The Milky Way
The Milky Way is our home: the galaxy that contains our Sun and every star you can see with the naked eye. It is a barred spiral galaxy about 100,000 light years across, with somewhere between 100 and 400 billion stars, a supermassive black hole at its centre, and a giant invisible halo of dark matter wrapping it all up. From here on Earth, we can see the disc of our own galaxy as a hazy band of light arching across the sky, the original milky way after which the whole galaxy is named.
- TypeBarred spiralType SBc
- Diameterapprox. 100,000 light yearsThe disc
- Number of starsapprox. 100 to 400 billionPlus possibly a trillion planets
- Number of arms4 mainPerseus, Sagittarius, Centaurus, Outer
- Sun's distance from centreapprox. 27,000 light yearsAbout halfway out
- Central black holeSagittarius A*approx. 4 million solar masses
What our galaxy looks like
The Milky Way is shaped like a flat disc with a bulge in the middle, similar to two fried eggs stuck back to back. The disc has long curving spiral arms wrapping around the centre. A straight bar of stars runs through the middle of the bulge, and the spiral arms come off the ends of the bar (which makes the Milky Way a barred spiral).
If you were able to look down on the galaxy from far above, you would see four main spiral arms, named after the constellations they appear to point at from Earth: the Perseus Arm, the Sagittarius (or Carina) Arm, the Scutum-Centaurus Arm and the Outer Arm. Our Sun sits in a smaller "side street" called the Orion Spur, between the Perseus and Sagittarius arms.
Our place in the galaxy
Our Sun sits about 27,000 light years from the centre of the Milky Way, roughly halfway out from the bulge to the edge of the disc. We orbit the galactic centre at around 220 km/s, and one full lap takes about 225 million years. The dinosaurs were just appearing the last time the Sun was in this part of the Milky Way.
The Milky Way contains somewhere between 100 and 400 billion stars, and current estimates suggest there are at least as many planets as stars: possibly a trillion planets in total. Most stars are far smaller and dimmer than our Sun, but every one of them is a star.
Sagittarius A*: the central black hole
At the exact centre of the Milky Way sits a supermassive black hole called Sagittarius A* (said "Sagittarius A-star"). It has the mass of about 4 million Suns packed into a region only about the size of Mercury's orbit. We cannot see Sagittarius A* directly because thick clouds of gas and dust between Earth and the centre block visible light.
But astronomers can see the effects of its gravity on nearby stars. By tracking the orbits of about 30 stars whipping around the galactic centre at thousands of kilometres per second, the German astronomer Reinhard Genzel and the American astronomer Andrea Ghez proved a black hole had to be there. They shared the 2020 Nobel Prize in Physics for this work. In 2022 the Event Horizon Telescope released a direct image of Sagittarius A*, the second-ever real photograph of a black hole.
How to see the Milky Way in the night sky
The disc of our galaxy is visible from anywhere on Earth, as long as the sky is dark enough. Get well away from city lights on a clear, moonless night, and look up. You will see a pale band of soft light stretching across the sky from one horizon to the other. That band is what people on Earth call the milky way: the combined glow of millions of stars in the disc of our galaxy, too faint and crowded for your eyes to separate one from another.
The brightest, thickest part of the band is in the constellation Sagittarius, and that is the direction of the centre of our galaxy. (You are looking directly towards Sagittarius A*, although the black hole itself is hidden behind clouds of dust.) The Milky Way is at its best in the summer evenings of the Northern Hemisphere and the autumn evenings of the Southern Hemisphere.
The dark matter halo
The bright stars and gas of the Milky Way are only about 10% of its total mass. The remaining 90% is an invisible halo of dark matter that wraps around the disc and bulge, stretching at least three times further than the visible galaxy. We cannot see this dark matter directly: we only know it is there because the gravity of the visible stars is not enough to explain how fast the outer parts of the galaxy are rotating.
The Milky Way also has a smaller halo of older red and metal-poor stars, plus around 150 globular star clusters: tight balls of hundreds of thousands of old stars that orbit the galaxy in all directions. Globular clusters are some of the oldest objects in the universe, and studying them tells us about the early days of our galaxy.
Deeper dive: the Milky Way's satellite galaxies
The Milky Way is the gravitational boss of a small empire of dwarf galaxies that orbit it. The two biggest and easiest to see are the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), both visible to the naked eye from the Southern Hemisphere. The LMC and SMC look like dim detached patches of the Milky Way, hence their name (people thought they were chunks of the Milky Way before recognising them as separate galaxies).
Beyond the Magellanic Clouds, the Milky Way has at least 50 known smaller dwarf galaxies, with names like Sagittarius, Sculptor, Draco, Carina, Fornax, Leo I and II. Some of these are slowly being torn apart by our galaxy's gravity. The Sagittarius Dwarf is being stretched into a long ribbon of stars that already wraps right around the Milky Way several times. Eventually all its stars will be absorbed into the main galaxy.
One of the missions of the European Space Agency's Gaia telescope, launched in 2013, is to map the positions and motions of over a billion stars in and around the Milky Way. The data has already revealed long streams of stars from old galaxy mergers and is letting astronomers reconstruct the early history of our home galaxy in extraordinary detail.
For our nearest big neighbour, see the Andromeda Galaxy. For the general types, see spiral galaxies and elliptical galaxies.