skyandtelescope.com • NOVEMBER 2019 19Fraction of light blocked by dustSun- 9,000 – 6,000 –3,000 0 3,000 6,000 9,000
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Red Antares and its blue neighbors in Scorpius also reside )
in the Local Arm. They spangle the Scorpius-Centaurus
Association, the nearest group of massive stars, 400 to 500
light-years from Earth, right on the inner edge of the Local
Arm. Beyond is the darkness of the interarm region between
the Local and Sagittarius arms.
A 2017 paper analyzed different studies and plotted how
far the Sun and various arms lie from the galactic center.
The researchers put the middle of the Sagittarius Arm at
23,800 light-years, the Sun at 27,000 light-years, the middle
of the Local Arm at 28,000 light-years, and the middle of the
Perseus Arm at 33,600 light-years. These distances are along
the line joining the galactic center with the anticenter. Each
distance is probably uncertain by at least 1,000 light-years.
Alas, the separations between the Sagittarius, Local, and
Perseus arms carry some bad news for us Earthlings. In other
barred spiral galaxies, the major arms usually lie farther
apart, which implies that the Local Arm — though lively and
lustrous over its length — is not a full-fl edged arm that encir-
cles much of the galaxy. “I call it an armlet,” says Jacques P.
Vallée (Herzberg Astrophysics, Canada).
“The best word would be arm segment,” says Reid, who(continued on page 22)thinks the Local Arm could be a branch from the Perseus
Arm in the constellation Cygnus. Such forks split spiral arms
in other galaxies, too, including beautiful M101 in Ursa
Major. Still, over its length, the Local Arm seems as vigorous
as its neighbors in Sagittarius and Perseus.A Call to Arms
It’s not just radio observations that have sharpened our view
of the Milky Way. So have observations in the infrared. In
2005 data from the Spitzer Space Telescope led to a controver-
sial conclusion: “All arms are not created equal,” Benjamin
says. Instead, he explains, two of the main arms are grander
than the other two; they extend from either end of the Milky
Way’s central bar, while the two lesser arms originate else-
where in the galaxy’s inner sanctum.
Theory says a spinning galaxy can indeed develop two dif-
ferent types of spiral arms. The fi rst and more massive type
is a density wave, full of stars old and new. These arms are
so massive their gravity whips up additional arms midway
between them in the galaxy’s gas. These gas arms spawn new
stars but have no excess of old ones.
Benjamin considers the Milky Way’s two greater arms to
be the Perseus Arm and its twin, the Scutum-Centaurus Arm.
The latter arises from the near end of the central bar and
winds between the galactic center and the Sagittarius Arm,
approaching us in Scutum, a small constellation squeezed
between Aquila and Sagittarius. In Scutum we view the arm’s
tangent point, which explains the fi rst half of the Scutum-
Centaurus Arm’s name. The arm continues through Sagit-
tarius and Scorpius, then curls away from us in Centaurus,
where we view the arm’s other tangent point.uDUSTY ARMS Because dust is more concentrated within spiral arms
than between them, astronomers can map our galaxy’s structure by de-
termining how much light the dust blocks from stars of known distances
and luminosities. The Sun sits at center, and outside the dotted line
sources are too scarce for precise mapping. The burst pattern is merely a
byproduct of the analysis.1976
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