Sky & Telescope - USA (2019-11)

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skyandtelescope.com • NOVEMBER 2019 17

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f the Milky Way were just an average galaxy, mapping its
structure would be a cinch. The average galaxy is a dim,
ghostly dwarf roughly a thousand light-years across, so
astronomers could easily chart it by measuring the parallaxes
of its stars to determine their distances.
But the Milky Way is a galactic colossus, one of the great
Goliaths of the cosmos. From end to end the spiral arms
inhabit a disk spanning some 120,000 light-years. So huge
is the galaxy that nearly every star the naked eye can see
belongs to the local spiral arm in which the Sun and Earth
dwell. As a result, many astronomers have long despaired of
ever knowing how our galaxy would appear from afar.
In recent years, however, visible, infrared, and radio
observations have delivered the crispest views yet of the
Milky Way’s structure. Surprises abound. At least one spiral
arm seems to make a complete turn around the galaxy. And
precise parallaxes of far-off stellar nurseries have upgraded
the status of the spiral feature lodging the Sun.
Best of all, the Milky Way may be much more majestic
than many of its spiral peers. “We potentially live in quite a
lovely barred spiral,” says Thomas Dame (Center for Astro-
physics, Harvard & Smithsonian), “and I wouldn’t have
thought that 15 years ago.”

Open Arms
The Milky Way has four main spiral arms, winding out from
its central regions and whipped up by its rapid rotation (S&T:
Apr. 2019, p. 14). If you were above the galactic plane, you’d
see the Milky Way’s disk spinning clockwise — opposite the
direction planets orbit the Sun. Because of this clockwise
rotation, the arms wend their way counterclockwise from the
inner galaxy to the outer. The same thing happens when you
stir cream into coffee clockwise: The resulting spiral arms
coil counterclockwise from center to edge.
Spiral arms squeeze interstellar gas and dust, making it
collapse and give birth to stars. The most massive of these
newborn stars are hot blue O- and early B-type stars that light

the arms. That’s what Ireland’s Lord Rosse saw in 1845 when
he spotted the fi rst spiral “nebula,” the Whirlpool Galaxy.
Because we are tangled in the thicket of the Milky Way’s
arms, a full century passed before astronomers established
that our galactic home is also a spiral. In 1951 William Mor-
gan (Yerkes Observatory) and his colleagues searched for
the red light of gas clouds that newborn O and early B stars
had ionized. These stellar nurseries lay along two parallel
arms (S&T: Jul. 1984, p. 10). Two years later, he discovered a
third spiral arm.
The names of these three arms fi rst appeared in a 1954
scientifi c paper from the Netherlands, where astronomers
were mapping them by observing radio waves from neutral
hydrogen gas. Unlike light, radio waves zip through the dust
that cloaks the Milky Way’s disk. After consulting Morgan,
the Dutch astronomers named the spiral arm that winds
through the Sun the Orion Arm for its best-known stellar
nursery, the Orion Nebula, which lies farther from the galac-
tic center than we do. Today, however, astronomers are more
likely to call this arm the Local Arm instead. The Sun is near
the Local Arm’s inner edge.
The next spiral arm between us and the Milky Way’s heart
took the name of the galactic center’s home, Sagittarius. The
Sagittarius Arm boasts such stellar nurseries as the Lagoon,
Omega, and Trifi d nebulae in Sagittarius, the Cat’s Paw
Nebula in Scorpius, and the Eagle Nebula in Serpens Cauda.
We view this arm’s fi rst tangent point, where it approaches

Recent years have seen tremendous strides
in unraveling the galaxy’s spiral arms, and the
next decade promises the best maps ever
made of our celestial home.

us, in Aquila, and its second tangent point, where it winds
away, in Carina. In fact, Carina hosts the Sagittarius Arm’s
famous resident Eta Carinae, a star system so luminous the
naked eye can see it despite its distance of 7,500 light-years.
On the other side of us, the arm just outside our own
became the Perseus Arm, after the young Double Cluster
h and Chi Persei. This, too, is visible to the eye, even though
no individual star in the Double Cluster surpasses the stan-
dard naked-eye threshold of magnitude 6. Despite coursing
through the outer galaxy, the Perseus Arm starts closer to
the galactic center than we are, arising from the far end of
the Milky Way’s central bar. The arm then spirals behind the
galactic center and curves halfway around the galaxy before
passing behind us at the galactic anticenter, the point oppo-
site the Milky Way’s core.
At least one spiral arm lurks beyond even Perseus. The
so-called Outer Arm is about 49,000 light-years from the
galactic center, measured along a straight line from center to
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