Galaxy cluster

Huge bunches of galaxies live in space.

These bunches are called clusters. They stay together because of a strong pull.

Clusters have three main parts. They have many galaxies. They also have very hot gas. Most of a cluster is made of dark matter.

We cannot see this dark matter.

Groups of clusters make even bigger things. These are called superclusters.

Clusters are some of the biggest things in the universe. They are truly amazing to see.

A galaxy cluster is a huge group of galaxies. These groups hold hundreds or even thousands of galaxies together. They stay together because of gravity. Gravity is the force that pulls things toward each other.

Clusters have three main parts. The first part is the galaxies. They are the only part we can see with light. The second part is hot gas. This gas sits between the galaxies. It is very hot, reaching 100 million degrees Celsius.

The third part is dark matter. This makes up 90% of the mass in a cluster. We cannot see dark matter directly. We only know it is there by how it pulls on other things.

Clusters can act like a cosmic magnifying glass. This is called gravitational lensing. The heavy mass of a cluster bends light. This helps telescopes see very far away galaxies.

Small groups of galaxies can join to form superclusters. These are even larger structures in space.

A galaxy cluster is a huge structure in space. It holds hundreds to thousands of galaxies together. Gravity is the force that binds them. These clusters are the biggest things in the universe held by gravity.

Before the 1980s, scientists thought these were the largest structures. Then, they discovered even bigger things called superclusters. Small groups of galaxies are called galaxy groups. Groups and clusters join together to form superclusters.

Clusters are made of three main parts. Galaxies are only 1% of the total mass. They are the only part we can see with light.

The next part is hot gas between the galaxies. This is called the intracluster medium. It is very hot, between 30 and 100 million degrees Celsius. It sends out X-ray radiation. The biggest part is dark matter. It makes up 90% of the mass. We cannot see it with light, but we know it is there.

Clusters change and grow over time. As they form, they release a lot of energy. This happens because of shock waves and gas heating. Gas hits other material and creates these shock waves. This makes the gas very hot. Galaxies inside the cluster also interact with each other. They can merge together into one. Sometimes, gas is stripped away from a galaxy.

Scientists use clusters to study the universe. Radek Wojtak studied 8,000 clusters at the Niels Bohr Institute. He looked at something called gravitational redshift. This is when light loses energy escaping gravity. Light from the center of a cluster loses more energy than light from the edge. This happens because gravity is stronger in the center. His work supports the Lambda-Cold Dark Matter model. Clusters also act like a cosmic magnifying glass. This is called gravitational lensing. The mass of a cluster bends the path of light. This helps telescopes see very distant galaxies.

There are many famous clusters in our sky. The Virgo Cluster is the nearest massive cluster. The Norma Cluster sits at the heart of the Great Attractor. The Bullet Cluster shows dark matter separating from normal matter. Some clusters are very far away. SPT-CL J0546-5345 and SPT-CL J2106-5844 are very massive. They are found in the early universe. Even the very first clusters were forming long ago.

A galaxy cluster is a massive cosmic structure held together by gravity. These structures consist of anywhere from hundreds to thousands of individual galaxies. They are considered the largest known gravitationally bound structures in the entire universe. Before the 1980s, scientists believed these clusters were the biggest things in existence. However, the discovery of even larger superclusters changed that understanding. Smaller collections of galaxies are known as galaxy groups. When these groups and clusters combine, they form the massive superclusters we see today.

Galaxy clusters are composed of three distinct parts with very different properties. The most visible part consists of the galaxies themselves. Surprisingly, galaxies only account for about 1% of a cluster's total mass. The second component is the intracluster medium, or ICM. This is a hot, intergalactic gas that exists between the galaxies. It reaches peak temperatures between 30 and 100 million degrees Celsius. This plasma emits X-ray radiation through a process called thermal bremsstrahlung.

The third and most significant component is dark matter. Dark matter makes up roughly 90% of the mass in a cluster. We cannot detect dark matter using optical telescopes because it does not emit light. Instead, scientists infer its presence through gravitational interactions with other objects. Because dark matter is so massive, it dominates the cluster's behavior. The total mass of a cluster typically ranges from 10^14 to 10^15 solar masses. These clusters can reach diameters of 1 to 5 megaparsecs.

Clusters undergo constant evolution and change through violent physical processes. As they form, massive amounts of energy are released into the surroundings. This energy comes from shock waves, heating of gas, and galaxy interactions. When gas collides with existing material, it generates shock waves. These waves heat the gas to tens of millions of degrees. Inside the cluster, galaxies also interact through mergers. During these events, gas can be stripped away from a galaxy. This complex environment helps govern how galaxies evolve over billions of years.

Astronomers use these massive structures to test the laws of physics. Radek Wojtak from the Niels Bohr Institute studied 8,000 galaxy clusters. He investigated a phenomenon called gravitational redshift. This occurs when light loses energy while escaping a strong gravitational field. Photons emitted from the center of a cluster lose more energy than those from the edge. This happens because gravity is much stronger at the cluster's center. Consequently, light from the center has a longer wavelength. Wojtak's research supports the Lambda-Cold Dark Matter model of the universe.

Galaxy clusters also function as cosmic magnifying glasses through gravitational lensing. The massive gravitational potential of a cluster distorts the surrounding space-time. This distortion bends the path of passing photons. This effect allows telescopes to observe incredibly distant galaxies from the early universe. Such distant objects would otherwise be too faint to detect. This lensing works across many wavelengths, from optical light to X-rays. For example, the Phoenix galaxy cluster helped scientists observe a dwarf galaxy during its early star formation stages.

Many notable clusters exist within our observable universe. The Virgo Cluster is the nearest massive galaxy cluster to us. The Norma Cluster is a significant structure that dominates the Great Attractor. The Great Attractor is a massive aggregation of galaxies that affects local expansion. Other famous examples include the Fornax, Hercules, and Coma clusters. In the distant, high-redshift universe, clusters like SPT-CL J0546-5345 are among the most massive ever found. Even in the very early universe, precursors called protoclusters like JADES-ID1 were already forming.