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How Are Scientists Able to Classify Galaxies?

Scientists have developed a number of classification systems that categorize galaxies based on their observable characteristics. One of the earliest and most widely used methods for classifying galaxies was developed by Edwin Hubble. Hubble's classification system puts galaxies into three main categories based on their apparent shape as seen from the Earth. This system further divides these galaxies based on their specific shape using a system of letters and numbers.

Spiral Galaxies
- Spiral galaxies like our own Milky Way galaxy are classified by their distinctive shape. These galaxies consist of a bulging central region of stars surrounded by a flattened disc. This type of galaxy is distinguished by curving arms that extend outward from the center of the disc. Typically the oldest stars in a spiral galaxy are found in the central bulge, while middle aged stars like our own sun are located in the disc. Newly formed stars in spiral galaxies are usually located in the outer arms of the galaxy. Modern classification systems describe the degree of the spiral shape and whether it has a central bar of stars extending from the central bulge.
Elliptical Galaxies
- Elliptical galaxies include a variety of shapes ranging from nearly circular to elongated shapes commonly compared to a football. These galaxies consist of older stars that have very little overall rotation, unlike spiral galaxies. Most elliptical galaxies consist primarily of stars with very little matter located in the spaces between them. As a result, most elliptical galaxies do not have a disc and are similar to the bulge of a spiral galaxy in appearance.
Irregular Galaxies
- This classification of galaxy is used to describe groups of stars that do not fit into the other classifications. These galaxies are often distorted forms of elliptical or spiral galaxy that have been pulled into unusual shapes by the gravity of other galaxies.
Lenticular Galaxies
- The lenticular galaxy is a specific type of galaxy is that is halfway between an elliptical and a spiral galaxy. These galaxies are distinguished by their distinct central bulge and disc, but they do not possess the curving arms associated with a spiral galaxy.
Classification System
- Hubble's system of classifying galaxies describes several basic features that are common among galaxies. Spiral galaxies are denoted with a capital S followed by the letter a, b or c. Spiral galaxies that have a large bar of stars running through their disc are denoted by a capital B. The lower case letters are indicators of the size of the central bulge and the amount of dust in the disc with the letter a indicating a large disc and the letters b and c denoting successively smaller discs. Elliptical galaxies are described using a similar system using a capital E followed by a number describing the degree of elongation. An E0 galaxy is nearly round in shape while an E7 is significantly elongated. Lenticular galaxies are special types of galaxies that are denoted as S0, while galaxies with irregular shapes are simply marked Irr.

Astronomy