The Milky Way is a barred spiral galaxy containing our Sun, Earth, and every star visible to the naked eye. It stretches roughly 100,000 light-years across and holds an estimated 100 to 400 billion stars, all orbiting a supermassive black hole at the center. When you look up on a clear night and see that hazy band of light stretching across the sky, you’re looking at the disk of the galaxy from the inside.
Shape and Structure
The Milky Way belongs to a category called barred spiral galaxies, which make up about two-thirds of all spiral galaxies. “Barred” refers to a thick, elongated bar of stars running through the center, with spiral arms extending from the bar’s ends. If you could view the Milky Way from above, it would look like a pinwheel with a bright, stretched core.
The galaxy has two major spiral arms, called Scutum-Centaurus and Perseus, attached to the ends of the central bar. Two smaller, less distinct arms called Norma and Sagittarius sit between them. Our solar system lives in a small, partial structure called the Orion Arm (sometimes called the Orion Spur), which sits between the Sagittarius and Perseus arms. Think of it as a side branch off one of the main arms rather than a full arm itself.
From the side, the Milky Way would look like a thin disk with a bulge in the middle, somewhat like two fried eggs pressed together. The disk is where most of the stars, gas, and dust reside, while the central bulge is a dense, roughly spherical collection of older stars surrounding the galactic center.
The Black Hole at the Center
At the very heart of the Milky Way sits a supermassive black hole called Sagittarius A* (pronounced “A-star”). It has a mass about 4 million times that of our Sun. Despite being extraordinarily massive by any human standard, it’s actually modest compared to the supermassive black holes found in larger galaxies, some of which are billions of times the Sun’s mass.
Sagittarius A* doesn’t actively consume large amounts of material right now, so it’s relatively quiet. The region immediately around it spans about half a light-year and is detectable in X-ray observations. In 2022, the Event Horizon Telescope collaboration released the first direct image of this black hole’s shadow, confirming decades of indirect evidence for its existence.
How Big and Heavy Is the Milky Way?
The stellar disk spans about 100,000 light-years in diameter. To put that in perspective, light traveling at 186,000 miles per second would take 100,000 years to cross from one side to the other. Our solar system sits roughly 26,000 light-years from the center, placing us a little more than halfway out from the core to the edge.
The total mass of the galaxy is about 1.5 trillion times the mass of our Sun, based on measurements combining data from the Hubble Space Telescope and the European Space Agency’s Gaia mission. Earlier estimates ranged anywhere from 500 billion to 3 trillion solar masses, so the current figure lands near the middle of that range. The vast majority of this mass isn’t stars, gas, or planets. It’s dark matter, an invisible substance that doesn’t emit or absorb light but exerts gravitational pull. Dark matter may account for up to 95% of all matter in the Milky Way, forming an enormous halo that extends far beyond the visible disk.
Where We Are and How Fast We’re Moving
Our solar system orbits the galactic center at about 250 kilometers per second, which works out to roughly 570,000 miles per hour. Even at that speed, one full orbit takes approximately 225 to 250 million years. The last time Earth was in this same position relative to the galactic center, dinosaurs hadn’t yet appeared.
You wouldn’t feel any of this motion. Everything in our neighborhood, including nearby stars and interstellar gas, is moving along at a similar speed and direction, so there’s no sensation of travel. It’s the same reason you don’t feel the Earth spinning. From our vantage point on the Orion Arm, we see the galaxy’s disk as that familiar milky band of light because we’re embedded within it, looking along the plane of the disk rather than from above or below.
What the Milky Way Is Made Of
Stars are the most visible component, but they represent only a small fraction of the galaxy’s total mass. The space between stars contains vast clouds of gas (mostly hydrogen and helium) and dust, collectively called the interstellar medium. These clouds are the raw material for new stars. In denser regions, gravity pulls gas and dust together until it collapses and ignites nuclear fusion, forming a new star.
The galaxy also contains stellar remnants: white dwarfs (the cooling cores of dead Sun-like stars), neutron stars (the ultra-dense remains of larger stars), and stellar-mass black holes scattered throughout the disk. Then there’s the dark matter halo, invisible but gravitationally dominant, forming a roughly spherical shell that envelopes the entire visible galaxy and extends well beyond it. Without dark matter’s gravitational pull, the outer stars would fly off into intergalactic space because the visible matter alone doesn’t generate enough gravity to hold the galaxy together at the speeds those stars are traveling.
The Milky Way’s Neighborhood
The Milky Way is part of a collection of galaxies called the Local Group, which contains more than 50 galaxies spread across roughly 10 million light-years. The two largest members are the Milky Way and the Andromeda galaxy (also a barred spiral), which is about 2.5 million light-years away. Most of the remaining galaxies in the group are small dwarf galaxies, many of which orbit the Milky Way or Andromeda as satellites.
The Milky Way is currently pulling in material from some of its smaller companion galaxies. The Sagittarius Dwarf Elliptical Galaxy, for example, is being stretched apart and absorbed by our galaxy’s gravity, a process that has been ongoing for hundreds of millions of years.
Collision With Andromeda
The Milky Way and Andromeda are heading toward each other, pulled by their mutual gravity. Hubble Space Telescope observations indicate the two galaxies will collide head-on about 4 billion years from now. Around 6 billion years from now, they will fully merge into a single, larger elliptical galaxy (sometimes nicknamed “Milkomeda”).
Despite the dramatic language of a “collision,” individual stars are so far apart that almost none of them would actually hit each other. The gravitational interaction would reshape both galaxies, flinging some stars into new orbits and triggering waves of new star formation as gas clouds compress. Our solar system would likely survive the merger intact, though it could end up in a very different part of the resulting galaxy. By that point, the Sun will be nearing the end of its life anyway, so the collision wouldn’t be humanity’s most pressing concern even if we were still around.