From the Earth\u2019s surface \u2014 if you are somewhere very dark \u2014 you can only see the bright stripe of the Milky Way\u2019s galactic disk, edge-on. But the galaxy we live in is so much more complicated.<\/p>\n

A supermassive black hole churns at its center, surrounded by the \u201cbulge,\u201d a knot of stars containing some of the galaxy\u2019s oldest stellar denizens. Next comes the \u201cthin disk\u201d \u2014 the structure we can see \u2014 where most of the Milky Way\u2019s stars, including the sun, are partitioned into gargantuan spiraling arms. The thin disk is encased in a wider \u201cthick disk,\u201d 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.<\/p>\n

To make maps of these structures, astronomers turn to individual stars. Each star\u2019s composition records its birthplace, age and natal ingredients, so studying starlight enables a form of galactic cartography \u2014 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.<\/p>\n

The first major effort to study the primordial Milky Way\u2019s formation began in the 1960s, when Olin Eggen, Donald Lynden-Bell and Alan Sandage, who was Edwin Hubble\u2019s 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.<\/p>\n

Everything changed in 2016, when the first data from the European Space Agency\u2019s 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 \u2014 one that revealed many surprises.<\/p>\n

The bulge is not spherical but peanut-shaped, and it\u2019s 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\u2019t even sure how many spiral arms the galaxy really has.<\/p>\n

The map of our island universe is not as neat as it once seemed. Nor as calm.<\/p>\n