Astronomy

billion light-years

2.75 3

SOONER OR LATER ON ANY

CLEAR, DARK NIGHT, AN ETHE-

REAL BAND CALLED THE MILKY

WAY ARCHES ACROSS THE SKY.

Although recognized since antiquity, phi-

losophers and scientists could only guess at

what it represented until fairly recently (see

“How the Milky Way Galaxy got its name,”

p. 33). With the invention of the telescope,

it became clear that the Milky Way was the

collective glow of stars too faint to be seen

by the naked eye. More than a century later,

English astronomer Thomas Wright sug-

gested that this glowing band was precisely

what one would expect to see if the Sun were

embedded in a flat disk of stars.

We now know that the Milky Way is the

primary structure of our galaxy seen edge-

wise. Additional detail and especially the

physical scale of the galaxy took another two

centuries to work out. The process continues

today as astronomers wrestle with conf lict-

ing evidence and make new discoveries.

Much like mapping a fogbound city from a

single intersection, scientists must decipher

the galaxy’s structure while viewing it from

inside a disk where dust clouds dim and

block starlight.

The true scale of the Milky Way Galaxy

— and, indeed, the universe as a whole —

became dramatically clearer in the 1920s.

That’s when a new generation of large tele-

scopes coupled with photography revealed

that “spiral nebulae” were actually entire

galaxies like our own — “island universes”

in the evocative parlance of the time. Sur-

veys showed that most disk-shaped galaxies

possessed winding spiral arms where young

stars, gas, and dust were concentrated.

Astronomers assumed our galaxy was a

spiral too. In the 1950s, radio telescopes

produced the first crude maps of the Milky

Way’s spiral arms by tracking how gas

clouds moved around the galaxy.

Over the past two decades, surveys using

dust-penetrating infrared light have brought

the general picture of our galaxy into better

focus. These projects include the ground-

based Two Micron All-Sky Survey and Sloan

Digital Sky Survey (SDSS) as well as two

NASA spacecraft, the Wide-field Infrared

Survey Explorer (WISE) and the

Spitzer Space Telescope. These

observations have helped astron-

omers better define our galaxy’s

spiral arms, take a census of star

clusters and other phenomena in

Giant clouds of gas and dust sprinkled

with splashy star clusters adorn the Milky Way’s

spiral arms, while the galaxy’s vast halo teems

with darker matter. by Francis Reddy

THE MILKY WAY

Earth’s home galaxy

Francis Reddy is the senior science writer for the

Astrophysics Science Division at NASA’s Goddard

Space Flight Center in Greenbelt, Maryland.

FAST FAC T

27,200

LIGHT-YEARS

The Sun’s distance

from the galaxy’s

center