30 Scientific American, April 2020
Mark J. Reid is a senior radio astronomer at the Smithsonian Astrophysical
Observatory at the Center for Astrophysics | Harvard & Smithsonian. He was
recently elected to the U.S. National Academy of Sciences.
Xing-Wu Zheng is a professor of astronomy
at Nanjing University in China. He has studied
astronomical masers and star formation for decades.
Recent efforts, however, have begun to map the
Milky Way from the inside out, allowing us to assemble
an accurate snapshot of its structure for the first time.
This emerging vista is the result of several large proj-
ects involving advanced radio and optical telescopes,
including our program, the Bar and Spiral Structure
Legacy (BeSSeL) Survey. For this effort, we were grant-
ed an unprecedented amount of observing time—5,000
hours—on the Very Long Baseline Array, a system oper-
ated by the National Radio Astronomy Observatory
and funded by the National Science Foundation.
Our initial results offer a new and improved view of
the Milky Way. In addition to gaining a better under-
standing of what the Milky Way looks like, we are
starting to clarify why galaxies such as ours exhibit
spiral structure and how our astronomical home fits
into the universe as a whole.
THE COSMIC NEIGHBORHOOD
In the early 1800 s William Parsons, the third Earl of
Rosse, built a 72-inch telescope—huge for its time. He
observed and drew what we now call the Whirlpool
Galaxy, which clearly had a spiral pattern. Without
knowledge about how far away it was or about the scale
of the Milky Way, however, it was unclear whether the
Whirlpool was a small structure inside our galaxy or a
large nebula similar to it. Debate on these points con-
tinued into the early 1900s, until Edwin Hubble, using
a technique developed by Henrietta Leavitt to measure
the distance to bright stars, showed that the Whirlpool
and similar spirals were far outside the Milky Way. This
revelation upended the notion that the Milky Way
might encompass the entire universe.
Astronomers figured out that we live in a spiral gal-
axy by measuring the motions of gas throughout the
disk—the large, pancake-shaped region that makes up
the main body of the Milky Way. Spirals, along with
rounded ellipticals, are common types of galaxies. The
nearby spirals NGC 1300 and Messier 101 (M101) pro-
vide good examples of how the Milky Way might look
from afar. NGC 1300 has a bright, linear structure in
its center, which astronomers call a bar, and two bluish
spiral arms that start at the ends of the bar and wend
slowly outward as they encircle it. Bars are seen in the
majority of spiral galaxies and are thought to form
from gravitational instabilities in a galaxy’s dense disk.
In turn, the stirring action of the rotating central bar
may give rise to spiral arms. (Other processes—such as
instabilities associated with large mass concentrations
inside a disk or gravitational perturbations from near-
by galaxies—can also lead to arms.) Spiral arms tend to
glow in blue light, which comes from gigantic stellar
nurseries where massive stars are forming. M101, the
other potential Milky Way match, is known as the Pin-
wheel Galaxy; although it lacks the bright bar of NGC
1300, it boasts more spiral arms.
Astronomers have long thought that the Milky Way
has characteristics of both these galaxies. It probably
has a significant bar as seen in NGC 1300, as well as
multiple spiral arms as in M101. Beyond these basic
IN BRIEF
Astronomers know
surprisingly little
about the structure
of our Milky Way
galaxy, including
the number of spiral
arms it has and the
location of our sun
within it.
Recently, though,
scientists have
pieced together the
best map yet of our
galaxy by using data
from several new
research projects,
particularly the Bar
and Spiral Structure
Legacy (BeSSeL)
radio survey.
The map reveals
at least four major
spiral arms in the
Milky Way, as well as
some smaller fea
tures, and shows
that the sun lies
almost exactly on the
central plane of the
disk of the galaxy.
H
undreds of years ago explorers saIled across oceans and traversed
uncharted continents to map Earth, and in the past half a century space
probes have photographed most of our solar system. Yet as well as we
have come to know our astronomical backyard, our image of the larger
neighborhood—our Milky Way galaxy—is blurry. The reason is obvious:
we cannot get outside it to take a peek. Imagine sending a spacecraft on
a multimillion-year journey to go beyond our galaxy, look back and snap
a picture: clearly impractical. We are left with many open questions about
our cosmic home, such as how many spiral arms the galaxy has, whether the large structure
closest to the sun counts as an arm and where in the galaxy our solar system lies.
© 2020 Scientific American
