Sky & Telescope - November 2019 - 18

Spiraling In

The Local Arm: Spurning the Spur
Decades ago, some astronomers had downgraded the Local
Arm we inhabit to just a spur jutting off from either the Sagittarius or Perseus arms. But new observations have partially
revived the original claim that the local feature is an arm in its
own right - which is great news for us Earthlings, because it's
far more exciting to live in a bright and well-lit spiral arm than
a mere spur or, even worse, a dark and dreary interarm region.
You can thank radio astronomers for our new and
improved standing in the galaxy. "The Local Arm is bigger
and more massive and more active in star formation than
people had thought," says radio astronomer Mark Reid (Center for Astrophysics, Harvard & Smithsonian). "I don't think
this is anything like a spur."
Radio astronomers can link their telescopes into arrays
that span entire continents, enabling the measurement of
precise parallaxes across vast distances. Furthermore, the
same young massive stars tracing the spiral arms also energize electrons in interstellar molecules such as water and
methanol. The molecules then emit intense microwaves and

BUILDING THE MAP Plotting the locations of young, massive collections of stars provided early glimpses of our galaxy's spiral arms
(left). Maps of neutral hydrogen gas extended our view farther (center).
Astronomers eventually identified four large arms (right). Note: Galactic
coordinate systems changed between the second and third maps.

become masers, allowing a radio astronomer to measure the
parallaxes and thus the distances of the stellar nurseries.
"Maser parallaxes are vital," says Robert Benjamin (University of Wisconsin, Whitewater), who is not part of a team
measuring them. "They remove one of the major uncertainties of trying to do galactic cartography."
The recent work has found new distances to numerous
star-forming regions in Cygnus, where we look down the
Local Arm but also see the more distant Perseus Arm. "Many
masers thought to be in the Perseus Arm are instead associated with the Local Arm," says team member Ye Xu (Purple
Mountain Observatory, China).
"That was a surprise," says Dame, another team member.
"The Local Arm is a much larger feature than previously
thought. The amount of star formation in the Local Arm
seems comparable to Sagittarius and Perseus on either side.
That's another surprising result."
Reid estimates the Local Arm is about 16,000 light-years
long and runs about 30° around the galaxy. Xu says its length
could exceed 20,000 light-years.
The Local Arm is certainly impressive to stargazers. All
stars ﬁrst-magnitude and brighter are in the Local Arm, even
the four farthest: Antares in Scorpius, Deneb in Cygnus, and
Rigel and Betelgeuse in Orion. So are nearly all other nakedeye stars. For example, when you look at Sagittarius, the
eight stars that compose its beautiful teapot are foreground
members of the Local Arm rather than distant denizens of
the Sagittarius Arm.

1954

1953

N OV E M B E R 2 019 * SK Y & TELESCOPE

HISTORICA L M A PS: 1953: W. W. M ORG A N, A . E. WHITFORD, A . D. CODE / ASTR OPHYSICAL JOUR NAL 1953;
1954: H. C. VA N DE HULST, C. A . M ULLER, J. H. OORT / BULLE TIN OF THE ASTR ONOMICAL INSTITUTES OF THE
NE THER L ANDS 1954; 1976: Y. M. G EORG ELIN & Y. P. G EORG ELIN / ASTR ONOMY & ASTR OPHYSICS 1976

anticenter. In 2019, astronomers reported signs of the Outer
Arm's gravitational inﬂuence, ﬁnding that stars near the
Outer Arm tend to move toward it, whether on our side of the
arm or beyond it. If you follow the Outer Arm as it winds its
way inward, it probably joins up with an inner segment called
the Norma Arm, so named because we view the arm's tangent
point in Norma, a small constellation southwest of Scorpius.

Sky & Telescope - November 2019

Table of Contents for the Digital Edition of Sky & Telescope - November 2019