Barred spiral galaxy

A barred spiral galaxy is a massive system of stars with a distinct central structure. This structure is a bar-shaped formation made of stars.

These bars are quite common in our local universe. Surveys show that about two-thirds of all spiral galaxies possess them. The bar is not just a decoration. It actively influences the motion of stars and interstellar gas. It can even change the shape of the spiral arms. Our own home, the Milky Way Galaxy, is classified as a barred spiral galaxy.

The formation of a bar is a complex physical process. It is generally thought to begin with a density wave. This wave radiates outward from the center of the galaxy. As it moves, it reshapes the orbits of the inner stars. This effect gradually moves to stars orbiting farther out. This process eventually creates a self-perpetuating bar structure.

The bar acts as a type of stellar nursery. It uses orbital resonance to channel gas from the spiral arms toward the center. This influx of gas can fuel intense star birth near the galactic center. This mechanism may also explain why many barred spirals have active galactic nuclei. The Southern Pinwheel Galaxy is one such example.

Astronomers use specific categories to organize these galaxies. Edwin Hubble created the Hubble sequence to classify them. He labeled barred spiral galaxies as "SB," which stands for spiral barred. He then divided them into sub-categories based on their arm structure. SBa types have tightly bound arms. SBc types are at the opposite extreme with loosely bound arms. SBb types represent the middle ground between these two.

There is also a category called SBm. This describes somewhat irregular barred spirals. The Magellanic Clouds were once called irregular galaxies, but they are now known to have barred spiral structures.

There are several other specialized types within this classification system. An SB0 galaxy is known as a barred lenticular galaxy. An example of this is NGC 2787.

There are also more irregular versions like the SBdm type. The galaxy NGC 5398 is an example of this type. Some galaxies, like NGC 55, are classified as SBm, which is a type of Magellanic spiral. These various categories help scientists understand the different shapes and stages of galactic life.

Research shows that bars are a sign of galactic maturity. In the distant past, bars were much less common. A 2008 investigation found that only 20 percent of spiral galaxies in the early universe had bars. In contrast, about 65 percent of local spiral galaxies possess them today.

This suggests that galaxies develop bars as they grow older. However, bars are also thought to be temporary phenomena. They do not last forever in a single continuous state. The bar structure eventually decays over time. This transforms the galaxy back into a more regular spiral pattern.

As a bar accumulates mass, it can lose its stability. Once the bar reaches a certain size, the mass can compromise the entire structure. This can lead to what scientists call a "buckling" event. During buckling, a disturbance in the orbital resonances of stars causes an inward collapse. This makes the bar become both thicker and shorter.

Galaxies with high mass accumulated in their centers often have short, stubby bars. A supermassive black hole at the galactic center can suppress or delay this buckling. Despite this, the instability usually occurs eventually.

Because bars can form and then decay, they are likely recurring phenomena. A galaxy may move through an oscillating evolutionary cycle. This cycle from a regular spiral to a barred spiral takes about two billion years on average.

This means a single galaxy might develop bars multiple times throughout its history. This cycle connects the study of individual galaxy shapes to the broader evolution of the universe. It shows that even the largest structures in space are constantly changing and reshaping themselves over billions of years.