28 AUSTRALIAN SKY & TELESCOPE May | June 2018
These galaxies have proved to be so rare that I Zwicky 18
remained the most oxygen-poor star-forming galaxy known
for three decades. Eventually, however, it lost its crown to a
blue galaxy three times farther away in Eridanus. The galaxy,
named SBS 0335-052W, is part of a larger, oxygen-poor
system and is creating new stars. In 2005, Yuri Izotov, Natalia
Guseva (Main Astronomical Observatory, Kiev, Ukraine), and
Thuan proclaimed this upstart galaxy the new champion. It
has an oxygen level of 7.13.Blue but not new
Astronomers once suspected that I Zwicky 18 might be
a galactic infant, having formed all of its stars recently.
Indeed, the galaxy derives its blue hue from bright, massive
newborn stars, which wouldn’t have had time to enrich their
surroundings with oxygen.
Deeper observations, however, have revealed much older
stars. Now astronomers think that the galaxy stands outIn the Milky Way’s disk,
gas makes up 99% of
interstellar matter and
dust 1%. In I Zwicky 18,
however, dust accounts
for a mere 0.001% of
interstellar matter.
has spent decades hunting for more in the hopes of finding a
galaxy so extreme it has no oxygen at all. The search is worth
it, because these curious systems also carry news from the
very first minutes of the universe’s life.Fresh air
Oxygen is an excellent element to study in star-forming
galaxies. Of all the ‘metals’ in the universe, oxygen is the
most abundant. Oxygen is also the second most common
element in Earth’s air (after nitrogen) and its interior (after
iron). In most stars, however, oxygen produces few spectral
lines, making its abundance difficult to gauge. But when hot
stars ionise interstellar gas, oxygen atoms in the gas glow at
visible and near-ultraviolet wavelengths that astronomers
can detect. Comparing the strengths of these emission lines
with those of hydrogen reveals oxygen’s abundance relative to
hydrogen, the most common element in the universe.
Astronomers often express abundances using a scale on
which the hydrogen level is always 12. This scale is logarithmic,
so 11 means an element is one-tenth as abundant as hydrogen,
10 means the element is one-hundredth as abundant as
hydrogen, and so on. The Milky Way is far bigger and brighter
than most other galaxies, and its many stars have blessed it
with lots of oxygen — good news for those of us who like to
breathe it. Surprisingly, the Sun’s exact oxygen abundance is
controversial, but it is probably around 8.76. If so, the Sun has
1 oxygen nucleus for every 1,740 hydrogen nuclei.
Because stars create oxygen, galaxies with fewer stars
have lower oxygen levels. For example, the Milky Way’s two
brightest satellite galaxies — the Large and Small Magellanic
Clouds — have oxygen abundances around 8.35 and 7.95,
respectively, giving them levels that are 39% and 15% solar. I
Zwicky 18, the long-time champ among oxygen-deprived star-
making galaxies, has an oxygen level of only 7.17. That’s just
2.6% of the solar value.PRIMORDIAL PROXIES