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The Milky Way Galaxy

The Milky Way is our galactic home, part of the story of how we came to be. Astronomers have learned that it’s a large spiral galaxy, similar to many others, but also different in ways that reflect its unique history. Living inside the Milky Way gives us a close-up view of its structure and contents, which we can’t do for other galaxies. At the same time, this perspective makes it difficult for astronomers to obtain a complete picture of galactic structure. Modern research on the Milky Way refines our understanding of how the galaxy formed and what continues to shape our galactic home.

Learning From Our Own Galaxy

On a dark clear night, the Milky Way is a strikingly beautiful stream of light stretching across the sky. Every culture has assigned it a name and meaning, but the word that stuck for scientists came from Greek mythology: galaxy is derived from the word for milk. Over the centuries, astronomers recognized that the stream runs all the way across the sky because we’re inside it.

In the early 20th century, Harvard College astronomer Henrietta Swan Leavitt discovered the relationship between the period of fluctuation and the brightness of a type of star called a Cepheid variable. That discovery let astronomers map the Milky Way well enough to know roughly how big it was, and locate its center. With the discovery that “spiral nebulae” were other far-off galaxies, our place in the universe was established: we live in one galaxy much like many others.

Today, astronomers study the Milky Way to understand its history, structure, contents, and our place in the galaxy.

  • Like most other large galaxies, the Milky Way harbors a supermassive black hole at its center. Known as Sagittarius A* — abbreviated as Sgr A* — this galaxy is about four million times the mass of the Sun. While it is fairly quiet today, astronomers have found traces of gas clouds and stars torn apart by Sgr A*. Some stars that wandered too close were flung out of the galaxy at high speed by the black hole’s gravity. Researchers study the feeding habits of Sgr A* to understand how these supermassive black holes behave. The Event Horizon Telescope (EHT) captured an image of the black hole in the nearby galaxy M87, and is collecting data to do the same for Sgr A*.

  • The part of the Milky Way containing the Sun is the disk, which is a thick platter of stars, gas, and dust about 100,000 light-years across. The galaxy’s spiral arms are part of this disk, where the youngest and brightest stars of the galaxy live. Astronomers study the populations of stars and material between them to understand the evolution of the galaxy.

  • The most massive part of the Milky Way and any galaxy is the halo, which is a roughly spherical region surrounding the galactic disk. This halo consists of two parts, which may or may not be related. The “dark” halo contains most of the galaxy’s mass, which takes the form of dark matter; astronomers infer the mass and properties of this part by how it affects the motion of stars in the disk. The stellar halo is much lower in mass, containing the oldest stars and star clusters in the galaxy. This part may be the remnants of other galaxies disrupted by the Milky Way’s gravity. For those reasons, the halo is important for understanding our galaxy’s overall behavior and interactions with other galaxies, as well as the nature of dark matter.

  • The Milky Way contains stars that came from other galaxies, which merged with or were eaten by our galaxy. Astronomers have observed galactic thievery taking place right now, where the Milky Way is stripping stars and gas from its satellite galaxies, as well as traces of past interactions. Tracing populations of stars around the galaxy provide a history of these mergers and thefts, helping us understand both our own galaxy’s evolution and how this process works for other galaxies.

The Milky Way's supermassive black hole is quiet today, but observations indicate that wasn't always the case. This artist's impression shows the Milky Way as it may have appeared 6 million years ago during a "quasar" phase of activity.

Mark A. Garlick/CfA