How big is the Milky Way?

Its stellar disc is about 100,000 light-years across and roughly 1,000 light-years thick. Faint outer stars and the surrounding dark-matter halo extend much further.

How many stars are in the Milky Way?

Estimates range from about 100 billion to 400 billion. The exact number is uncertain because most stars are faint red dwarfs and many are hidden behind dust.

Where is Earth in the Milky Way?

The Solar System lies in the Orion Spur, a minor spiral arm, about 26,000 light-years from the galactic centre. It sits close to the mid-plane of the disc.

How long does it take the Sun to orbit the Milky Way?

About 230 million years, a period sometimes called a galactic year. The Sun travels at around 230 km/s along its orbit.

Will the Milky Way collide with Andromeda?

Possibly, but it is not certain. A 2025 study found roughly a 50% chance of a merger within the next 10 billion years, and if it happens it will most likely be well beyond 5 billion years from now.

Our home galaxy, a spiral of billions of suns

The Milky Way

Barred spiral galaxy · We are inside it

The Milky Way is the galaxy that contains the Sun, Earth and every star you can see with the naked eye. It is a barred spiral galaxy, a flattened disc of stars, gas and dust about 100,000 light-years across, with a bar of older stars through its centre and spiral arms winding outwards. It holds somewhere between 100 and 400 billion stars, and it is wrapped in a far larger halo of invisible dark matter.

We live inside it, about 26,000 light-years from the centre, so we see the disc edge-on as a pale band of light arching across a dark night sky. That band gave the galaxy its name. Mapping a galaxy from the inside is difficult, and many details of its shape are still being worked out.

Key facts

Type: Barred spiral galaxyClassified around SBbc
Diameter of the disc: ≈ 100,000 light-yearsFaint outer stars extend further
Number of stars: 100–400 billion
Total mass: ≈ 1–1.5 trillion M☉Mostly dark matter
Sun's distance from centre: ≈ 26,000 light-years
Sun's orbital period: ≈ 230 million yearsOne 'galactic year'
Sun's orbital speed: ≈ 230 km/sAbout 830,000 km/h
Age of oldest stars: ≈ 13 billion years or more
Central black hole: Sagittarius A*≈ 4.3 million M☉

Structure of the galaxy

Seen from above, the Milky Way would look like a pinwheel. Its central bulge is crossed by a bar of mostly older, yellowish stars several thousand light-years long. Spiral arms rich in gas, dust and young blue stars wind out from the ends of the bar. Two major arms, Scutum–Centaurus and Perseus, carry many of the stars, with smaller arms and spurs between them.

The thin disc where the arms lie is only about 1,000 light-years thick, very slim compared with its width. Around it lies a sparse, roughly spherical halo of old stars and about 150 globular clusters. All of this sits inside a dark-matter halo that extends hundreds of thousands of light-years and outweighs the visible stars and gas many times over.

The galactic centre

At the very centre lies Sagittarius A*, a black hole of about 4.3 million solar masses, surrounded by the densest cluster of stars in the galaxy. Thick dust hides the centre from visible-light telescopes, so it is studied with radio, infrared and X-ray instruments.

A warped disc

The outer disc is not flat. It bends up on one side and down on the other, like the brim of a hat. Astronomers think gravitational tugs from satellite galaxies or the dark-matter halo may cause this warp. Gaia's measurements suggest the warp itself slowly rotates, or precesses, around the galaxy over hundreds of millions of years.

Where the Sun fits in

The Sun lies in a minor arm called the Orion Spur, between the larger Sagittarius and Perseus arms, about 26,000 light-years from the centre. It is a fairly quiet location, away from the crowded inner galaxy, where close encounters with other stars and nearby supernovae are rare.

The Sun orbits the galactic centre at roughly 230 km/s and takes about 230 million years to complete one lap. Since it formed 4.6 billion years ago, it has gone around about 20 times. The last time the Sun was at its current position in the galaxy, the first dinosaurs were appearing on Earth. As it travels, it also bobs gently up and down through the disc.

How we learned what the Milky Way is

The ancient Greeks called the band of light galaxias kyklos, the milky circle, and the word galaxy comes from it. In 1610 Galileo turned his telescope on it and found it was made of countless faint stars. In the late 1700s William Herschel counted stars in different directions and concluded that we live inside a flattened system, though he placed the Sun near its centre.

In 1918 Harlow Shapley used globular clusters to show the Sun lies far from the centre. Whether the Milky Way was the entire universe remained debated until Edwin Hubble showed in the 1920s that spiral nebulae such as Andromeda are separate galaxies. Today, radio maps of hydrogen gas and the Gaia spacecraft's precise measurements of more than a billion stars are revealing the galaxy's shape in unprecedented detail.

Satellites and swallowed galaxies

The Milky Way is circled by dozens of smaller satellite galaxies. The largest are the Large and Small Magellanic Clouds, visible to the naked eye from the Southern Hemisphere at about 160,000 and 200,000 light-years away. The Sagittarius Dwarf galaxy, on the far side of the centre, is being stretched into long streams of stars as it orbits through the disc.

The galaxy grew partly by swallowing others. Data from Gaia revealed that about 8 to 11 billion years ago the Milky Way merged with a sizeable galaxy, named Gaia-Enceladus or Gaia-Sausage, whose stars still move on distinctive orbits through the halo. The Milky Way, Andromeda, the Triangulum Galaxy and dozens of dwarf galaxies together form a group called the Local Group.

The future: Andromeda

The Andromeda Galaxy, about 2.5 million light-years away, is approaching the Milky Way at roughly 110 km/s. For years, the standard forecast was a head-on collision in about 4 to 5 billion years, ending in a single large elliptical galaxy sometimes nicknamed Milkomeda. The idea dates back to measurements of Andromeda's motion that seemed to point it almost straight at us.

A 2025 study that included the gravity of other Local Group galaxies, especially the Large Magellanic Cloud and Triangulum, found the outcome is far less certain. It gave roughly even odds of a merger within the next 10 billion years, and only a small chance of one within 5 billion years. Even in a collision, stars are so widely spaced that direct hits between them would be extremely rare.

Surprising facts about the Milky Way

The Sun has completed only about 20 laps of the galaxy since it formed.
Most of the Milky Way's mass is not in stars or gas but in dark matter that has never been seen directly.
If the Milky Way's disc were shrunk to the width of a football pitch, the whole Solar System out to Neptune would be about the size of a speck of fine dust.
Light from the far side of the galactic disc takes tens of thousands of years to reach us, so we see that part of the galaxy as it was long before human civilisation began.
The Milky Way has absorbed several smaller galaxies, and remnants of them still travel through its halo as streams of stars.

Frequently asked questions

How big is the Milky Way?: Its stellar disc is about 100,000 light-years across and roughly 1,000 light-years thick. Faint outer stars and the surrounding dark-matter halo extend much further.
How many stars are in the Milky Way?: Estimates range from about 100 billion to 400 billion. The exact number is uncertain because most stars are faint red dwarfs and many are hidden behind dust.
Where is Earth in the Milky Way?: The Solar System lies in the Orion Spur, a minor spiral arm, about 26,000 light-years from the galactic centre. It sits close to the mid-plane of the disc.
How long does it take the Sun to orbit the Milky Way?: About 230 million years, a period sometimes called a galactic year. The Sun travels at around 230 km/s along its orbit.
Will the Milky Way collide with Andromeda?: Possibly, but it is not certain. A 2025 study found roughly a 50% chance of a merger within the next 10 billion years, and if it happens it will most likely be well beyond 5 billion years from now.

Keep exploring

The 3D views in Ember’s Universe are scientific visualisations: physically based where possible, compressed or exaggerated where the real scales would hide everything.

Sources