Our galaxy is far larger, brighter, and more massive than most other galaxies. From end to end, the Milky Way's starry disk, observable with the naked eye and through optical telescopes, spans 120,000 light-years. Encircling it is another disk, composed mostly of hydrogen gas, detectable by radio telescopes. Engulfing all that our telescopes can see is an enormous halo of dark matter. While it emits no light, this dark matter far outweighs the Milky Way's hundreds of billions of stars, giving the galaxy a total mass one to two trillion times that of the sun. Indeed, our galaxy is so huge that dozens of lesser galaxies scamper about it, like moons orbiting a giant planet.
As a result of its vast size, the Milky Way can boast at least one planet with intelligent life. Giant galaxies like the Milky Way and the nearby, even larger Andromeda galaxy possess the power to create and retain a rich supply of iron, oxygen, silicon, magnesium, and other elements heavier than helium. Forged by the Milky Way's abundant stars, such heavy elements are the building blocks of terrestrial planets.
Arrow points toward the center of the galaxy.
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