Sky & Telescope - March 2019 - 10

NEWS NOTES

Puny Star Might Be Survivor from Early Universe
THE FIRST STARS to light up the uni-

verse - known as Population III stars
- were likely giants, hundreds of times
heftier than the Sun. But a recently
discovered puny star in our galaxy
might be an ancient specimen that
shows how the ﬁ rst stellar generation
could have sprouted some runts that
still live among us.
The star in question, estimated to be
about 13.5 billion years old, contains
few elements heavier than helium, a

p The 13.5-billion-year-old binary system
2MASS J18082002-5104378 lies in the yellow
box just off the plane of our galaxy.

sign that it was born before other stars
spewed lots of atoms such as carbon,
oxygen, and iron into space. Astronomers know of similarly pristine stars,
but two things make this one stand
out. It's just 14% as massive as the
Sun, barely weighty enough to ignite
fusion, and it's the slimmer half of a
binary system, which gives a clue about
how it formed.
Kevin Schlaufman (Johns Hopkins
University) and colleagues report in the
November 10th Astrophysical Journal
that the star likely formed via disk fragmentation - gas clumping together in a
disk that once swirled around the more
massive of the duo. Researchers have
debated whether or not this can happen
around stars with a paucity of elements
heavier than helium.
If disk fragmentation can happen for
10

M A RCH 2 019 * SK Y & TELESCOPE

stars as pristine as these two - which
aren't themselves from the ﬁrst stellar
generation - then it should also have
been possible for Sun-sized stars to form
in disks around gargantuan Population
III stars, the team argues. Unlike ﬁrstgeneration behemoths, which would
have shone brieﬂy then exploded long

ago, their low-mass bretheren might
still be with us today.
"That's the reason we should keep
looking for low-mass Population III
stars in the galaxy," says Schlaufman.
"We shouldn't despair. There's good reason to think they're still there."
■ CHRISTOPHER CROCKETT

COSMOLOGY

Starlight Tally Reveals History of Star Formation
FOUR THOUSAND octillion octillion

octillion - or 4x1084. That's roughly the
number of photons emitted by all the
stars in all galaxies throughout time.
Using this new tally, astronomers have
pieced together an updated timeline of
how the rate of star formation has waxed
and waned over cosmic history, researchers report November 30th in Science.
The new timeline jibes with earlier
independent estimates: The cosmic starforming factories were at their most
proliﬁc about 3 billion years after the
Big Bang and have been gradually slowing down ever since. At its peak, the
universe churned out 10 times as many
stars per year as it does now. "Our
galaxy and the entire universe were lit
up like a Christmas tree," says coauthor
Marco Ajello (Clemson University).
Starlight that escapes galaxies contributes to a haze of photons known as
the extragalactic background light. This

light is tricky to measure directly. So a
team of researchers known as the Fermi
LAT collaboration looked at how this
light interferes with gamma rays emitted from blazars, wellsprings of radiation powered by supermassive black
holes at the cores of galaxies.
To make their tally, researchers
analyzed data on 739 blazars (and one
gamma-ray burst) collected over 9
years by the Fermi Gamma-ray Space
Telescope. The light from these blazars
took anywhere from 200 million to
11.6 billion years to reach Earth. By
measuring how speciﬁc wavelengths of
gamma-ray light from each blazar have
been suppressed, the team estimated the
intensity of the extragalactic background light at various epochs. This in
turn enabled the researchers to reconstruct the rate of star formation across
most of cosmic time.
■ CHRISTOPHER CROCKETT

q This map of the entire sky from the Fermi Gamma-ray Space Telescope marks the location of 739
blazars (green crosses) used to estimate the extragalactic background light.

PUN Y STA R: BELE TSK Y (ESO) / 2M ASS / DIGITA L SK Y SURV E Y;
STA R FOR M ATION: N ASA / DOE / FER MI L AT COLL A BOR ATION

STARS

Sky & Telescope - March 2019

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