Sky & Telescope - USA (2019-11)

Spiraling In

anticenter. In 2019, astronomers reported signs of the Outer

Arm’s gravitational infl uence, fi nding 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.

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 Sagit-

tarius 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 (Cen-

ter 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 ener-

gize electrons in interstellar molecules such as water and

methanol. The molecules then emit intense microwaves and

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 (Uni-

versity of Wisconsin, Whitewater), who is not part of a team

measuring them. “They remove one of the major uncertain-

ties 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 associ-

ated 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 fi rst-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 naked-

eye 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.

BUILDING THE MAP Plotting the locations of young, massive col- 1954

lections of stars provided early glimpses of our galaxy’s spiral arms

(left). Maps of neutral hydrogen gas extended our view farther (center).

Astronomers eventually identifi ed four large arms (right). Note: Galactic

coordinate systems changed between the second and third maps.

18 NOVEMBER 2019 • SKY & TELESCOPE

1953

HIS

TO

RIC

AL

M

AP

S:^

19

53

:^ W

.^ W
.^ M

OR

GA

N,^

A.^

E.^

WH

ITF

OR

D,^

A.^

D.^

CO

DE

/^ A

ST

RO

PH

YS

ICA

L^ J

OU

RN

AL

19

53 ;

195

4 :^

H.

C.

VA

N^ D

E^ H

UL

ST

,^ C

.^ A
.^ M

UL

LE

R,^

J.^

H.^

OO

RT

/^ B

UL

LE

TIN

O

F^ T

HE

AS

TR

ON

OM

ICA

L^ I

NS

TIT

UT

ES

OF

TH

E^

NE

TH

ER

LA

ND

S^1

95

4 ;^

197

6 :^

Y.^

M.

G

EO

RG

EL

IN^

&^ Y

.^ P
.^ G

EO

RG

EL

IN^

/^ A

ST

RO

NO

MY

&^

AS

TR

OP

HY

SIC

S^1

976

1953