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Irregular galaxies have no particular shape. They are among the smallest galaxies and are full of gas and dust. Having a lot of gas and dust means that these galaxies have a lot of star formation going on within them. This can make them very bright.

The bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars. This process is also thought to explain why many barred spiral galaxies have active galactic nuclei, such as that seen in the Southern Pinwheel Galaxy.

The Andromeda Galaxy (IPA: /ænˈdrɒmɪdə/), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), is a barred spiral galaxy approximately 2.5 million light-years (770 kiloparsecs) from Earth and the nearest large galaxy to the Milky Way.

Types of Galaxies

Astronomers classify galaxies into three major categories: elliptical, spiral and irregular.

The Milky Way is a huge collection of stars, dust and gas. It's called a spiral galaxy because if you could view it from the top or bottom, it would look like a spinning pinwheel. The Sun is located on one of the spiral arms, about 25,000 light-years away from the center of the galaxy.

• The Milky Way is a barred spiral galaxy, which is the most common kind of galaxy in the universe. A barred spiral galaxy, as the name suggests, is a galaxy that forms a spiral with a bar running through it. Seen from the side, though, it simply looks like a circle.

• barred spiral galaxy. n. A spiral galaxy whose inner spiral arms do not extend all the way to its center, but are connected with a bar-shaped structure that spans the galaxy center.

• A barred spiral galaxy is just the name that we have for the appearance of a galaxy , with a bar in it's center. It just so happens that the Milky Way, where our solar system is located, is a barred spiral galaxy. We can tell this through several different things, from the dust content and color of our galaxy, to the velocity of the stars.

• Barred spiral galaxy IC 5201, located more than 40 million light-years from Earth. It was discovered by Joseph Lunt. Barred galaxies are apparently predominant, with surveys showing that up to two-thirds of all spiral galaxies contain a bar.

A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure made of stars. Bars are found in around half of all spiral galaxies, and appear to influence the motions of stars, dust and gas.

Bars are perhaps one of the most important catalysts for changing a galaxy. They force a large amount of gas towards the galactic center, fueling new star formation, building central bulges of stars, and feeding massive black holes. "The formation of a bar may be the final important act in the evolution of a spiral galaxy," Sheth said.

We know this after a study of more than 2,000 galaxies revealed that there were far fewer bar spiral galaxies 7 billion years ago, showing that a bar is a sign of maturity. We think that it takes approximately 2 billion years for a bar to form in a spiral galaxy.

"The recently forming bars are not uniformly distributed across galaxy masses, however, and this is a key finding from our investigation," Sheth explained. "They are forming mostly in the small, low-mass galaxies, whereas among the most massive galaxies, the fraction of bars was the same in the past as it is today."