Star cluster

Stars can live in big groups.

These groups are called star clusters. They form at the same time. Gravity holds the stars together.

Some clusters are tight and round. They have many old stars. These are called globular clusters. Other groups are loose. They have fewer stars. These are called open clusters.

Some stars are born inside clouds of dust. These are very young clusters. Most stars were born in groups too. Even our Sun was born in one.

Stars often live in groups called star clusters. These stars share a common origin. They form at nearly the same time. Gravity holds the stars together in a group.

There are two main types of clusters. The first type is the globular cluster. These are tight, round groups. They can have millions of old stars. They stay together for a very long time.

The second type is the open cluster. These groups are less tight. They usually have only a few hundred stars. They are often young. Over time, gravity from large clouds can pull them apart.

Some very young stars live in embedded clusters. These stars are wrapped in dust and gas. This dust can hide them from our eyes.

Most stars were born in these young groups. Even our Sun was born in one. Most of those groups broke apart long ago. Studying clusters helps us learn how stars change. It also helps us measure how far away galaxies are.

Stars often live together in groups called star clusters. These stars share a common origin because they form at roughly the same time. Gravity is the force that holds these stars together in a group.

There are two main types of clusters to know. Globular clusters are tight, round groups of ten thousand to millions of old stars. Open clusters are less tight groups that usually have fewer than a few hundred members.

Open clusters work in a specific way as they move through a galaxy. They are often young, sometimes only a few tens of millions of years old. Over time, the gravity from giant molecular clouds can pull them apart. This process can even cause stars to be thrown out of the group. This is sometimes called "evaporation." When they are no longer held together by gravity, they are called stellar associations. They still move in the same direction through space.

Some stars start their lives in very special groups called embedded clusters. These very young stars are wrapped in clouds of gas and dust. This dust can hide them from our eyes. These clusters form when molecular clouds begin to collapse. Stars like protostars live inside these clouds. The phase might last for several million years. Eventually, the gas is blown away by stellar winds or big star explosions.

Globular clusters are much different because they are very old. They are made of stars that are nearly as old as the universe itself. These stars are mostly yellow or red. Because these clusters are so crowded, stars can sometimes crash into each other. This can create rare blue stars called blue stragglers. In our Milky Way, these clusters orbit the center in long, oval paths. We can see some of them with just our eyes, like 47 Tucanae.

Scientists use star clusters to learn many secrets about space. By looking at clusters, they can find the distance to far-off galaxies. They use special stars called Cepheids to help with this work. The cluster NGC 7790 even hosts three of these important stars. Studying clusters also helps us understand how our own Sun was born. Most stars, including our Sun, began in embedded clusters that broke apart.

A star cluster is a group of stars that share a common origin. These stars formed at roughly the same time and are held together by self-gravitation. Gravity acts as the glue that keeps the members of a cluster in a group.

By studying these groups, astronomers can learn how stars evolve and how galaxies grow. They provide a way to measure distances across the vastness of space.

Clusters often begin their lives as embedded clusters. These are groups of very young stars partially or fully encased in interstellar dust and gas. This gas is often impervious to optical observations, meaning we cannot see them with normal light.

These clusters form when molecular clouds begin to collapse. During this phase, the cluster may contain young stellar objects like protostars. This stage lasts for several million years. Eventually, radiation pressure, stellar winds, or supernova explosions disperse the gas. Usually, less than 30% of the cloud mass is converted into stars before this happens.

Once the gas is gone, the stars may become part of an open cluster. Open clusters are less tightly bound than other types. They generally contain fewer than a few hundred members and span up to 30 light-years.

These clusters are often found within the spiral arms of a galaxy. They are typically young, often only a few tens of millions of years old. However, rare exceptions like Messier 67 can be as old as a few billion years. Over time, the gravity of giant molecular clouds can disrupt these groups. Close encounters between stars can also cause "evaporation," where stars are ejected from the cluster. When they are no longer gravitationally bound, they are called stellar associations.

Globular clusters are much larger and more permanent structures. They are roughly spherical groupings that contain between 10,000 and several million stars. These stars are packed into regions only 10 to 30 light-years across.

Most members are very old Population II stars. These stars are mostly yellow and red, with masses less than two solar masses. The hotter, massive stars have already ended their lives as supernovae or white dwarfs. In the dense inner regions, stars may even merge to create rare blue stars called blue stragglers. In the Milky Way, these clusters orbit the galactic center in highly elliptical orbits.

History has shown how much globular clusters can teach us. In 1917, astronomer Harlow Shapley used their distribution to estimate the Sun's distance from the Galactic Center. For a long time, there was a mystery regarding their age. Theories of stellar evolution suggested they were older than the universe itself. This paradox was resolved in the mid-1990s using the Hipparcos satellite. Better measurements of the Hubble constant helped confirm the universe is about 13 billion years old. This allowed scientists to align the age of the stars with the age of the universe.

Clusters also serve as vital tools for measuring the scale of the universe. Astronomers use open clusters to calibrate the period-luminosity relationship of Cepheid variable stars. These luminous stars act as "standard candles" to find the distance to remote galaxies. For example, the cluster NGC 7790 hosts three classical Cepheids. By using main-sequence fitting, scientists can also estimate distances to other clusters. This involves comparing the luminosity of known clusters to those with unknown distances.

Beyond the Milky Way, different types of clusters exist. In the Andromeda Galaxy, astronomers found extended globular clusters. These contain hundreds of thousands of stars but are much less dense than our globular clusters. They are several hundred light-years across.

There are also "faint fuzzies," which are large, old objects found in lenticular galaxies. Finally, some large groups of stars called star clouds appear closely packed. While they look like clusters, they are actually large groups spread over many light-years.

They do not form a single bound structure like a true star cluster.