd their way into the ﬁrst Pop II stars.
So where did the iron go? One possibility suggests mixingfallback supernovae. In this scenario, which might affect
relatively small Pop III stars of 10 to 20 solar masses, mixing
inside the collapsing core transports some heavy elements to
the core's outer layers, where they are ejected in the explosion
along with lighter elements such as carbon. But most of the
heaviest end products of nuclear fusion in the core - such as
iron, nickel, and zinc - would fall back into the black hole or
neutron star formed at the star's core, explaining the relative
lack of iron. A 2017 study by Ke-Jung Chen (National Astronomical Observatory of Japan) and colleagues made detailed
computer simulations of mixing-fallback supernovae for
Pop III stars. Using stars of 10 to 60 solar masses, they successfully explained the observed metal abundances in stars
such as SMSS 1605-1443 and SMSS 0313-6708.

because Pop I stars, which are younger, formed
from gas in the interstellar medium that was
enriched by heavy elements ejected from many
generations of dying stars. The Sun is a Pop I
star, as are most other bright stars visible to
the unaided eye. The older Pop II stars have far
lower metal abundances than the Sun because
they formed from less enriched material, earlier
in the history of the universe. The 7th-magnitude
star HD 140283 in Libra (see image above) is an
example of a Pop II star.
Astronomers introduced Population III stars
as a concept in the 1960s. These stars have yet
to be observed, but they would have no heavy
elements because they formed out of primordial hydrogen, helium, and tiny traces of lithium
only 100 to 200 million years after the Big Bang,
before heavier elements existed.

Methuselah Star

Zubenelgenubi
γ

ν
Antares

p METHUSELAH STAR Astronomers realized in the 1950s that the 7thmagnitude star HD 140283 in Libra has a surprisingly low level of iron.

Another explanation involves pulsational pair-instability
supernovae, which would afﬂict Pop III stars of 100 to 140
solar masses. According to Stan Woosley (University of
California, Santa Cruz), who has extensively modelled these
supernovae, such stars ﬁrst lose mass to stellar winds that
might themselves be rich in carbon, nitrogen, and oxygen. Then they pulse violently as the dense soup of nuclei
and gamma rays in their cores form pairs of electrons and

Halo
Globular
cluster

Bulge

Disk

Globular
cluster

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Sky & Telescope - June 2020

Table of Contents for the Digital Edition of Sky & Telescope - June 2020