Hubble’s Galaxy Classification

FIELDS OF STUDY: Extragalactic Astronomy; Observational Astronomy

ABSTRACT: Hubble’s galaxy classification is a system devised by astronomer Edwin Hubble for categorizing different types of galaxies. The major types included are elliptical, spiral, barred spiral, lenticular, and irregular. Although modern astronomers consider some parts of Hubble’s theories to be flawed, the galaxy classification system provided an important step toward a greater understanding of the universe.

Studying the Galaxies

Hubble’s galaxy classification was devised by American astronomer Edwin Hubble (1889–1953) around 1926. The major types classified by Hubble included elliptical galaxies, spiral galaxies, and barred spiral galaxies. He also noted additional types such as lenticular and irregular galaxies. This system marked a revolutionary advance in the study of the galaxies.

Long before Hubble, astronomers had little knowledge of galaxies, which are mostly extremely distant and difficult to see. Even when simple telescopes became available in the seventeenth and eighteenth centuries, they revealed little more than fuzzy patches of light in space. Early astronomers, puzzled, referred to these mysterious cloud-like bodies as "nebulae." Only much later, in the twentieth century, did new technology and scientific understanding lead to a greater ability to study these space features. At that time, astronomers realized that some of them were actually galaxies.

Most of these galaxies appeared to be held together by gravity, with a denser area in the middle and an outer region surrounding it called a halo. However, there was great diversity among the galaxies in size, shape, complexity, and color. Astronomers believed these changes were due to many factors, including the contents of the galaxies and their ages. They realized that galaxies were a vastly important feature of space with much to teach humans about the formation of the universe.

In the first decades of the twentieth century, the study of galaxies increased. Astronomers quickly identified two main varieties: the elliptical and the spiral galaxies. During Hubble’s research on galaxies, he identified additional varieties, such as the barred spiral galaxy, the arms of which form a relatively straight line or "bar" where they emerge from the center. Hubble classified galaxies whose traits overlap those of both elliptical and spiral galaxies as lenticular. Finally, unusually shaped galaxies that did not share characteristics with other kinds of galaxies he termed "irregular." Hubble noted different ranges of shapes within each category, such as spiral galaxies with loose or tight spirals.

Hubble plotted these categories onto a diagram that was shaped like a horizontal letter Y. (Fellow astronomers called this diagram the "tuning fork" because of its two-pronged shape.) On the single flat plain of the diagram, Hubble placed the different varieties of elliptical galaxies. On the upper divergent line, he placed spiral galaxies; on the lower divergent line, barred spiral galaxies. Hubble placed lenticular galaxies at the point where the lines diverge. He generally did not include irregular galaxies on the diagram.

Determining the Class of a Galaxy

Hubble’s galaxy classification greatly simplified the process of studying and categorizing galaxies. Now astronomers could classify galaxies using simple, standardized criteria. Those basic criteria included galactic regularity or symmetry, central shape types, and the presence and arrangement of spiral arms on their disks. For each new galaxy, scientists have only to gather information on those features in order to determine whether the galaxy is elliptical, spiral, barred spiral, lenticular, or irregular.

If the galaxy being examined appears to have the shape of a flattened circle and has no evidence of spirals, it will most certainly be an elliptical galaxy. Hubble took that designation further by rating elliptical galaxies on their level of ellipticity (E), which is determined using the equation

E = 10 × (1 – b / a)

where a is the size of the larger axis of the ellipse and b is the size of the smaller axis. Hubble’s chart rates the least elliptical galaxies, which appear almost round, at E0, or zero ellipticity. Levels of ellipticity increase from there to E7, which is highly elliptical and resembles a football shape when observed from Earth.

If the galaxy exhibits any spiral arms, it will likely be classified as a spiral galaxy. Again, Hubble divided this group into classes based on their characteristics. Spiral galaxies of the Sa class have arms that tightly hug a large central bulge. Sb galaxies have arms slightly looser and longer around a medium-sized bulge. Sc galaxies have the loosest arms and the smallest central bulge.

If a spiral galaxy features a line or bar through the bulge from which the spirals emerge, it is classified as a barred spiral. Hubble denoted barred spiral galaxies as SB and noted their particular size and shape in a manner similar to regular spiral galaxies. SBa galaxies have tight spirals and large bulges. SBb galaxies have looser arms and medium bulges. SBc galaxies have the loosest arms and smallest bulges.

Galaxies that are shaped like spirals but have no visible spiral arms are classified as S0, or lenticulars. Lastly, if the galaxy does not fit into any of the above models, it is classified as an irregular galaxy. Irregular galaxies come in a countless variety of shapes and sizes and often appear asymmetric and disordered. Although irregular galaxies are not always included in Hubble’s diagram, Hubble classified them as well, with the abbreviation Irr. Varieties that demonstrate some basic structure are designated Irr I, while Irr II varieties appear to be completely random in structure.

Examples of Classified Galaxies

Using Hubble’s criteria, astronomers have classified thousands of galaxies based on their sizes, shapes, and compositions. For example, Earth’s galaxy, known as the Milky Way, is a barred spiral. Astronomers studying the arrangement of the galaxy noted that it has a center bulge leading to two bars that in turn lead to spiral arms. Other galaxies identified as barred spirals include New General Catalog 5101 (NGC 5101), a type SBa galaxy; NGC 1365, type SBb; and NGC 7479, type SBc. These galaxies share similar features but vary somewhat in structure, hence the different letter classifications.

Many other galaxies are regular spirals that do not have bars. The galaxy Messier 104 (M104), for instance, has a bright, large central bulge and large spirals surrounded by space dust. This shape has earned the galaxy the nickname the Sombrero galaxy. Spiral galaxies are also subcategorized by their details. The galaxy M81 is usually classified as Sa because its spirals are tightly wound around the central bulge. M51 has looser spirals, so it is classified as Sb. Finally, NGC 2997 has very loose spirals and a small bulge and is therefore classified as Sc.

If a regularly shaped galaxy is found to lack spiral arms, it is generally classified as an elliptical galaxy. Ellipticals are subcategorized by their shape. For example, the galaxy M59 is highly elliptical, resembling a football, and is therefore designated E5. Meanwhile, M89 is more circular than elliptical and therefore is rated E0, for zero ellipticity.

When astronomers discovered M82, they could clearly see an arrangement of stars and other space objects bound by gravity. However, the galaxy resembles a strange splash of light, lacking a clear shape or defining features. Therefore, astronomers classified it as an irregular galaxy. Due to its totally random-looking shape, M82 was subclassified as Irr II. An unusual-looking galaxy that has some semblance of order, such as the Large Magellanic Cloud, is categorized as Irr I.

The Importance of Hubble’s System

When Hubble first created his classification system, he suggested that the different forms of a galaxy might correspond to its age. Modern astronomers believe that this is not necessarily correct. Galaxies form, age, and develop in different ways depending on their initial collapse, interactions with one another, and star formation. Astronomers have also determined that Hubble’s system of classifying galaxies was somewhat subjective. For that reason, some astronomers have applied different classifications to the same galaxies due to differences of personal opinion.

Despite its flaws, Hubble’s galaxy classification established a strong launching point from which researchers could embark on more in-depth studies of the galaxies. Over time, scientists continue to discover more about galactic types, formation, and evolution, thanks largely to the early innovations of Edwin Hubble.

PRINCIPAL TERMS

• bulge: the central round part of a galaxy that contains old stars, dust, and gas.
• disk: the flat part of a spiral galaxy that forms the arms and surrounds the central bulge. Its stars are generally younger.
• elliptical galaxy: a massive group of stars and other space objects that is shaped like an ellipse, or a flattened circle.
• halo: the outer portion of a galaxy that contains old stars, star clusters, and dark matter.
• irregular galaxy: a massive group of stars and other space objects that has an unusual shape and does not share characteristics with other kinds of galaxies.
• lenticular galaxy: a massive group of stars and other space objects that shares traits of both elliptical and spiral galaxies.
• spiral galaxy: a massive group of stars and other space objects that takes on a pinwheel-like spiral form comprising a bright central bulge with swirling arms emerging from it.

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