What is the Milky Way? Our galaxy, explained

The Milky Way is the galaxy that contains the Sun, Earth and every star you can see with the naked eye. It is a flat disk of somewhere between 100 billion and 400 billion stars, about 100,000 light-years across, with a bar of stars across its middle and spiral arms winding out from it. The pale band called the Milky Way in a dark sky is that disk, seen edge-on from inside.

In short

• The Milky Way is a barred spiral galaxy, one of hundreds of billions in the universe, and the Sun is about halfway out from its center.
• The glowing band across the night sky is the rest of the galaxy's disk, seen from within it; its brightest part lies toward the center, in the constellation Sagittarius.
• At the very center is Sagittarius A*, a black hole of about 4 million Suns, first photographed in 2022.

What does the Milky Way look like?

From outside, it would look like a flat, bright pinwheel. Its stars, gas and dust lie mostly in a thin disk, 100,000 light-years wide, according to NASA, but only about a thousand light-years thick in the thin layer where most of its stars are, by most estimates. If the disk were shrunk to the width of a dinner plate, it would be about as thick as a coin. In the middle is a bulge of older stars crossed by a bar, which is why astronomers call it a barred spiral.

Spiral arms wind out from the ends of the bar. NASA's map of the galaxy shows two major arms, Scutum–Centaurus and Perseus, with smaller ones between them. The arms are not solid structures that rotate like a wheel. They are more like traffic jams: zones where stars and gas crowd together as they pass through, and where new stars form, which makes the arms bright. All of this sits inside a much larger, roughly spherical halo of old stars, star clusters and dark matter, the invisible mass that makes up most of the galaxy's weight.

No one has ever seen the Milky Way from outside, and no spacecraft could go far enough. Its shape is worked out from inside, by measuring the positions and motions of stars and gas. The European Space Agency's Gaia satellite has measured the positions and motions of more than 1.5 billion stars, a small fraction of the total but enough to map our part of the galaxy in three dimensions.

How many stars are in the Milky Way?

Estimates run from about 100 billion to about 400 billion. The range is wide because most stars are faint red dwarfs that can't be counted directly over most of the galaxy, and because dust hides much of the disk. Astronomers estimate the number from the galaxy's total mass and from how stars of different sizes are spread, so the count depends on assumptions.

To feel the size of those numbers, count. At one star per second, without stopping to sleep:

1. A day has 86,400 seconds, so a year has about 31.6 million.
2. 100 billion stars ÷ 31.6 million per year ≈ 3,200 years of counting.
3. 400 billion stars would take about 12,700 years, longer than all of recorded history.

The galaxy probably holds at least as many planets as stars. Whether any of them host life, and why we have seen no sign of it, is the question known as the Fermi paradox.

Where is the Sun in the Milky Way?

The Sun sits about 26,000 light-years from the center, roughly halfway to the edge; published estimates range from about 25,000 to 27,000 light-years. It lies in the Orion Arm, a small, partial arm between two larger ones. The Sun, carrying the solar system with it, orbits the galaxy's center about once every 230 million years, by NASA's count. On that clock, the last time the Sun was where it is now, the first dinosaurs were appearing.

Why does the Milky Way look like a band across the sky?

Because we're inside the disk. Look along the disk and your line of sight passes through thousands of light-years of stars, too far and too many to see one by one, which blur into a glowing stripe. Look up or down out of the disk and you see only nearby stars against dark space. Dark lanes in the band are clouds of dust blocking the light behind them.

The band is brightest toward the galaxy's center, in the constellations Sagittarius and Scorpius. From mid-northern latitudes, that part is best seen on summer evenings, low in the south. From the Southern Hemisphere it passes high overhead. But you need a dark sky. A 2016 world atlas of light pollution found that the Milky Way is hidden from more than one-third of humanity, including 60% of Europeans and nearly 80% of North Americans, as Falchi and colleagues reported in Science Advances.

What is at the center of the Milky Way?

A supermassive black hole called Sagittarius A*, with about 4 million times the Sun's mass. It is about 27,000 light-years away, according to the European Southern Observatory, which helped release the first image of it on 12 May 2022. Despite its mass, it is quiet: it swallows little gas, so it gives off far less light than the bright cores of some other galaxies.

How did we learn all this?

Each step moved us further from the center of things.

• Ancient Greece. The Greeks called the band galaxías kýklos, "milky circle", which gave us the word galaxy.
• 1610. Galileo turned an early telescope on the band and found it was made of countless faint stars.
• 1918. Harlow Shapley used clusters of stars to show that the Sun is not at the galaxy's center, but far out in its disk.
• 1923–1925. Edwin Hubble found pulsing stars in the Andromeda "nebula" and used them to show it is far outside the Milky Way, a separate galaxy. The Milky Way stopped being the whole universe and became one galaxy among many.
• 1974. Radio astronomers found a compact radio source at the center, later named Sagittarius A*.
• 2020. Reinhard Genzel and Andrea Ghez shared a Nobel Prize for tracking stars around it and showing it must be a black hole.
• 2022. The Event Horizon Telescope released its image.

Still argued

The Milky Way and Andromeda, about 2.5 million light-years away, are approaching each other, and a collision in about 5 billion years was long treated as near-certain. A 2025 study in Nature Astronomy, led by Till Sawala, reran the calculation with the measurement uncertainties included, and with the pull of two smaller galaxies, and found only about a 50% chance of a merger within the next 10 billion years, as its authors explained. The biggest unknown is Andromeda's sideways motion, which better measurements may settle. Even if the two merge, stars are so far apart that they would almost never collide.

The Milky Way is one galaxy among hundreds of billions in the universe, and a fairly large one. It is the only one we will ever see from the inside.

Check yourself

• Why does the Milky Way appear as a band across the sky rather than spread evenly?
• Why is the number of stars in the Milky Way given as a range as wide as 100 to 400 billion?
• Where is the Sun in the galaxy, and how long does one orbit around the center take?

Sources

• The Milky Way galaxy — NASA Imagine the Universe
• The Milky Way galaxy, annotated map — NASA
• Astronomers reveal first image of the black hole at the heart of our galaxy — European Southern Observatory, 2022
• The new world atlas of artificial night sky brightness — Falchi and others, Science Advances, 2016
• Astronomers thought the Milky Way was doomed to crash into Andromeda. Now they're not so sure — University of Western Australia, The Conversation, 2025
• Milky Way — Wikipedia