Ring galaxy

Some star groups look like rings.

These rings are very bright. They have many young blue stars. The middle may be empty.

One ring may form from a hit. A small group can pass through a big one. This works like a rock in a pond. It pushes stars out into a ring.

Other rings come from gas. Gas can flow into a star group. This makes a ring shape too.

These shapes do not last forever. They can change back into other shapes. It is fun to look at the sky!

Some star groups look like rings. We call these ring galaxies.

These rings are very bright. They contain many young blue stars. The center of the ring may be empty. Arthur Hoag found a famous one in 1950. It is called Hoag's Object.

How do these rings form? One way is a collision. A small galaxy can pass through a big one. This is like dropping a rock into a pond. The gravity of the small galaxy pushes the arms out. This creates a ring shape. These rings may only last for a short time. They might turn back into spiral shapes later.

Another way is through accretion. Accretion is when a galaxy pulls in new gas. This gas can flow into the galaxy. The gas can make new stars. This can form a ring instead of a spiral. Some rings even form around the poles of a galaxy. This can happen to old elliptical galaxies too. These rings can be very stable.

Some star groups in space look like giant circles. We call these ring galaxies. They have a large ring shape called an annular structure. The center of the galaxy might be separate from the ring. Sometimes the center and the ring form one continuous disc. The rings are very bright because they have many young blue stars. These stars are quite massive. The middle part of the galaxy often has very little light.

How do these rings form? One way is through a galactic collision. This often happens when a smaller galaxy passes through a larger one. This is like dropping a rock into a still pond. The gravity of the small galaxy creates a wave. This wave pushes the arms of the larger galaxy outward. This process can create an empty-looking ring. Sometimes the core gets shoved toward the disc. This can make an oval-shaped ring.

Other scientists think rings form through a thing called accretion. Accretion is when a galaxy pulls in new material from space. This material might be gas from the intergalactic medium. As the gas flows in, it can cool down. This gas can form a ring of hot young stars. This can happen even in very old elliptical galaxies. Some rings even form around the poles of a galaxy. These are called polar-ring galaxies.

There is also a way called bar instability. This happens in a barred spiral galaxy. The bar is a shape in the middle of the galaxy. If the bar becomes unstable, it can move matter around. Gravity and pressure push stars and gas into a ring. This can ignite the birth of new stars. This process can change a galaxy over hundreds of millions of years.

We have seen many famous examples of these shapes. Arthur Hoag discovered a famous one in 1950. It is known as Hoag's Object. Other examples include the Cartwheel Galaxy. You can also find ring galaxies named AM 2026-424 and Arp 147. These rings do not always last forever. They might only stay for a few hundred million years. Later, they might turn back into spiral arms.

A ring galaxy is a unique type of galaxy defined by a large, circular structure called an annular structure. In these systems, the central galactic core may sit separately from the ring. In other cases, the center and the ring form one continuous disc shape. These rings are often very bright because they contain many massive, young blue stars. In contrast, the central region of the galaxy often contains very little luminous matter. Understanding these galaxies helps astronomers learn how stars form and how galaxies change over time.

One way these rings form is through galactic collisions. This often happens during a "bullseye collision." In this scenario, a smaller donor galaxy passes directly through the disc of a larger spiral galaxy. This event works much like dropping a stone into a still pond. The gravity of the smaller galaxy creates an outward push on the larger galaxy's arms. This shockwave can launch the core away from the main disc. This results in a ring that looks almost empty. If the collision shoves the core toward the disc, the galaxy may form an oval-shaped ring. These collisional rings are often transient features. They may only last for a few ten to a hundred million years. After this time, they might turn back into spiral arms or fall apart.

Another theory involves a process called bar instability. This occurs within a barred spiral galaxy, which has a central bar-shaped structure. Usually, gravitational density waves create spiral arms in these galaxies. However, if the rotational velocity of the bar increases, it can become unstable. This instability causes the density waves to migrate outward. The pressure and gravitational influence of baryonic and dark matter push stars, gas, and dust into a torus-like region. This movement forms the ring and often ignites new star formation. Some rings form where the bar dominates and "carves out" the disc. Other rings appear where a bar has collapsed into a flattened bulge.

Accretion is a third method for creating these structures. Accretion is the process of a galaxy pulling in new material from its surroundings. One type is intergalactic medium accretion. This involves the inflow of gas from the space between galaxies. UV-light observations show faint, ring-like structures of hot, young stars forming along these cooling gas flows. If the conditions are right, a ring can form instead of a spiral structure. This can happen to protogalaxies that are just beginning to form. It can also happen to old galaxies that move into areas with more gas.

There is also a specific type called tidal accretion. This happens when a gas-rich host galaxy interacts with a donor galaxy in a polar orbit. These interactions can lead to the formation of polar-ring galaxies. These rings often intersect with the poles of the central portion of the galaxy. Because some angular momentum is lost during this process, these rings often have a greater inclination angle. This means the ring sits at a different angle than the original donor galaxy. These processes show how much a galaxy's shape depends on its environment.

Arthur Hoag discovered one of the most famous examples in 1950. This galaxy is known as Hoag's Object. It serves as a primary example of a ring galaxy with a separate center. Other notable examples include the Cartwheel Galaxy. Astronomers have also identified ring galaxies known as AM 2026-424 and Arp 147. These specific examples help scientists test their theories about collisions and gas accretion. Studying these objects provides a window into the life cycles of galaxies.

All these formation processes have a major effect on stellar populations. Observations of the spectrum of ring structures show very high star formation rates. This is caused by the outwards pressure wave during formation. These regions also show very high metallicities. This high level of metals suggests that the stars within the ring have undergone significant evolution. Whether through collisions, instability, or accretion, ring galaxies demonstrate the powerful, changing nature of the universe.