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April 2020, ScientificAmerican.com 31

NASA, ESA AND HUBBLE HERITAGE TEAM (STS

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conclusions there is considerable debate. Infrared ob ser va tions

made more than a decade ago with the Spitzer Space Telescope

have suggested that the galaxy might have only two spiral arms.

But radio-wavelength observations of atomic hydrogen and car-

bon monoxide, which are concentrated in the spiral arms of oth-

er galaxies, indicate that the Milky Way has four arms. Addition-

ally, astronomers have debated how far the sun is from the cen-

ter of the galaxy and how high it sits above the Milky Way’s

midplane—the central plane of the disk.

Nearly 70 years ago scientists calculated the distances to

some nearby luminous blue stars. Plotting these points on a

map revealed segments of three nearby spiral arms, which we

call the Sagittarius, Local and Perseus arms. Around the same

time, starting in the 1950s, radio astronomers observed atomic

hydrogen gas, which releases a telltale light signature at a wave-

length of 21 centimeters. When this gas is moving relative to

Earth, the frequency of this atomic hydrogen signature shifts

because of the Doppler phenomenon, allowing astronomers to

measure the velocity of the gas to provide clues to its location in

the galaxy. Using such measurements, galactic cartographers

employ a convenient coordinate system for our Milky Way as

viewed from the sun: by analogy to Earth’s longitude and lati-

tude, galactic longitude ( l ) is zero toward the galactic center and

increases along the “equatorial” plane of the Milky Way as

viewed from the Northern Hemisphere; galactic latitude  ( b )

denotes the angle perpendicular to the plane. So-called longi-

tude-velocity plots of 21-centimeter light signatures from hydro-

gen gas (and later from carbon monoxide) revealed continuous

arcs of emission that very likely trace spiral arms. This mapping

method, however, is plagued by ambiguities and lacks the accu-

racy necessary to clearly reveal the galaxy’s spiral structure.

A NEW VIEW

one reason that we know so little about the Milky Way is that the

galaxy contains an enormous amount of dust. Dust absorbs opti-

cal light efficiently, so along most lines of sight through the disk,

we cannot see very far—dust is blocking the view. Another reason

is the Milky Way’s mind-numbing vastness. Light from stars on

the other side of the galaxy takes more than 50,000 years to reach

Earth. Such distances make it hard to even sort out which stars

are near and which are far away.

New telescopes operating at optical wavelengths in space and

at radio wavelengths across the globe are now making great

strides toward answering our questions about the Milky Way.

The Gaia mission, launched in 2013, seeks to measure distances

to more than a billion stars in the galaxy and will undoubtedly

revolutionize our understanding of the different stellar popula-

tions involved in the Milky Way’s formation. But because it uses

optical light, which is absorbed by interstellar dust grains, Gaia

cannot freely probe distant spiral arms. In contrast, radio waves

easily pass through dust and allow us to explore the entire disk

and map its structure.

Two major projects now mapping the Milky Way use a tech-

nique in radio astronomy called very long baseline interferome-

try (VLBI). The VERA (VLBI Exploration of Radio Astrometry)

project operates four radio telescopes spanning Japanese terri-

tory from the north of the country (Mizusawa) to its southern-

most (Ishigaki) and easternmost (Ogasawara) islands. And the

BeSSeL Survey uses the Very Long Baseline Array, which in -

cludes 10 telescopes and spans much of the Western Hemi-

sphere, from Hawaii to New England to St. Croix in the U.S. Vir-

gin Islands. Because their telescopes are separated by nearly the

diameter of Earth, these arrays can attain an angular resolution

LIKE THE MILKY WAY, the nearby galaxy NGC 1300 is a barred spiral of stars stretching across more than 100,000 light-years.

But our celestial neighbor is not an exact mirror image: studies indicate the Milky Way has four major spiral arms rather than two.

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