Our galaxy, seen from inside
On a dark night away from city lights, a pale, hazy band stretches across the sky. This is the Milky Way as we see it, and the name is also used for the whole galaxy that contains the Sun, the planets and every individual star you can see with the naked eye. When Galileo turned an early telescope on the band in 1610, he found that it was made of vast numbers of faint stars too dim to see separately. That discovery was among the results of the Scientific Revolution that changed ideas about our place in the universe.
We live inside the galaxy, so we cannot step back and photograph it. Its shape has been worked out by combining many measurements: star counts, radio and infrared observations that see through dust, and careful distance measurements. It is a bit like working out the layout of a forest while standing among the trees.
Shape and structure
The Milky Way is generally classified as a barred spiral galaxy. Its main parts are:
- The disk. A flat, rotating structure containing most of the stars, gas and dust. Seen edge-on, it is thin compared with its width.
- The spiral arms. Curving lanes in the disk where stars form more often and where gas is dense. Their exact number and arrangement are still debated, and the pattern is harder to map from inside than for other galaxies.
- The central bulge and bar. Around the center is a thicker, roughly elongated concentration of older stars, which is the bar.
- The halo. A roughly spherical region around the disk, with older stars, globular clusters (tight, ancient groups of stars) and a large amount of unseen matter.
The band across our sky is the disk seen from within. We look along its length and so see many stars piled up in the same direction. Dark patches in the band are clouds of dust blocking the light behind them.
How big is it?
The stellar disk is roughly 100,000 light years across, though estimates of its edge vary depending on how it is defined. Its thickness is around 1,000 light years in the main part of the disk, so it is very flat for its size. A light year is the distance light travels in a year, which gives a sense of how vast this is.
The number of stars is only known roughly. A typical quoted range is somewhere from about 100 billion to a few hundred billion, since faint small stars are hard to count. The total mass, including unseen matter, is estimated at around a trillion times the mass of the Sun, though measurements differ.
Where does the Sun sit?
The Sun is located about 26,000 light years from the center of the galaxy, which puts it roughly two thirds of the way out from the middle to the edge of the disk. It lies in a smaller structure of stars and gas sometimes called the Orion Spur or Local Arm, between two of the larger spiral arms.
The Sun is not stationary. It orbits the galactic center at a speed of roughly 220 kilometers per second, and one full circuit takes on the order of 200 to 250 million years. By that rough measure, the Sun has made only about twenty trips around the galaxy since it formed. For scale, the farthest space exploration probes ever launched have covered only a tiny fraction of the distance to the galactic center.
Our location is not special in any way we can identify. It is a quiet suburb: far enough from the crowded center to avoid frequent close encounters with other stars, yet inside the region where heavy elements are plentiful enough to build rocky planets.
What is in the center?
At the heart of the galaxy is a compact radio source called Sagittarius A*. Studies of stars orbiting it for years have shown that they move as if circling an unseen object of about four million times the Sun’s mass. The best explanation is a supermassive black hole. In 2022, the Event Horizon Telescope collaboration released a radio image of the glowing ring around it, which matched predictions.
The center is hidden from optical telescopes by dust, so it is observed with infrared, radio and X-ray instruments. Many are space telescopes, which are not bothered by Earth’s atmosphere.
What fills the galaxy?
Apart from stars, the Milky Way contains:
- Gas and dust. Mostly hydrogen, forming the raw material for new stars.
- Star clusters. Open clusters of young stars in the disk and globular clusters in the halo.
- Planets. Surveys suggest that planets are very common around stars, though a complete count does not exist.
- Dark matter. Stars in the outer disk orbit faster than visible matter alone would explain, which is one of the main reasons astronomers infer a large unseen halo.
New stars are still being born in the disk, at a modest rate, as the stellar life cycle continues.
Neighbors and the future
The Milky Way belongs to a small collection of galaxies called the Local Group. The largest member is the Andromeda galaxy, about 2.5 million light years away. The Milky Way is the other big spiral, and the smaller Triangulum galaxy is a third. Two small companion galaxies, the Large and Small Magellanic Clouds, are visible from the Southern Hemisphere.
Andromeda is approaching us, and calculations suggest the two galaxies will probably merge in several billion years. Even then, stars are so widely spaced that direct collisions between them would be rare. Forecasts of this kind carry real uncertainty, because they depend on measurements of small sideways motions.
Why it matters
Studying our own galaxy allows astronomers to look at individual stars in a way that is impossible for distant galaxies. It serves as a test bed for ideas about how galaxies form and grow. And the galaxy is only one of a very large number in the observable universe, each with its own history.