skyandtelescope.com • NOVEMBER 2019 23The molecular clouds reside beyond even the Outer Arm,
lying 46,000 to 71,000 light-years from the galactic center.
The discovery suggests the Scutum-Centaurus Arm makes
more than a full turn around the Milky Way.
Recent work has further elucidated the Scutum-Centaurus
Arm. In 2017, Alberto Sanna (Max Planck Institute for Radio
Astronomy, Germany) and colleagues measured the parallax
of a maser on the far side of this arm, in the constellation
Sagittarius, 67,000 light-years from Earth. This work indicates
how fast the outer Milky Way spins on the opposite side of
the galaxy and thus provides a glimpse of the Milky Way’s
rotation curve there.
Other recent work has illuminated the Perseus Arm.
European astronomers looking near the galactic anticenter
reported in 2015 that they saw an excess of stars in the Per-
seus Arm, some of which were old enough to have circled the
galaxy more than once. The astronomers also detected dust
in front of these stars. Similar dust lanes often line the inner
edges of arms in other spiral galaxies.
But some parts of the Perseus Arm seem barren. “I’m
beginning to get the feeling, especially for the Perseus Arm,
that once you go too far into the outer galaxy, demanding
that things are continuous and well-ordered is maybe asking
too much of the Milky Way,” Benjamin says. The Perseus
Arm lacks much molecular gas and star formation over long
stretches in Vulpecula and Cygnus. Such gaps exist in the
outer arms of other spiral galaxies, too, including M101.
Despite recent progress, maser parallaxes currently suffer
from a blind spot. Most radio telescopes are in the Northern
Hemisphere, but mapping the Milky Way’s southern domain
requires radio telescopes in the Southern Hemisphere. To
rectify this problem, Reid is working with astronomers in
Australia and hopes to have a better map of the Milky Way
in fi ve years.
Still, even with our present understanding, the Milky Way
now appears to be a giant and fairly symmetric barred spiral
galaxy with four main arms — perhaps equal to one another,
perhaps not — that may be a little ragged around the edges.
And we ourselves belong to the vibrant Local Arm, whose
massive stars, leading bright but brief lives, add splendor to
the night.The Milky Way’s Twin
Of all the galaxies that populate the cosmos, there must be
some that look like the Milky Way. With our new knowledge
of spiral structure, which galaxy do astronomers think most
resembles ours?
Dame and Reid both cite UGC 12158, an attractive barred
spiral 450 million light-years away in Pegasus. “You can
match it up pretty well with the Milky Way in many ways,”
Reid says.
Vallée points to another barred spiral, NGC 6744 in Pavo,
30 million light-years from us. “It’s a grand design with four
arms,” he says, and the arms are equally prominent, like his
model for the Milky Way.
“My fave is NGC 3953,” Benjamin says, referring to a barred
spiral in Ursa Major some 50 million light-years away. “It’s the
kind of galaxy that has enough order so that you can see it as
orderly but enough messiness that you can see it as messy.”
And this may be the best news of all for Earthlings: The
three galaxies these scientists choose are all stunning and
striking celebrations of light. Thus, not only do we live in
one of the largest of galaxies, but we may also be fortunate
enough to inhabit one of the most beautiful.¢KEN CROSWELLearned his PhD at Harvard University for
studying the Milky Way, and his 1995 book about the galaxy,
The Alchemy of the Heavens, was a Los Angeles Times Book
Prize fi nalist.What’s in a Name?
Astronomers have sometimes used different names for
individual spiral arms, which can cause confusion. Here’s
a list of some spiraliferous pseudonyms:Scutum-Centaurus Arm:
Scutum, Centaurus,
Scutum-Crux
Sagittarius Arm:
Sagittarius-Carina
Local Arm: Orion Arm,
Orion Spur, Local SpurPerseus Arm: always
called Perseus
Outer Arm: Perseus+1,
Cygnus-Norma
New Arm: always called
New (at least for now)UGC 12158