April 2020, ScientificAmerican.com 35Local spur, suggesting a minor structure similar to smaller append-
ages seen branching off spiral arms in other galaxies. This “spur”
interpretation is probably incorrect, however. In our data, this
fragment appears to be an orphan segment of an arm that wraps
around less than a quarter of the Milky Way. Over its short length,
though, it has amounts of massive star formation comparable to
what we see in a similar length of the nearby Perseus arm. Inter-
estingly, some astronomers have thought the Perseus arm to be
one of the two dominant arms (the other is the Scutum-Centau-
rus–Outer-Scutum-Centaurus arm) in the Milky Way. We find,
however, that massive star formation decreases significantly as
the arm wends inward away from the sun, suggesting that it would
not appear as a very prominent arm to an external observer.
By using the three-dimensional locations of our massive young
stars and modeling the measured motions, we can estimate val-
ues for fundamental parameters of the Milky Way. We find that
the distance from the sun to the galaxy’s center is 8,150 ± 150 par-
secs (or 26,600 light-years). This is smaller than the value of 8,500
parsecs recommended decades ago by the International Astro-
nomical Union. Also, we find that the Milky Way is spinning at
236 kilometers per second, which is about eight times the speed at
which Earth orbits the sun. Based on these parameter values, we
find that the sun circles the Milky Way every 212 million years. To
put this in perspective, the last time our solar system was in this
part of the Milky Way, dinosaurs roamed the planet.
The part of our galaxy interior to our sun has a very thin and
nearly flat planar shape. Although this has long been known, the
location of the sun relative to this plane has been controversial.
Recently astronomers settled on a value of 25 parsecs (82 light-
years) above the plane, but our results strongly disagree with this
estimate. By fitting a plane through the locations of massive stars
for which we have accurate distances, we determine that the sun
is only about six parsecs (20 light-years) above that plane. This
distance is only 0.07 percent of the sun’s distance from the plane’s
center, meaning it is extremely close to the midplane. We also
confirmed previous observations that farther out in the Milky
Way the plane starts warping upward on its northern side and
downward on its southern side, a bit like a potato chip.
When describing their observations, astronomers divide the
Milky Way into quadrants, with our sun at the center. Using that
convention, we have traced spiral arms in the first three quad-
rants. To complete the map in the fourth quadrant, we need
observations from the Southern Hemisphere. These are being
planned and will be obtained with telescopes across Australia and
New Zealand. While awaiting those results, we can extrapolate
the known arms into the fourth quadrant by using auxiliary infor-
mation from observations of atomic hydrogen and molecular car-
bon monoxide. The architecture revealed by these observations
coincides with previously theorized structures named the Norma-
Outer, Scutum-Centaurus–Outer-Scutum-Centaurus, Sagittarius-
Carina and Perseus arms. We caution, though, that we have only
one distance measurement to a star-forming region well beyond
the galactic center. The measured location of this region, coupled
with its position in galactic longitude-velocity plots of carbon
monoxide emission, gives us some confidence in how we connect-
ed arms on the far side of the galactic center. We will need more
such measurements to be certain of our model, however.
We now have a clearer picture of our cosmic neighborhood. It
seems we live in a four-armed spiral galaxy with a bright centralbar and a reasonable degree of symmetry. Our sun is located
almost exactly in its midplane but far from its center, about two
thirds of the way out. In addition to arms that wrap approximate-
ly all the way around, the Milky Way has at least one additional
arm segment (the Local arm) and probably has numerous spurs.
These features make our galaxy appear fairly normal, but it cer-
tainly is not typical. About two thirds of spiral galaxies exhibit
bars, so in this way the Milky Way is in the majority. Yet its pos-
session of four clearly defined and fairly symmetric spiral arms
makes it stand out from most other spiral galaxies, which have
fewer, messier arms.MORE MYSTERIES
although we have some new answers, we are also left with signif-
icant questions. Astronomers are still actively debating how spi-
ral arms arise in the first place. Two competing theories are that
gravitational instabilities on the scale of the entire galaxy form
long-lasting spiral-wave patterns or that smaller-scale instabilities
stretch and amplify over time into arm segments that then link
up to form long arms. In the former theory, spiral arms can last for
many billions of years, whereas in the latter theory, arms are short-
er-lived and new ones emerge many times over a galaxy’s lifetime.
It is also difficult to set an age for the Milky Way because it has
no clear birth date. Current thinking is that it gradually coalesced
over eons as many smaller protogalaxies that had formed earlier
in the history of the universe collided and merged. The Milky
Way probably would have been recognizable as a large galaxy
about five billion years ago, but it might have looked quite differ-
ent then because major mergers would have been likely to scram-
ble any existing spiral structure.
Improving on our latest image of the Milky Way will require
many more observations and will be facilitated by the next gener-
ation of radio telescope arrays capable of VLBI. Such arrays are
being planned now and include the Square Kilometer Array in
Africa and the Next Generation Very Large Array in North Ameri-
ca. Both are giant arrays of radio telescopes projected to span their
continents, and they could be fully operational by the end of this
decade. By greatly increasing the telescope collecting area com-
pared with that of current arrays, they will allow the detection of
much fainter radio emissions from stars and hence will see farther
across the Milky Way. Ultimately we hope to definitively trace our
galaxy’s large-scale architecture to confirm or reject the compet-
ing theories of how its grand, spiraling structure came to be.MORE TO EXPLORE
Studies in Galactic Structure. I. A Preliminary Determination of the Space
Distribution of the Blue Giants. W. W. Morgan et al. in Astrophysical Journal, Vol. 118,
pages 318–322; September 1953.
The Milky Way in Molecular Clouds: A New Complete CO Survey. T. M. Dame et al.
in Astrophysical Journal, Vol. 547, No. 2, pages 792–813; February 1, 2001.
Mapping Spiral Structure on the Far Side of the Milky Way. Alberto Sanna et al. in Science,
Vol. 358, pages 227–230; October 13, 2017.
Trigonometric Parallaxes of High-Mass Star-Forming Regions: Our View of the
Milky Way. M. J. Reid et al. in Astrophysical Journal, Vol. 885, No. 2, Article 131;
November 10, 2019.
FROM OUR ARCHIVES
The Spiral Structure of the Galaxy. W. W. Morgan; May 1955.
Fossil Hunting in the Milky Way. Kathryn V. Johnston; December 2014.
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