Sky and Telescope - April 2018 - 29

p ONE MORE FOR LEO The dwarf galaxy DDO 68 (also known as UGC 5340) is a ragged collection of stars and gas clouds. At 40 million light-years
away, it's one of the closer oxygen-poor star-forming galaxies. This image combines visible and infrared observations.

Astronomers could try and subtract the stellar contribution
from the amount of helium the Sun was born with - about
27% by weight - and thereby extrapolate all the way back to
the Big Bang abundance.
"Extrapolate: We hate that word," says Evan Skillman (University of Minnesota). Far better to measure the helium level
in star-making galaxies so pristine their chemical composition
almost reflects that of the early universe. "It's really the only
way we have to estimate the primordial helium abundance,"
he says. In these galaxies, new stars emit ultraviolet light that

Star-Forming Galaxies with
the Lowest Oxygen Levels
Galaxy

Constellation

Oxygen Abundance
(hydrogen = 12)*

Approx. Distance
(light-years)

Lynx

6.98

620 million

Leoncino

Leo Minor

7.02

40 million?

SBS 0335-052W

Eridanus

7.13

190 million

Little Cub

Ursa Major

7.13

60 million?

I Zwicky 18

Ursa Major

7.17

60 million

Leo P

Leo

7.17

5 million

DDO 68

Leo

7.20

40 million

N ASA / ESA

J0811+4730

*Oxygen abundances are generally the average of the values for multiple regions within a single
galaxy. Question marks indicate a notably high uncertainty in distance.

strips electrons from helium atoms in the interstellar gas;
electrons that rejoin the helium atoms create spectral lines
revealing the element's abundance. In contrast, the many dim
dwarf galaxies that orbit the Milky Way, some of which have
still less oxygen, provide no such information because they
lack both gas and hot young stars (S&T: Mar. 2017, p. 16).
Past measurements of I Zwicky 18 and its kin have found
that helium made up a quarter of the chemical elements
emerging from the Big Bang, but different astronomers derive
slightly different numbers. In 2014, Izotov and Thuan's team
obtained a primordial helium abundance of 25.5%, which
is so high it suggests the number of neutrino types is more
likely to be four than three. But Skillman's team favors a
lower primordial helium level, around 24.5%, which implies
three neutrino flavors, in accordance with standard physics.
Now, having spotted three new oxygen-poor star makers in just the past three years, astronomers have three more
chances to derive the primordial helium abundance. If that
number comes in on the high side, physicists should be seeking a fourth type of neutrino. Thus these odd little galaxies
might tell us not only what the first galaxies looked like but
also how many flavors of "little neutral particles" there are
zipping through the cosmos.

¢ KEN CROSWELL earned his PhD for studying a decidedly
oxygen-rich galaxy, the Milky Way. His book about our galaxy,
The Alchemy of the Heavens, was a Los Angeles Times Book
Prize finalist. He has also written for National Geographic, New
Scientist, and Scientific American.
sk yandtelescope.com * A PR I L 2 018

29


http://www.skyandtelescope.com

Sky and Telescope - April 2018

Table of Contents for the Digital Edition of Sky and Telescope - April 2018

Contents
Sky and Telescope - April 2018 - Cover1
Sky and Telescope - April 2018 - Cover2
Sky and Telescope - April 2018 - 1
Sky and Telescope - April 2018 - Contents
Sky and Telescope - April 2018 - 3
Sky and Telescope - April 2018 - 4
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