Astronomy 106

Western Kentucky University


In the 1600's and 1700's, the first astronomers to use telescopes to study the heavens noted the presence of fuzzy patches of brightness which they called nebulae. We now realize this term includes many different sorts of objects, whose only similarity is that they appear in small telescopes as celestial clouds of light. In the 1920's, astronomers came to understand that some of these nebulosities are actually galaxies -- systems of tens, hundreds, or even thousands of billions of stars (and interstellar gas and dust) organized by their common gravitational field.

Almost all galaxies are so incredibly distant that they are too faint to be seen with the naked eye (the Large and Small Magallenic Clouds and the Andromeda Galaxy are the only three exceptions). Small telescopes allow humans to directly view a few score of galaxies. However, to study the faint details of galaxies more closely astronomers generally collect the light with photographic film or digital image detectors. Many such images of galaxies are now available on the world wide web. In this lab you will use a web browser to navigate the world wide web and access images of galaxies.

After astronomers discovered what galaxies really were, they began cataloguing them according to their appearance. In the early 1900's astronomers divided the galaxies into two general categories: elliptical and spiral galaxies. Elliptical galaxies are shaped like ellipses, and spiral galaxies are shaped like spirals, with arms winding in to a bright center. Astronomers also realized that galaxies can appear very different based on our view of each galaxy -- differences in distance and its orientation to our line of sight. In general, however, astronomers ignore differences due to viewing angle.

Edwin Hubble introduced a classification scheme for galaxies that is referred to today as the "Hubble Tuning Fork Diagram." This scheme provides for subcategories of both elliptical and spiral galaxies, and introduces two new primary types of galaxies, lenticular and irregular. Hubble realized that elliptical galaxies could be classified by how round or flat they look. He classified spiral galaxies by how large and bright their central regions are and how tightly their arms are wound. Hubble also noticed that some spiral galaxies have a bright line, or bar, running through their central regions, and called these barred spiral galaxies. A transition type between the elliptical and spiral galaxies, with a central bulge and a disk but no spiral arms, are known as lenticular galaxies. The final classification, irregular galaxies, are neither spiral nor elliptical, and can have any number of shapes.


Elliptical Galaxies are designated as type E, with a subclassification based on a measure of the ellipticity of their appearance. In Hubble's classification scheme, an E0 galaxy is very round and an E7 galaxy is very elongated. The number after the E can be numerically determined by the ratio of the galaxy's major axis (a) to minor axis (b): n = 10(a - b)/a. Galaxies with higher ellipticities have higher numbers.

Spiral Galaxies with spiral arms (type S) are sub-classified as type "a" through "d". Type Sa galaxies have their arms wound very tightly and have large central bulges. Type Sd galaxies have very loosely wound arms and have small central bulges. Because these classifications are subjective, astronomers often use inbetween designations of Sab, Sbc, or Scd. A galaxy that is obviously a spiral, but has some distince peculiarity can have a "p" suffixed to its classification. A spiral galaxy that is encircled by a filamentary ring of brightness can have a "r" suffixed to its classification.

Barred spirals are designated by SB, with divisions into SBa through SBd defined the same as for normal spirals. Note that the capitalization of the B for a barred spiral is important: an SBc galaxy is not the same as an Sbc galaxy!

Lenticular galaxies are designated as S0 (not SO!). Galaxies in this transition classification can be recognized as having bars, where they are catalogued as SB0.

Irregular galaxies, designated by Irr, form the catch-all, none-of-the-above category.

A number of excellent introductions to the morphological classification of galaxies are available on the web. A brief, interactive classification exercise is available at: The Hubble Tuning Fork - classification of galaxies. A more complete introduction to classifying galaxies according to their appearance is: COSMOS Hubble Classification. Visit the COSMOS site and carefully read the descriptions of spiral, lenticular, elliptical, and irregular galaxy classifications. Be sure to note the description of sub-classifications such as the presence of bars in spiral galaxies or the elongation of elliptical galaxies. Carefully note the classification for the various example images, until you are comfortable recognizing the different types.

When you think you are ready, you can take our interactive quiz:

Take the practice quiz
- or -
Take the real quiz