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Dr. Edwin Hubble: Study of Extragalactic Nebulae, 1939

Classification

Edwin Hubble continued his observations and found galaxies at increasingly further distances. He devised a classification system, known as the Hubble sequence, for these galaxies using pictures he took of their structure through his telescope. This is a system that is still used today. Galaxies are generally classified based on overall shape: elliptical, spiral, or barred spiral. Further classification is given depending on certain properties of the specific galaxy (for example, the degree of its ellipse). Elliptical galaxies are generally characterized by random motion and an older population of stars. Spiral galaxies are composed of a central bulge (contains older stars) surrounded by a disk (young stars and open star clusters), and have bright arms of star formation within the disk that entend from the bulge. Barred spiral galaxies are spiral galaxies with a band of bright stars that emerge from the center and extend across the middle of it. Spiral arms emerge from the ends of the "bar," as opposed to emerging from the core of ordinary spiral galaxies. Earth's galaxy, the Milky Way, has been classified as a barred spiral galaxy (SBb).

Also called the Hubble "tuning fork" diagram, the classification system can be seen in the illustration at right. It reads from left to right with elliptical galaxies as the base. The ellipticals can be named E0 through E7, with the number indicating the degree of the oval shape of the ellipse ("0" is ball-shaped and "7" is discus-shaped).

The spiral branch is indicated as follows:1

  • "0" means no arms
  • Sa - tightly-wound, smooth arms, and a bright central disc
  • Sb - better defined spiral arms than Sa
  • Sc - much more loosely wound spiral arms than Sb
  • Sd - very loose arms, most of the luminosity is in the arms and not the disc

The barred spiral branch is indicated as follows:2

  • SBa - a bright center and tight spirals
  • SBb - better defined arms than SBa galaxy and are more loosely wound
  • SBc - even looser arms, and a much dimmer central portion of the galaxy

Irregular galaxies can show spiral structure but are deformed in some way, and do not fit into other categories.

1&2 en.wikipedia.org/wiki/Galaxy_morphological_classification