The Milky Way is made of stars, gas, dust, and a massive envelope of dark matter that outweighs everything visible. Current estimates place its total mass between 1.2 and 1.9 trillion times the mass of our sun, but the roughly 100 billion stars you might picture when you think of a galaxy account for only a fraction of that figure.
Stars: The Visible Backbone
Astronomers estimate the Milky Way contains around 100 billion stars. These range from red dwarfs smaller than half our sun (by far the most common type) to blue supergiants dozens of times more massive. Together with gas and dust, the visible material in the galaxy adds up to about 60 billion solar masses. That means the average star in the Milky Way is actually less massive than our sun, which makes sense given how overwhelmingly red dwarfs dominate the stellar population.
Not all stars in the galaxy are alike in age or chemistry. The thin disk, where our solar system sits, contains relatively young, metal-rich stars traveling in orderly circular orbits. The thick disk, a puffier layer surrounding it, holds older stars with intermediate metal content. Far above and below the disk, the stellar halo is home to the galaxy’s oldest, most metal-poor stars, many of them born within the first billion years after the Milky Way began forming. The central bulge, a dense concentration of stars around the galactic core, contains a wide mix of ages and compositions.
Gas and Dust Between the Stars
The interstellar medium, the material scattered between stars, makes up 10 to 15 percent of the Milky Way’s visible mass. About 99 percent of that material is gas, with the remaining 1 percent being tiny solid particles known as dust.
The gas itself exists in several distinct phases. In the dense inner regions of the galaxy, especially along spiral arms, most of the gas is molecular hydrogen: pairs of hydrogen atoms bound together in cold, dense clouds. These molecular clouds are where new stars form. Farther out toward the galaxy’s edges, the gas becomes predominantly atomic, meaning lone hydrogen atoms drifting in a thinner, warmer state. There’s also a hot, ionized phase where hydrogen atoms have been stripped of their electrons by radiation from nearby massive stars or by shockwaves from supernovae. Gas cycles between these phases as it passes through spiral arms, gets compressed, forms stars, and is blown apart again.
Dust grains are microscopically small, mostly made of carbon and silicates. Though they represent a tiny share of the interstellar medium by mass, they punch above their weight in terms of influence. Dust blocks and reddens visible light, which is why the center of our own galaxy is invisible to optical telescopes. It also provides the surfaces where hydrogen atoms meet and bond into molecules, making dust essential to the star-formation process.
The Supermassive Black Hole at the Center
At the very core of the Milky Way sits Sagittarius A*, a supermassive black hole with a mass of about 4.15 million suns. That sounds enormous, but it’s actually modest compared to the black holes found in many other large galaxies, some of which are billions of solar masses. Relative to the galaxy’s total mass, Sagittarius A* is a tiny fraction, less than a rounding error. Its gravitational influence dominates only the innermost few light-years of the galaxy. Still, it plays a role in shaping the orbits of stars near the galactic center and likely influenced the galaxy’s evolution during earlier, more active periods when it was consuming material and releasing tremendous energy.
Dark Matter: The Invisible Majority
Most of the Milky Way’s mass isn’t stars, gas, or dust. It’s dark matter, a substance that doesn’t emit, absorb, or reflect light but exerts gravitational pull. Dark matter forms an enormous, roughly spherical halo that extends far beyond the visible disk of the galaxy.
How much dark matter the Milky Way contains is still debated. The visible material totals about 60 billion solar masses. If the galaxy’s total mass is in the range of 1.2 to 1.9 trillion solar masses (a widely cited estimate from 2019 combining data from the Hubble and Gaia space telescopes), dark matter would account for the vast majority, roughly 90 percent or more of the total. However, a more recent Gaia-based study arrived at a significantly lower total mass of around 200 billion solar masses. If that figure holds, the dark-to-ordinary matter ratio drops to roughly 2.3 to 1, far below the 10-to-1 ratio typically found in galaxies of similar size. This would make the Milky Way surprisingly light on dark matter compared to its peers.
Resolving this discrepancy is one of the bigger open questions in galactic astronomy. The answer depends on how far the dark matter halo extends and how researchers model the motions of the most distant stars and satellite galaxies orbiting the Milky Way.
How It All Fits Together
Picture the Milky Way as a set of nested components. At the center is the bulge, a dense ball of old and young stars surrounding the supermassive black hole. Extending outward is the disk, a flat structure about 100,000 light-years across containing spiral arms rich in gas, dust, and young stars. The disk itself has two layers: a thin disk where most star formation happens and a thicker, older layer above and below it. Surrounding the whole thing is the stellar halo, a sparse sphere of ancient stars and globular clusters. And enveloping everything, extending perhaps 300,000 light-years or more from the center, is the dark matter halo, invisible but gravitationally dominant.
The composition varies depending on where you look. In the spiral arms, cold molecular gas is abundant and new stars are actively forming. Between the arms, the gas is thinner and more atomic. In the halo, there’s almost no gas or dust at all, just old stars on long, looping orbits and the pervasive dark matter holding the whole structure together.