In the Milky Way’s Stars, a History of Violence

With the cleaving of the cosmos into a home galaxy and a larger universe, the study of our finite home — and how it exists within that universe — could begin in earnest. Now, a century later, astronomers are still making unexpected discoveries about the only cosmic island we’ll ever inhabit. They may be able to explain some of the Milky Way’s characteristics by reimagining how it formed and grew in the early universe, by scrutinizing its uneven shape, and by studying its ability to form planets. The latest results, amassed over the past four years, are now painting a picture of our home as a unique place, at a unique time.

We have been lucky, it seems, to live near a particularly quiet star on the calm fringes of a middle-aged, oddly tilted, loosely spiraling galaxy that has been largely left alone for most of its existence.

Our Island Universe

From the Earth’s surface — if you are somewhere very dark — you can only see the bright stripe of the Milky Way’s galactic disk, edge-on. But the galaxy we live in is so much more complicated.

A supermassive black hole churns at its center, surrounded by the “bulge,” a knot of stars containing some of the galaxy’s oldest stellar denizens. Next comes the “thin disk” — the structure we can see — where most of the Milky Way’s stars, including the sun, are partitioned into gargantuan spiraling arms. The thin disk is encased in a wider “thick disk,” which contains older stars that are more spread out. Finally, a mostly spherical halo surrounds these structures; it is mostly made of dark matter, but also contains stars and diffuse hot gas.

To make maps of these structures, astronomers turn to individual stars. Each star’s composition records its birthplace, age and natal ingredients, so studying starlight enables a form of galactic cartography — as well as genealogy. By situating stars in time and place, astronomers can retrace history and infer how the Milky Way was built, piece by piece, over billions of years.

The first major effort to study the primordial Milky Way’s formation began in the 1960s, when Olin Eggen, Donald Lynden-Bell and Alan Sandage, who was Edwin Hubble’s former graduate student, argued that the galaxy collapsed from a spinning gas cloud. For a long time after that, astronomers thought that the first structure to emerge in our galaxy was the halo, followed by a bright, dense disk of stars. As more powerful telescopes came online, astronomers built increasingly precise maps and started refining their ideas about how the galaxy came together.

Everything changed in 2016, when the first data from the European Space Agency’s Gaia satellite came back to Earth. Gaia precisely measures the paths of millions of stars throughout the galaxy, allowing astronomers to learn where those stars are located, how they move through space, and how fast they are going. With Gaia, astronomers could paint a sharper picture of the Milky Way — one that revealed many surprises.

The bulge is not spherical but peanut-shaped, and it’s part of a larger bar spanning the middle of our galaxy. The galaxy itself is warped like the brim of a beat-up cowboy hat. The thick disk is also flared, growing thicker toward its edges, and it may have formed before the halo. Astronomers aren’t even sure how many spiral arms the galaxy really has.

The map of our island universe is not as neat as it once seemed. Nor as calm.

“If you look at a traditional picture of the Milky Way, you have this nice spherical halo and a nice regular-looking disk, and everything is kind of settled and stationary. But what we know now is that this galaxy is in a state of disequilibrium,” said Charlie Conroy, an astronomer at the Harvard-Smithsonian Center for Astrophysics. “This picture of it being simple and well ordered has been really tossed out in the past couple of years.”

A New Map of the Milky Way

Three years after Edwin Hubble realized Andromeda was a galaxy unto itself, he and other astronomers were busy imaging and classifying hundreds of island universes. Those galaxies seemed to exist in a few prevailing shapes and sizes, so Hubble developed a basic classification scheme known as the tuning fork diagram: It divides galaxies into two categories, ellipticals and spirals.

Astronomers still use this scheme to categorize galaxies, including ours. For now, the Milky Way is a spiral, with arms that are the main nurseries for stars (and therefore planets). For a half-century, astronomers thought there were four main arms — the Sagittarius, Orion, Perseus and Cygnus arms (we live in a smaller offshoot, unimaginatively called the Local Arm). But new measurements of supergiant stars and other objects are drawing a different picture, and astronomers no longer agree on the number of arms or their sizes, or even whether our galaxy is an oddball among islands.

“Strikingly, almost no external galaxies present four spirals extending from their centers to their outer regions,” Xu Ye, an astronomer with China’s Purple Mountain Observatory, said in an email.

To trace the Milky Way’s spiral arms, Ye and colleagues used Gaia and ground-based radio telescopes to look for young stars. They found that, like other spiral galaxies, the Milky Way only has two main arms, Perseus and Norma. Several long, irregular arms also wind around its core, including the Centaurus, Sagittarius, Carina, Outer and Local arms. It seems that, at least in shape, the Milky Way may be more similar to distant cosmic islands than astronomers thought.

“Studying the spiral-shaped Milky Way may reveal whether it is unique among the billions of galaxies in the observable universe,” Ye wrote.

Cosmic Shores

Hubble’s study of Andromeda and its variable star stemmed from his fierce rivalry with another famed astronomer at Mount Wilson, Harlow Shapley. The Harvard astronomer Henrietta Swan Leavitt had pioneered the use of Cepheid variable stars to measure distances, and using her method, Shapley had calculated that the Milky Way was 300,000 light-years across — an astonishing claim in 1919, when most astronomers believed the sun was at the galaxy’s center, and that the whole galaxy spanned 3,000 light-years. Shapley thus insisted that other “spiral nebulae” must be gas clouds and not separate galaxies, because their sizes would mean they were inconceivably far away.

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