NASA — Milky Way Anatomy

If we could see our galaxy, the Milky Way, from the outside, it would look like an enormous, bedazzled pinwheel. Vast sprays of stars form spiral arms that curl outward from a bright center that bulges like the yolk in a fried egg. Dark, dusty tendrils darken some regions, while glowing pink gas clouds light up others.

We have a pretty good idea of the Milky Way’s overall structure, but since we’re nestled inside it, fine details are hard to see. Those clouds of gas and dust strewn throughout interstellar space block our view, especially of the far side of the galaxy.  Astronomers have used observations from different telescopes to piece together our galaxy’s anatomy. Let’s scrub up and dive in!

An artist’s concept of our Milky Way galaxy’s central bulge.

At the heart of our galaxy, an enormous swarm of about 10 billion mostly old stars crowd into a slightly peanut-shaped region around 10,000 light-years across called the bulge. The innermost stars dance around an invisible object. By measuring the stars’ orbits, scientists have calculated that the central object must be as hefty as about 4 million Suns.

This unseen behemoth is a monster black hole called Sagittarius A* (A* is pronounced “A-star”). Its gravity is so powerful that if you came within 7 million miles (12 million kilometers) or so –– less than a tenth of Earth’s distance from the Sun –– you’d never be able to escape its grip, no matter how hard you tried! But don’t worry, Sagittarius A* is a pretty friendly giant; it’s largely dormant, releasing only faint flickers of X-rays and radio waves.

An artist’s concept of our Milky Way galaxy’s disk.

The disk, which is home to the bulk of the Milky Way’s stars, extends out from the bulge like the brim of a sombrero. It’s around 100,000 light-years wide and divided into two parts. The thin disk is about 1,000 light-years from top to bottom, and the thick disk (which isn’t as densely populated by stars) extends above and below it for another few thousand light-years. So, the thick disk is like a bagel, and the thin disk is like a generous layer of cream cheese spread inside it.

The thin disk hosts our galaxy’s spiral arms, which look like they spin around the Milky Way like bicycle spokes, although they actually work more like galactic traffic jams. We live along one of these dense areas in an arm called the Orion Spur. All of the Milky Way’s arms extend outward from the bar –– a rotating structure of stars in the middle of the galaxy that’s about 16,000 light-years long.

The edge of a nearby stellar nursery called NGC 3324, found at the northwest corner of the Carina Nebula, forms the “mountains” and “valleys” spanning this image captured by the James Webb Space Telescope.

The spaces between stars in the disk aren’t quite empty –– they’re home to the interstellar medium, which is made of dust and gas. Dark, smoky ribbons of dust wind through the starlight, clumping up here and there to form clouds of molecules. To some astronomers, the dust is a nuisance that blocks things they’d like to study. But for others, the dust is the target –– interstellar dust is both the leftover crumbs from stars long dead and raw material from which new stars and planets may form.

An artist’s concept of our Milky Way galaxy’s stellar halo.

A sparse smattering of incredibly old, faint stars lives in a football-shaped “halo” that’s about 300,000 light-years across, encasing the disk and bulge. Stars there are tiny, which means they burn through their nuclear fuel so slowly they can live 12 billion years or even longer! Many of them formed early in the universe’s history, before many generations of stars enriched the galaxy with heavier elements than hydrogen and helium.

This Hubble Space Telescope image shows one of the Milky Way’s many globular clusters. Known as NGC 6388, the cluster is more than 10 billion years old.

The stellar halo is also home to at least 150 globular clusters –– huge, spherical collections of ancient stars bound to each other by their mutual gravity. These groups of tens of thousands or even millions of stars are the ultimate squad goals. They’re so tightly packed together, sometimes just a fraction of a light-year apart, that from Earth they look like glittery disco balls. And they’re practically inseparable, sticking together for billions of years.

This artist’s concept visualizes gamma-ray bubbles discovered by NASA’s Fermi Gamma-Ray Space Telescope. From end to end, the bubbles extend 50,000 light-years, or roughly half of the Milky Way’s diameter. Hints of the bubbles’ edges were first observed in X-rays (blue) by ROSAT, a Germany-led mission operating in the 1990s. The gamma rays mapped by Fermi (magenta) extend much farther from the galaxy’s plane.

Vast “bubbles” of gamma rays, each about 25,000 light-years long, stretch into the stellar halo from the center of the galaxy. Scientists found them by surprise in data from NASA’s Fermi Gamma-ray Space Telescope. The mysterious structure may be only a few million years old, perhaps leftover from a massive burst of star formation or an eruption from Sagittarius A*.

An artist’s concept of our Milky Way galaxy’s dark matter halo.

An even larger halo of dark matter (about a million light-years across) cocoons the stellar halo. This mystery material has mass, so its gravity pulls on things we can see. But it isn’t visible itself, and no one knows exactly what it’s made of. This strange stuff makes up about 90 percent of our galaxy’s mass.

This illustration, taken from a computer simulation, visualizes the Milky Way’s dark matter halo (as well as several surrounding dark matter clumps) in blue.

Scientists know it’s there because if it weren’t, stars would orbit much faster near the galaxy’s center than on the outskirts. But for the most part, orbital speeds are pretty constant regardless of distance from the center. Stars toward the edge of the disk whirl around so quickly that they should be flung off into space if there weren’t something keeping them anchored to the Milky Way. Dark matter holds our galaxy together.

This collage shows the Milky Way in 10 different wavelengths of light, from radio waves to gamma rays. By studying our galaxy in different types of light, astronomers can learn far more than they could otherwise.

While astronomers have mapped much of our galaxy’s bulge, disk, and stellar and dark matter halos, key details about its structure and hidden components remain unknown. NASA is tackling the Milky Way’s mysteries with a fleet of space telescopes designed to explore the universe in different ways.

For example, our upcoming Nancy Grace Roman Space Telescope will peer through dust with a large field of view to map stars, dust and gas clouds on the far side of the galaxy, revealing hidden structures, spiral arms, and stellar nurseries. Our picture of our home galaxy will soon be clearer than ever before!

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