Australian Sky & Telescope - May 2018

34 AUSTRALIAN SKY & TELESCOPE May | June 2018

NASA / ESA

Astronomerscouldtryandsubtractthestellarcontribution

fromtheamountofheliumtheSunwasbornwith—about

27%byweight—andtherebyextrapolateallthewaybackto

the Big Bang abundance.

“Extrapolate:Wehatethatword,”saysEvanSkillman

(UniversityofMinnesota).Farbettertomeasurethehelium

level in star-making galaxies so pristine their chemical

composition almost reflects that of the early universe. “It’s

reallytheonlywaywehavetoestimatetheprimordial

helium abundance,” he says. In these galaxies, new stars emit

Star-Forming Galaxies with

the Lowest Oxygen Levels

Galaxy Constellation Oxygen Abundance

(hydrogen = 12)*

Approx. Distance

(light-years)

J0811+4730 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

*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.

ultraviolet light that 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.

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 flavours, 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 flavours 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 inalist. He has also written for National Geographic,

New Scientist and Scientific American.

SONE 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.

PRIMORDIAL PROXIES