How big is Milky Way? | Scale of the Universe

The Milky Way is the galaxy that contains the Sun, Earth and every star visible in the night sky. A galaxy is a huge group of stars, gas and dust held together by gravity. The Milky Way's main disk of stars is about 100,000 light-years across, where a light-year is the distance light travels in one year, about 9.46 trillion kilometers. Fainter stars reach much farther out.

Size

The Sun sits inside the Milky Way, so no one can photograph the whole galaxy from outside. Astronomers work out its shape and size by mapping the stars around the Sun and comparing the results with other galaxies. The European Space Agency (ESA) describes the disk that holds most of the galaxy's stars as about 50,000 light-years in radius and only about 1,000 light-years thick. That makes it about 100,000 light-years across, the width NASA also gives.

A galaxy has no sharp edge, because its stars thin out gradually. A 2018 study compared the chemical makeup of distant stars in the plane of the disk with that of stars in the halo around it. It found disk stars at least 85,000 light-years from the center, and probably more than 100,000. Counting those stars, the disk is about 170,000 to 200,000 light-years across.

Around the disk is a roughly round halo of old stars and star clusters. ESA says it reaches about 100,000 light-years from the center, which makes it about 200,000 light-years across.

Size comparisons

The Milky Way's disk is about 6 billion times the average distance from Earth to the Sun.
If the disk were shrunk to the width of Earth, the Sun and Earth would be about 2 millimeters apart, and Proxima Centauri, the nearest star to the Sun, would be about 540 meters (1,800 feet) away.
If the Sun were shrunk to the size of a basketball, the disk would be about 160 million kilometers across, a little more than the real distance from Earth to the Sun.
The disk is about 100 times as wide as it is thick. Shrunk to the width of a basketball, 24 centimeters (9.4 inches), it would be about 2.4 millimeters thick.
The bright disk of the Andromeda Galaxy, the nearest large galaxy and one of the objects in the explorer slightly larger than the Milky Way, is about 140,000 light-years across, about 1.4 times as wide.

What the Milky Way is made of

The Milky Way is a barred spiral galaxy. At its center is a bulge of about ten billion mostly old, red stars, stretched into a bar about 20,000 light-years long. The disk around it holds most of the galaxy's stars, arranged in spiral arms. Maps of young, hot stars and glowing clouds of hydrogen show four main arms.

At the very center is a supermassive black hole called Sagittarius A*, with the mass of about four million Suns. NASA estimates that the galaxy holds 100 to 400 billion stars in all. The whole galaxy sits inside a larger halo of dark matter, which gives off no light and is detected only by its pull on the stars.

The Sun's place in the galaxy

The Sun lies about 26,000 light-years from the center of the Milky Way, roughly halfway out to the edge of the disk. It circles the center of the galaxy, much as Earth circles the Sun. One trip around takes about 250 million years.

Seen from inside the flat disk, the galaxy's stars form a pale band across the sky on dark nights. That milky-looking band gave the galaxy its name. The band is also a clue to the galaxy's shape. If the Milky Way were a round elliptical galaxy, its stars would be spread all over the sky instead.