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Spiral galaxies, the most common type, have a distinctive spiral shape with a dense core and arms of young stars. They are surrounded by a halo of older stars and gas and dust. The Milky Way is a barred spiral galaxy, classified as SBb. Identifying a galaxy’s class can be difficult due to perspective.
A spiral galaxy is one of three types of galaxies cataloged by Edwin Hubble. These galaxies appear to be the most abundant in the universe and, as their name implies, have a distinctive spiral shape, with arms wrapping around a very dense core. Our home galaxy, the Milky Way, is an example of a spiral galaxy. Estimates of the percentage of galaxies that take on this shape vary widely, but most exceed 50%.
Several features distinguish a spiral galaxy from other types of galaxies. The first is the galactic core, which consists of a bulge of very old stars arranged around a supermassive black hole. When a spiral galaxy is viewed from the side, the characteristic bulge is very evident. The core is surrounded by arms that can wrap tightly or loosely, depending on a number of factors. These arms have stars much younger than the galactic core.
Spiral galaxies are surrounded by a halo of older stars, most of which are arranged in extremely dense groups known as globular clusters. These clusters can sometimes be difficult to identify because the stars are so densely packed that they can look like a single point. Throughout the galaxy gas and dust abound. The precise shape and size of a spiral galaxy can be influenced by the presence of nearby galaxies, and there can be many variations within the galaxy. Some areas, for example, seem hostile to planet formation, while others are more planet-forming friendly.
Many spiral galaxies take on a barred shape. In a barred spiral galaxy, instead of having a round core, the core of the galaxy is actually shaped like a bar, and when viewed from the side, the bulge strongly resembles a peanut. The Milky Way falls under this classification. Spiral galaxies are classified as Sa, Sb, or Sc depending on how tightly the arms are coiled, with Sc spiral galaxies having the loosest configuration. The barred spirals are designated SBa, SBb and SBc.
It can take some time to learn which class a galaxy belongs to. In part, the identification depends on perspective, because when viewed from the side, details of a galaxy’s structure can be obscured, with the galaxy looking like a bright line of stars. Scientists are constantly finding new galaxies and learning more about the structures within individual galaxies as observing equipment improves and more and more images of the universe accumulate.
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