Sky & Telescope - June 2020 - 62

The First Stars

positrons that self-annihilate, causing the core to partially
collapse and rebound. They eject what was left of their hydrogen envelope and the outer part (maybe up to 5 to 10 solar
masses) of their helium, along with appreciable C, O, Ne,
and Mg. After the pulsations, the remainder of the star collapses to a black hole, capturing the iron remaining near the
core. Very little of the iron - if any - escapes, and very little
silicon and calcium.
Explaining the overabundance of carbon in iron-poor stars
got more complicated in 2019, when Rana Ezzeddine (then at
MIT) and her collaborators measured UV absorption lines of

zinc in HE 1327-2326 using the Cosmic Origins Spectrograph
on the Hubble Space Telescope. In HE 1327-2326, zinc is not
only present but six times more abundant than iron. Zinc is
created from iron during supernova explosions, but standard
mixing-fallback and pulsational pair-instability supernovae
cannot produce such large amounts. Instead, the study suggests a significantly asymmetric supernova of a fast-rotating
25 solar-mass star would be energetic and unstable enough
to power a pair of jets that would emerge from the core itself.
The jets would pull up zinc from deep inside and eject it, even
as most of the iron falls into the newly made black hole.

MAKING THE OLDEST STARS
2. First stars die in powerful
supernovae, seed surrounding
gas with heavy elements

3. New stars form from this gas,
marked by first stars' chemical
fingerprints

Some low-mass first stars might survive
today, polluted by long-gone companions. Their atmospheres would have
unique chemical patterns.

Some first stars might form
with low-mass companions.

C
O
C

N
He,
H,

shells
ted
ec
O, Ne, M
C,
g

Ej

Based on the chemical patterns in the
oldest Population II
stars, astronomers
have suggested several possible ways
the universe's first
stars died.

W
ind

THE FIRST SUPERNOVAE

O
Si and
heavier
elements
Collapsing core
(Si and heavier
elements)

Not to scale

62

MIXING-FALLBACK: As the core explodes, the gravity of the newly formed neutron star/black hole drags on
expanding ejecta, causing it to fall back and mix. The star
ejects large amounts of carbon and oxygen, but elements
heavier than silicon mostly end up in the compact remnant.

JUNE 2 02 0 * SK Y & TELESCOPE

PULSATIONAL PAIR-INSTABILITY: Stars lose
their outer layers over time and in a series of
pulses, ejecting carbon, oxygen, and elements of
similar atomic mass. Heavier elements are trapped
in the core when it collapses.

G REGG DINDER M A N / S&T (2)

1. First-generation stars form



Sky & Telescope - June 2020

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