Sky & Telescope - February 2023 - 30

ALAN DYER
EXPLORE THE GALAXY by Ken Croswell
The Cepheus Spur
A
spectacular spiral galaxy is a majestic sight. The galaxy's
rapid rotation whips up spiral arms where interstellar
gas and dust spawn new stars, setting the arms
aglow with blue and red supergiants as well as pink clouds of
ionized hydrogen gas. In the greatest spirals, like the Whirlpool
(M51), arms wrap around the entire galaxy.
But take a closer look at an excellent image of the Whirlpool
and you'll also see plenty of irregularities, especially in
the galaxy's outskirts: A spiral arm splits in two; spurs and
feathers sprout from the arms; bridges of stars connect one
spiral arm to another. It's easy to discern all these details
on a Hubble image taken from M51's distance of 28 million
light-years, but it's a lot harder for any astronomers trapped
inside that galaxy to do the same.
Which brings us to the eternal problem of the Milky Way,
whose structure we cannot see from the outside. Because
of our location, astronomers discovered spiral arms in our
galaxy only in 1951, a full century after seeing them in the
Whirlpool. Even today, no one knows whether the Milky
Way is a stunning grand-design spiral like the Whirlpool,
a beautiful multi-arm spiral like M101, or a less impressive
THE OUTER MILKY WAY This panorama looks in the
direction opposite to the galactic center and covers about
200° of galactic longitude, from Cygnus (right, setting in
the evening sky) to Puppis (left, low in the southern sky
at midnight). Several of the naked-eye stars lie in a bridge
that links our spiral arm with the Perseus Arm just beyond.
A newly named feature in our galaxy drapes distant luminaries across the northern sky from
Cepheus and Camelopardalis to Auriga, Gemini, Taurus, and Orion.
flocculent spiral like M33.
We do know, however, that we live near the inner edge of
the Local Arm. This arm supplies almost every star the naked
eye can see (S&T: Nov. 2019, p. 16). Just as Earth lies between
the orbits of Venus and Mars, so the Local Arm lies between
the Sagittarius and Perseus arms. It's in these and other spiral
arms that the Milky Way Galaxy mints most of its stars.
But as in the Whirlpool Galaxy, there are exceptions,
because new stars can arise in interarm regions, too. Using
the Gaia spacecraft to obtain precise parallaxes and distances
of luminous blue O and B stars (see page 34), astronomers in
Spain, Chile, and the United States recently mapped a long
bridge of stars that stretches from the Local Arm out toward
the Perseus Arm. The astronomers named this 9,000-lightyear-long
feature the Cepheus Spur in their 2021 paper. At its
heart are six OB associations, which are loose groups of massive
young stars bearing spectral types O and B as well as the
white, yellow, orange, and red supergiants that have evolved
from such stars.
The six OB associations " are connected like a chain of
beads, connecting these two arms, " says Michelangelo Pan30
FEBRUARY 2023 * SKY & TELESCOPE

Sky & Telescope - February 2023

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