Globular cluster

A globular cluster is a massive, spheroidal collection of stars. These stars are held together by the force of gravity. This gravity creates a compact and stable formation. Within these clusters, stars are more concentrated toward the center. A single cluster can contain tens of thousands to many millions of stars.

While they resemble dwarf spheroidal galaxies, they are distinct objects. Their name comes from the Latin word for a small sphere. Astronomers often refer to them simply as "globulars."

Understanding how these clusters form remains a challenge for scientists. In the past, experts believed all stars in a cluster formed at once. They thought one giant molecular cloud created the entire group. However, modern observations show that most clusters contain multiple populations. This means the stars formed at different times or have different compositions.

Some clusters may have experienced multiple episodes of star formation. Others might be the remnants of smaller galaxies captured by larger ones. Some clusters might even start as "super star clusters" in very active star-forming regions.

Globular clusters are classified by how concentrated their stars are. In the early 20th century, Harlow Shapley and Helen Sawyer categorized them. They created the Shapley–Sawyer Concentration Class system. This system ranks clusters from Class I, which is highly concentrated, to Class XII, which is more diffuse.

This method helps astronomers describe the density of the stellar core. In 2015, researchers in Chile proposed a new type called "dark globular clusters." This shows that our understanding of these objects is still growing.

History shows how our view of the sky has changed with better technology. In 1665, the German astronomer Abraham Ihle discovered M 22. Early telescopes were weak, so clusters looked like fuzzy blobs. Charles Messier included many of these in a catalog to avoid confusing them with comets.

In 1789, William Herschel used large telescopes to resolve these blobs into individual stars. He was the first to use the term "globular cluster." Other notable discoveries include M 5 in 1702 and M 13 in 1714.

These clusters provide vital data about the scale of the universe. In 1914, Harlow Shapley studied RR Lyrae variables within the clusters. These are specific types of stars that change in brightness. By studying them, Shapley estimated how far away the clusters were.

He used the distribution of these clusters to map the Milky Way. He discovered that the Sun is not at the center of our galaxy. Instead, the center lies in the Sagittarius constellation. This was a major shift in how we view our place in space.

Globular clusters are found in nearly all galaxies. In our Milky Way, they mostly live in the galactic halo. The halo is the outer, spherical part of the galaxy. This is different from the flat disk where most ordinary stars reside.

The Milky Way has over 150 known globular clusters. In 2011, the VISTA survey discovered two more, named VVV CL001 and VVV CL002. Other galaxies hold even more. The Andromeda Galaxy may have 500 clusters. The giant galaxy M 87 can have as many as 13,000.

Studying these objects connects many different areas of astronomy. Because they are among the oldest objects in the universe, they help us estimate the age of the cosmos. They also help us understand galactic evolution. For example, the massive cluster Omega Centauri orbits the Milky Way in a retrograde motion. This means it moves in the opposite direction of the galaxy's rotation. This movement suggests it was once part of a separate dwarf galaxy. By studying these motions, scientists learn how galaxies grow and change over billions of years.