How big is Messier 54?

Messier 54, or M54, is a globular cluster, a dense ball of stars held together by their own gravity, in the constellation Sagittarius. It is about 86,000 light-years from Earth, where a light-year is the distance light travels in one year. It does not belong to the Milky Way but to a small galaxy that orbits it, the Sagittarius Dwarf Galaxy. Its bright main body is about 230 light-years across.

Size

Like all globular clusters, M54 has no sharp edge. Its stars are packed tightly at the center and thin out farther away, so its size depends on where the edge is drawn.

The New General Catalogue of nebulae and star clusters lists M54 as 9.1 arcminutes wide, where an arcminute is one-sixtieth of a degree. At the cluster's distance, that is about 230 light-years. The distance itself is known to within about 1,000 light-years.

Half of the cluster's light comes from within about 12 light-years of its center. Its faintest outer stars spread farther. A standard mathematical model of the cluster puts its outer edge about 10 arcminutes from the center, which makes the whole cluster about 490 light-years across.

Size comparisons

Seen from Earth, M54 is a little less than one-third as wide as the full Moon.
Omega Centauri, the most massive globular cluster known in the Milky Way, has a bright main body 150 to 190 light-years across, a little smaller than M54's. Omega Centauri looks four times as wide in the sky because it is only about one-fifth as far away.
M54 has about 1.6 million times the mass of the Sun, about two-fifths the mass of Omega Centauri. As of 2026, Omega Centauri is the only globular cluster around the Milky Way known to have more mass.
M54's bright main body is about 53 times as wide as the distance from the Sun to Proxima Centauri, the nearest star.
If the orbit of Neptune around the Sun were shrunk to the width of a U.S. penny, 19 millimeters (0.75 inches), M54's bright main body would be about 4.5 kilometers (2.8 miles) across.

A cluster from another galaxy

Charles Messier discovered M54 in 1778. More than 200 years later, astronomers discovered the Sagittarius Dwarf Galaxy and found that M54 belongs to it. Messier did not know it, but he had found the first globular cluster ever discovered outside the Milky Way.

M54 sits at the very center of the Sagittarius Dwarf Galaxy. It is the only center of a galaxy where telescopes can pick out separate stars in visible light. Stars there range from about 2 billion to about 12 billion years old. Astronomers suggest the oldest came from two or more globular clusters that sank to the center and merged, and the youngest formed at the center itself.

The future of M54

The Milky Way's gravity is pulling the Sagittarius Dwarf Galaxy apart. The small galaxy is stretched out along a line pointing toward the Milky Way's center. In time it will merge with the Milky Way, and M54 will become one of the Milky Way's own clusters.