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When you look up at the night sky from a place free of city lights, you may see thousands of stars twinkling above. But those are just a fraction of what’s truly out there. Every star you can see with your eyes belongs to our home galaxy: the Milky Way. And the Milky Way is just one of around 2 trillion galaxies in the observable universe.
What Is a Galaxy?
A galaxy is a vast system made up of stars, gas, dust, and dark matter, all bound together by the force of gravity. Our own galaxy, the Milky Way, holds around 200 billion stars, with the Sun being just one of them. When viewed from Earth, it appears as a soft, glowing band of light stretching across the night sky, interrupted by dark streaks of interstellar dust. The ancient Greeks called it Galaxias Kuklos, or “Milky Circle,” while the Romans referred to it as Via Lactea, meaning “Milky Road”—a name that eventually evolved into “Milky Way.”
If we could step outside the galaxy and look down on it from above, the Milky Way would resemble a vast spiral, shining like a cosmic pinwheel against the darkness of space.
The Structure of the Milky Way
The Milky Way is classified as a barred spiral galaxy, with a central bulge and multiple arms spiraling outward. Scientists have identified five major spiral arms:
- Norma Arm
- Scutum-Crux Arm
- Sagittarius Arm
- Perseus Arm
- Cygnus Arm
Our solar system sits within a smaller region known as the Orion Arm (or Local Arm), nestled between the larger Sagittarius and Perseus arms of the Milky Way. It’s about halfway from the center of the galaxy to its outer edge.
At the core of the Milky Way lies a dense central region known as the bulge, populated with older, cooler stars. The disk surrounding the bulge is much thinner and hosts younger, hotter stars, along with clouds of gas and dust.
If you could see the Milky Way from the side, it would resemble a fried egg—a flat disk with a bulging center.
How Do We Know What the Milky Way Looks Like?
Since we’re inside the Milky Way, we can’t take a picture of it from the outside. Instead, astronomers use observations of nearby galaxies, star movements, and mapping of radio waves and infrared light to piece together the galaxy’s shape—like figuring out the layout of a forest while standing inside it.
The Sun’s Journey Through the Galaxy
Just like Earth orbits the Sun, our entire solar system orbits the center of the Milky Way. We’re traveling at a speed of 250 kilometers per second, but the galaxy is so vast that it takes about 225 million years to complete one full orbit.
The Milky Way spans an incredible 950 quadrillion kilometers across (that’s 950,000,000,000,000,000 km)!
