To make maps of those buildings, astronomers flip to particular person stars. Each star’s composition data its birthplace, age, and natal substances, so learning starlight permits a type of galactic cartography—in addition to family tree. By situating stars in time and place, astronomers can retrace historical past and infer how the Milky Way was constructed, piece by piece, over billions of years.

The first main effort to check the primordial Milky Way’s formation started within the Nineteen Sixties, when Olin Eggen, Donald Lynden-Bell and Alan Sandage, who was Edwin Hubble’s former graduate pupil, argued that the galaxy collapsed from a spinning gasoline cloud. For a very long time after that, astronomers thought that the primary construction to emerge in our galaxy was the halo, adopted by a shiny, dense disk of stars. As extra highly effective telescopes got here on-line, astronomers constructed more and more exact maps and began refining their concepts about how the galaxy got here collectively.

Everything modified in 2016, when the primary knowledge from the European Space Agency’s Gaia satellite tv for pc got here again to Earth. Gaia exactly measures the paths of thousands and thousands of stars all through the galaxy, permitting astronomers to be taught the place these stars are situated, how they transfer by means of area, and how briskly they’re going. With Gaia, astronomers might paint a sharper image of the Milky Way—one that exposed many surprises.

The bulge just isn’t spherical however peanut-shaped, and it’s half of a bigger bar spanning the center of our galaxy. The galaxy itself is warped just like the brim of a beat-up cowboy hat. The thick disk can be flared, rising thicker towards its edges, and it could have fashioned earlier than the halo. Astronomers aren’t even certain what number of spiral arms the galaxy actually has.

The map of our island universe just isn’t as neat because it as soon as appeared. 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,” stated Charlie Conroy, an astronomer on 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 different astronomers have been busy imaging and classifying lots of of island universes. Those galaxies appeared to exist in just a few prevailing sizes and styles, so Hubble developed a fundamental classification scheme generally known as the tuning fork diagram: It divides galaxies into two classes, ellipticals and spirals.

Astronomers nonetheless use this scheme to categorize galaxies, together with ours. For now, the Milky Way is a spiral, with arms which might be the primary nurseries for stars (and due to this fact planets). For a half-century, astronomers thought there have been 4 essential arms—the Sagittarius, Orion, Perseus, and Cygnus arms (we dwell in a smaller offshoot, unimaginatively referred to as the Local Arm). But new measurements of supergiant stars and different objects are drawing a distinct image, and astronomers not agree on the variety of arms or their sizes, and even whether or not our galaxy is an oddball amongst 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, stated in an electronic mail.