Inner Workings: Astronomers are redrawing our corner of the Milky Way

We live in a giant barred spiral galaxy. The Milky Way’s fast-spinning disk of stars and gas whips up spiral arms that spawn new suns, while a bar of mostly older stars cuts through its heart. From afar, our galaxy likely resembles a glowing cosmic hurricane.

The stunning spiral structure of the Whirlpool Galaxy is easy to see from the outside, but it's far harder to map the spiral arms of our own galaxy. In both galaxies, though, hot blue stars and pink clouds of ionized hydrogen gas dot the spiral arms, as seen in this reprocessed image from the Hubble Telescope. Image Credit: NASA, Hubble Heritage Team, (STScI/AURA), ESA, S. Beckwith (STScI), additional processing by Robert Gendler.

Telescopes looking beyond the Milky Way see countless other spiral galaxies in exquisite detail. Yet the precise patterns of our own galaxy’s spiral arms have long eluded astronomers. Simply put, they cannot see the Milky Way from the outside.

Nevertheless, remarkable new observations are revealing the starry landscape of our galactic neighborhood and our place within it. Contrary to some previous claims, these discoveries demonstrate that our solar system resides in a bright and vigorous tendril of stars called the Local Arm, also known as the Orion Arm. Whereas some astronomers had once regarded this as merely a spur from another arm, recent studies have shown that it is a far more impressive entity.

“I think it definitely is a spiral arm,” says astronomer Ronald Drimmel at the Turin Astrophysical Observatory in Italy. “It may not be a major spiral arm, but certainly it is an important feature, at least in our part of the Milky Way.”

Nearly every star visible to the naked eye inhabits the Local Arm. Some were born here, whereas others, including the Sun, are simply passing through as they revolve around the galaxy’s center. So as the Local Arm’s full grandeur starts to unfold through the latest observations, the resulting star plots constitute the best maps yet of our cosmic backyard. These maps show that all the young stars nearby belong to a great structure that is giving birth to a plethora of new suns over a vast region of the galaxy.

Celestial Spirals

Astronomers spotted the first celestial spiral, now named the Whirlpool Galaxy, in 1845. It is one of the most spectacular spiral galaxies known, shining from a distance of 28 million light-years. But it wasn’t until 1951 that astronomers proved that our galaxy is also a spiral.

They did so by exploiting a basic trait of spiral arms: They squeeze interstellar clouds of gas and dust, prompting them to collapse and spawn new stars. This makes spiral arms in galaxies such as the Whirlpool glow blue and pink. The blue comes from the hottest newborn stars, which don’t drift far from their birthplaces. The pink comes from clouds of hydrogen gas that these same stars ionize by tearing off electrons. As the electrons rejoin the atoms, they can emit red light.

By estimating distances to the blue stars in regions of ionized hydrogen gas and then plotting the positions of these gas clouds on a map, William Morgan at Yerkes Observatory in Williams Bay, WI, and his colleagues discovered the three nearest spiral arms in the Milky Way (1, 2). According to this map, the Earth is near the inner edge of one of these, called the Orion Arm—named for the Orion Nebula, the brightest region of ionized hydrogen in the sky. The Sagittarius Arm lies closer to the galactic center than the Orion Arm does, whereas the Perseus Arm lies farther out.

Subsequent studies reaffirmed the existence of the Sagittarius and Perseus Arms, but some of this work relegated the Orion Arm to be nothing more than a spur off another arm—or even omitted it altogether (3). The latter case would consign the Earth to lurk in the dark region between the Sagittarius and Perseus Arms, leaving us only to gaze at the great spiral arms on either side of us that were forging new stars.

This bird's-eye view of the Milky Way plots O-type stars (small dots) and masers (triangles) as well as our own Sun (large yellow dot). The galaxy's center is at bottom and its bar appears as a gray ellipse. Spiral arms (labeled) are depicted in different colors, with the Local Arm orange. As shown here, the Local Arm stretches toward the constellation Vela (pictured at 9 o'clock), but some astronomers think our arm extends instead toward the constellation Puppis (at 10 o'clock). Image credit: Lucy Reading (artist), adapted with permission from ref. 6, © ESO.

A Local Revival

But all that has changed. Over the past decade, radio astronomers have determined precise distances to some of the hot blue newborn stars that line and light the spiral arms, producing a much better outline of the Milky Way’s spiral structure. This work reaches more distant stars than previous efforts because of the accuracy of the new observations and thanks to a major virtue of radio waves. Whereas interstellar dust absorbs visible light from these far-off stars, radio waves zip right through the dust, letting astronomers map spiral arms throughout the galaxy.

Radio astronomers gauge the young stars’ distances by using an old and trusted technique: measuring their parallaxes, an apparent movement that arises from the Earth’s motion around the Sun. Over the course of a year, observers see a star’s position shift slightly as they view it from different perspectives. This annual shift—the parallax—is smaller if the star is farther away.

Optical astronomers first measured the parallax of a star outside the solar system in 1838, when they deduced the distance to 61 Cygni, a double star just 11 light-years from Earth. Even a decade ago, the parallax method rarely worked beyond a distance of a few hundred light-years—a tiny fraction of our gargantuan galaxy—because a far-off star’s parallax was just too minute to measure.

But radio astronomers can link their telescopes together into huge arrays that span continents, giving researchers the ability to measure even the minuscule parallaxes of remote stars. Incredibly, this strategy can determine the distances of stars beyond the galaxy’s center, tens of thousands of light-years from Earth, as long as the stars emit radio waves. Fortunately, some of the powerful stars in spiral arms excite electrons in the water and methanol molecules found in surrounding gas clouds. As the electrons lose energy, they emit intense microwaves in the same way that lasers shoot out light. These so-called masers appear as small but bright points of radio radiation. Radio telescopes can monitor the masers over the course of a year and measure their parallaxes, yielding the distances to the stars.

A decade ago, these maser parallaxes uncovered a big surprise in Cygnus, a large constellation in the northern sky that is packed with stars located along one end of the Local Arm. Beyond these stars, also in the constellation Cygnus, lies the Perseus Arm. That arm was thought to host numerous stellar nurseries where hot blue stars are born. But the maser parallaxes revealed that many of Cygnus’s stellar newborns are closer, strung out along our line of sight in the Local Arm. One maser previously thought to be 19,000 light-years from Earth in the Perseus Arm actually had a distance of only 11,000 light-years, placing it firmly in the Local Arm instead (4).

“So all of a sudden it was clear that the Local Arm had an awful lot of massive star formation going on, and that it was a much longer structure than previously thought,” says Mark Reid, a radio astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, who is leading much of the maser work. “It may actually extend all the way to the Perseus Arm.”

The initial work on maser parallaxes suggested that the Local Arm was at least 16,000 light-years long. But since then, Reid and other astronomers have measured some 200 maser parallaxes across the galaxy, which indicate that the Local Arm’s length is at least 26,000 light-years (5). That’s comparable with the great distance between the Sun and the Milky Way’s center.

Unfortunately, astronomers don’t yet know the total length of the Local Arm, in part because the situation is less clear along its other end. There the Local Arm stretches toward Vela, a constellation so far south in the sky that most ground-based telescopes in the Northern Hemisphere can’t observe it. Still, Reid says the Local Arm probably extends farther toward Cygnus than it does toward Vela. To resolve the matter, he is currently working with astronomers in Australia and New Zealand to delineate the Local Arm’s southern stretch.

Gaia’s Eye on the Sky

Meanwhile, the Gaia spacecraft, which the European Space Agency launched in 2013, is able to scan the entire sky, north and south, measuring parallaxes and thus distances of countless stars. Although the spacecraft observes visible light, which doesn’t penetrate the dust cloaking distant parts of the Milky Way’s disk, Gaia has already sketched out the pattern of young stars in our own vicinity.

The latest Gaia data, released in December 2020, provide accurate distances for more than a thousand O-type stars, the hottest and most massive stellar newborns, which are ideal for tracing spiral arms. Because of their great mass, O-type stars shine very brightly, making them easy to see. Their brightness also dooms them to die within just 10 million years, giving them little time to drift away from their birthplaces in the spiral arms. That makes O-type stars excellent indicators of the true locations of the arms.

“Remarkably, the Local Arm traced by the distribution of O-type stars is distinct, it extends much longer than previously expected, and it seems more similar to a major spiral arm feature,” writes a team led by Ye Xu at the Purple Mountain Observatory in Nanjing, China (6). Xu thinks that the Milky Way’s arms resemble those of the beautiful spiral galaxy M101. From the new Gaia data, he estimates the Local Arm to be at least 25,000 light-years long, agreeing with Reid’s assessment from the maser parallaxes.

In March, a third team reported the same conclusion about the Local Arm’s length (7). Eloisa Poggio at the University of Côte d’Azur in France and her colleagues used Gaia data to map out hundreds of thousands of blue stars, most of which are younger than 100 million years old. In the north, toward Cygnus, the researchers see the Local Arm extend to the limit of their data, about 13,000 light-years from Earth. Unlike the other teams, however, they see the Local Arm extending equally far in the opposite direction, into the southern sky. That gives the Local Arm a total length of 26,000 light-years. “It may definitely be longer than that,” says Drimmel, a member of the team.

There are other differences, too. Poggio’s team found that in the southern sky, the Local Arm winds not through Vela but through another constellation called Puppis. If that’s correct, it means the Local Arm spirals farther outward there than astronomers had thought. But other astronomers have assigned those stars in Puppis to the Perseus Arm instead.

Nor have astronomers ascertained the blend of stars in the Local Arm. The Sagittarius Arm contains plenty of young stars, but it may not have an excess of old stars compared with the darker regions between the spiral arms. In contrast, the Perseus Arm abounds with stars both young and old. No one yet knows which type the Local Arm is.

Future work should clarify these issues. In particular, maser parallaxes in the southern sky promise a clearer view of the Local Arm there, and more observations from Gaia will make its parallaxes still more precise. “Each data release is going to get better and better,” Drimmel says. These advances will let astronomers map out even more of the Local Arm—and unveil the full extent of our surprisingly luminous galactic neighborhood.