Spiraling In
22 NOVEMBER 2019 • SKY & TELESCOPE
Both these great arms, he explains, possess not only gas
and young stars but also old stars: orange K and red M giants.
These cool aging stars glow in infrared light, and the Spitzer
data along the inner galactic plane show these old stars piling
up in Centaurus, one of the tangent points of the Scutum-
Centaurus Arm. Scutum may have a stellar excess, too, but
things are less clear because here the galaxy’s central bar also
contributes red giants.
In contrast, Benjamin saw no pileup of old stars in Aquila,
a tangent point of the Sagittarius Arm. This suggests the
Sagittarius Arm has gas and young stars but no surplus of old
stars. Ditto for the Outer Arm, because no rise in star counts
appears in Norma, the Outer Arm’s inner component.
In 2008, Benjamin’s team asked astronomer and art-
ist Robert Hurt (Caltech) to make a map that incorporated
recent discoveries. This map is so beautiful and informa-
tive that it has since appeared in publications ranging from
National Geographic to The Astrophysical Journal. The map
depicts the prominent Scutum-Centaurus and Perseus arms
leaving the Milky Way’s central bar, mirror images of each
other, with the Sagittarius and Outer arms less prominent.
In most other barred spiral galaxies, the bar also gives rise to
two dominant arms. This argues the Milky Way is similar.
“And my counterargument is the Milky Way is not like
them,” says Vallée, who thinks instead that the galaxy’s four
main arms — Scutum-Centaurus, Sagittarius, Perseus, and
Outer — are equal, each having both young and old stars. A
substantial minority of barred spirals, he notes, do sport arms
that are all equally prominent.Vallée says the Sagittarius Arm is so dusty it blocks
Spitzer’s view of the old stars there. Benjamin counters that
infrared light normally penetrates such dust. Also, in 2000
Ronald Drimmel (National Institute of Astrophysics, Italy)
saw only the two prominent arms in the infrared data he
analyzed.The Far Side: The Galaxy Imitates Art
In 2011, Dame and Patrick Thaddeus (Center for Astrophys-
ics, Harvard & Smithsonian) discovered that the Scutum-
Centaurus Arm wraps around not just the near side of the
galaxy but also its far side. The astronomers mapped molecu-
lar clouds some 50,000 light-years beyond the galactic center
and traced the arm from the constellation Sagittarius into
Aquila and Vulpecula.
Ironically, that extension is on the map Hurt had made
three years before. “I was shocked actually,” Benjamin says.
“I should have looked at that artist’s picture and had the
courage of its convictions.” Because the map shows symmetry
between the Scutum-Centaurus and Perseus arms, Dame and
Thaddeus argued that their discovery upped the chances that
the Milky Way is a fairly orderly galaxy, in contrast with the
many spiral galaxies whose structure is less striking.
Another breakthrough came in a 2015 paper, when Yan
Sun (Purple Mountain Observatory, China) and colleagues
detected a “New Arm” by tracking molecular clouds from
Cepheus and Cassiopeia into Perseus and Camelopardalis.
The team proposed that the feature may be a further exten-
sion of the Scutum-Centaurus Arm. “It’s just amazing that
the Scutum-Centaurus Arm would still be going,” Dame says.(continued from page 19)GALACTIC TWIN? Depending on their picture of the Milky Way, astrono-
mers favor one of these three galaxies as most resembling ours.NGC^395
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