The Milky Way: Reading Our Own Galaxy from Inside It
Every star you can see with your unaided eye is inside the Milky Way. That's a problem — mapping a galaxy from a single vantage point deep inside it is like mapping a forest from the middle of a thicket. Most of what we know took a century of clever indirect measurement.
The band across the sky
On a dark night, the Milky Way appears as a faint, mottled band of light stretching from horizon to horizon. That band is our line of sight along the plane of the galaxy's disk. Away from the plane there are fewer stars; within it, we look through tens of thousands of light-years of stellar population and glowing gas.
The brightest and richest section runs through Sagittarius and Scorpius in the summer sky. That is the direction of the galactic center, 26,000 light-years away, where a 4-million-solar-mass black hole named Sagittarius A* sits at the bottom of the gravitational well.
The structure, in numbers
| Property | Value |
|---|---|
| Disk diameter | ≈ 100,000 – 120,000 ly |
| Disk thickness (thin) | ≈ 1,000 ly |
| Stellar mass | ≈ 6 × 10¹⁰ M☉ |
| Total mass (incl. dark matter) | ≈ 1.5 × 10¹² M☉ |
| Stars | 100 – 400 billion |
| Sun's distance from center | ≈ 26,000 ly |
| Sun's orbital period | ≈ 225 million yr |
| Age of oldest stars | ≈ 13 billion yr |
A barred spiral
The Milky Way is a barred spiral galaxy — a flattened disk with a central bar of stars and at least four major spiral arms wound around it. The Sun sits on the inner edge of the Orion Spur, a minor arm segment between the larger Sagittarius Arm (inward) and Perseus Arm (outward).
We know this because of decades of radio observations of neutral hydrogen (which reveal gas kinematics through the entire disk, unlike visible light which is blocked by dust), and more recently through Gaia's precise measurements of the positions, distances, and motions of nearly two billion stars.
The four components
The Milky Way has four distinct structural components. The thin disk (where the Sun lives, ~1,000 ly thick) contains most of the young stars and all the star-forming gas. The thick disk, about 3,000 ly thick, holds an older stellar population.
The bulge at the center is a dense, roughly boxy region of mostly old stars around the bar. The halo is a diffuse spherical envelope stretching out to ~200,000 light-years, populated by ancient stars and about 150 globular clusters — the oldest structures in the galaxy.
Where we're headed
The Milky Way and the Andromeda Galaxy (M31), 2.5 million light-years away, are approaching each other at 110 km/s. In about 4–5 billion years the two will begin a merger, forming a larger elliptical galaxy that astronomers already jokingly call 'Milkomeda'. Individual stars almost certainly won't collide — galaxies are mostly empty space — but the gas will collide vigorously, triggering a final major starburst.
Frequently asked
- Can I see the Milky Way from Miami?
- The bright southern band, yes — you just have to leave the coast. From the Everglades and Big Cypress preserves an hour or two west of the city, the summer Milky Way is prominent and structured. From inner Miami itself, light pollution washes it out.
- How many galaxies are like ours?
- Roughly a hundred billion galaxies in the observable universe; the Milky Way is a fairly typical large spiral. Andromeda is the nearest one comparable in size.
- What's at the center?
- Sagittarius A*, a supermassive black hole of 4.15 million solar masses. Its shadow was imaged by the Event Horizon Telescope in 2022.