Sky & Telescope - March 2023 - 20

Galactic Pinwheels
A Complicated Case
To understand the origin of spiral arms, one would be forgiven
for thinking our own would be the easiest example to
study. After all, it's right here in front of us. But history has
shown otherwise.
The first firm evidence for our galaxy's spiral structure
came in the 1950s, as William Morgan (Yerkes Observatory)
mapped distances to the most massive, and thus the most
short-lived, stars (S&T: Feb. 2023, p. 30). The three stripes
these stars depict were later named the Perseus, Orion, and
Sagittarius arms.
But initially, observations of these arms showed only
stumps - it wasn't clear how far around the galactic center
they wound. Later, maps based on the distribution of hydrogen
gas added to this first crude picture. And later still, further
improvements came from increasingly precise measurements
of distances to millions of massive, young stars and
even protostars.
Those improvements didn't come easily, though. " There
were a lot of arguments with a lot of egos, " says Robert Benjamin
(University of Wisconsin, Whitewater). " And not enough
data to really distinguish between different points of view. "
Even now, there's still much debate when it comes to the
Milky Way's appearance. The points of contention played
themselves out when astronomer and graphic artist Robert
Hurt (Caltech) attempted to visualize the Milky Way in
2008. The concept depicted a four-armed spiral, with two of
the arms more prominent.
Benjamin, one of the astronomers who contributed to that
visualization of the Milky Way, says he received pushback
when it came out. Are there really four arms, or just two? Do
Density Waves
In the classical scenario of density waves, spiral
arms are persistent patterns. Stars form in arms but
ultimately pass through them, while the pattern itself
continues on. Watch a rendering of classical densitywave
theory at https://is.gd/DensityWaves.
two really dominate, or are all four equal? Some even argued
over whether the Milky Way is a spiral at all.
" I was just trying to make a nice, public-friendly artist's
picture that incorporated the data, " Benjamin says. " I did
not intend for it to be in the Astrophysical Journal. It sort of
became a default by accident, because people just needed to
grab something. "
The visualization has remained the default even though
other teams, and even Sky & Telescope (S&T: Nov 2019, p. 16,
reproduced on next page), have sought to refine the picture.
" My hope is to see it replaced in the next few years - then I
will be free of this burden, " Benjamin adds, laughing.
Understanding the shape of our own galaxy is difficult
in part because we're looking at the whale from inside its
belly. Our position in the galactic plane complicates distance
measurements, as does the dust that obscures our view of the
other side. Even the Gaia space telescope's extremely precise
distance measurements to more than 1 billion Milky Way
stars only extend to some tens of thousands of light-years
from the Sun - a few tenths of our galaxy's diameter.
Besides the difficult-to-make measurements, the Milky
Way is also far from an isolated and clear-cut example of a
spiral. Evidence shows recent interactions with a massive
dwarf galaxy, plus there's a bar at our galaxy's center with its
own subtle effects.
The nature of the Milky Way spiral complicates matters,
too. " If you don't have grand-design spiral structure, then the
number of arms that you count is likely to depend on which
patch of the galaxy you're looking at, " Tremaine says. " My
guess is that the right answer is that it's not really a question
you want to ask. "
Benjamin, too, takes an agnostic approach. " We rely on
models to try to come up with a synthesis of our data, " he
says. " And then we forget the data. " Ultimately, the data are
what we have to go back to, he adds. And because the case of
the Milky Way is so complex, he thinks it's not a great proving
ground, at least not yet. " If you don't know which model
to apply, I'm not sure the Milky Way is going to tell you
which one is right. "
Sellwood disagrees, arguing that only the Milky Way will
provide data detailed enough to differentiate between spirallaunching
scenarios.
To get a bigger and sharper picture of our galaxy, and thus
THE FIRST MAP Published in 1953, this initial map of star-forming regions
in the Milky Way presents three stripes, hinting at spiral structure in
our galaxy. Later observations of stars and gas bore this conclusion out.
20 MARCH 2023 * SKY & TELESCOPE
an improved testbed, astronomers have added other methods
of mapping the Milky Way. One of these is distance measurements
to masers, warm, radio-emitting gas associated with
W. W. MORGAN, A. E. WHITFORD, A. D. CODE / ASTROPHYSICAL JOURNAL 1953
https://www.is.gd/DensityWaves
Sky & Telescope - March 2023

Table of Contents for the Digital Edition of Sky & Telescope - March 2023
