A view from the inside
On a dark night away from city lights, a faint, cloudy band stretches across the sky. This is the Milky Way as we see it: the combined glow of an enormous number of stars, too distant to see individually, lying along a flat disk that we view edge-on from within. Ancient cultures told stories about it, and the English name comes from the Greek and Latin for “milky circle.” In 1610, Galileo’s telescope showed that the glow is made of countless stars.
Because we live inside the galaxy, we cannot photograph it from outside. Astronomers have built a picture of its shape by measuring the positions, distances and motions of stars and gas, and by comparing our galaxy to others that we can view from the side or face-on.
The overall shape
The Milky Way is generally classified as a barred spiral galaxy. Its main parts are:
- The disk. A thin, flat, rotating structure containing most of the stars, gas and dust. It is on the order of 100,000 light years across but only about a thousand light years thick in its thin part, comparable to a very wide, flat pancake. The numbers are approximate and depend on how the edge is defined.
- The central bulge and bar. Near the center, stars are packed more densely into a thicker bulge, and evidence suggests a bar-shaped structure of stars passes through it.
- Spiral arms. Curving lanes within the disk where gas is denser and star formation is more active. Astronomers still debate details such as how many major arms there are and exactly how they connect.
- The halo. A roughly spherical region surrounding the disk, containing old stars, globular clusters and a large amount of dark matter. The halo is extended and faint, but according to most estimates it holds a large share of the galaxy’s total mass.
How many stars are there? Estimates generally range from about one hundred billion to a few hundred billion. The uncertainty arises because many stars, especially dim ones, are hard to count.
The center
At the center of the galaxy lies a compact, massive object called Sagittarius A*. Astronomers tracked stars orbiting it for years, and their motions indicate a mass of around four million Suns packed into a very small region. This is the signature of a supermassive black hole. In 2022, a collaboration released a radio image of the glowing gas around it, consistent with expectations from general relativity. Our galaxy’s central black hole is relatively quiet today, in contrast to the extremely bright centers of some other galaxies.
Where we are
The Sun lies in the disk, roughly 26,000 light years from the center, which puts it about halfway out to the edge. Our position is on or near a minor spiral feature, often called the Orion Arm or Orion Spur, between two larger arms. We are neither at the center nor the outskirts.
The Sun orbits the galactic center, moving at several hundred kilometers per second. One full orbit takes roughly 220 to 250 million years, so since the Sun formed, it has circled the galaxy only about twenty times. Many details, such as how the Sun’s orbit has moved relative to the spiral arms, are still being studied.
All the stars you can see unaided in the night sky are our near neighbors in this vast structure, lying within a few thousand light years. Even our whole solar system is a tiny part of this galaxy.
Why we see a band
Because the disk is flat and we are inside it, looking along the plane of the disk shows us many more stars than looking up or down out of it. That produces the bright band. Dark patches in the band come from dust clouds that block starlight. Dust also prevents us from seeing the galaxy’s center in visible light; astronomers use infrared and radio telescopes to look through it. In the Northern Hemisphere, the brightest-looking part of the band is toward the constellation Sagittarius, in the direction of the center, and is best seen in summer.
Light pollution has made the Milky Way invisible to many people living in cities. If you want a better view, check a moon-phase calendar and look for a dark site away from town. A visit to a planetarium can also give a sense of what a dark sky looks like.
Our neighbors
The Milky Way has smaller satellite galaxies, including the Large and Small Magellanic Clouds, visible from the Southern Hemisphere. The nearest large spiral galaxy is Andromeda, about 2.5 million light years away. Andromeda and the Milky Way are approaching each other, and most models predict that they will merge in a few billion years, though the exact timing and details are uncertain. Even then, individual stars are so widely spaced that direct collisions between stars would be very rare.
The Milky Way, together with Andromeda and dozens of smaller galaxies, belongs to a gravitationally bound group called the Local Group. Our place in the universe is thus one galaxy among a very large number of others.