The Hubble tuning fork diagram was developed by Edwin Hubble as a way of classifying galaxies based on their shape. Basically, astronomers look through a telescope at a galaxy, and classify it as an elliptical, spiral, or barred spiral. These classifications were discussed in the last ADYK. If the galaxy looks like a fuzzy bright blob, it's classified as an elliptical (E), and a number is assigned based on how round it looks (1=circle, 9=cigar shaped). If a galaxy has deifned circular rings, but not exactly spiral arms, then it's an "S0". If the galaxy appears to have spiral arms, it's either a regular spiral galaxy (S), or a spiral with a bar like structure through the center (SB). They are also assigned a letter based on how tightly wound the spiral is. "Sa" means tightly wound arms, while "Sc" is very loosely wound arms. These classifications are handy for astronomers, as they can relate the properties of galaxies to their classification. Hubble originally thought that this diagram represented a time sequence, i.e. all galaxies started as round ellipticals (E0), and evolved over time into spirals (SBc or Sc). Astronomers are now able to measure the ages of many galaxies, and we've found some young ellipticals and old spirals, so the tuning fork diagram does not represent a galaxy evolution sequence. One thing to remember is that this classification is based on actual galaxy shape, not observed. For example, from Earth's perspective, you might view a spiral galaxy edge on, but think it's an E7 elliptical because it appears that way. Because of this, it's not easy to classify these galaxies accurately. You can actually help astronomers do this! Check out the next ADYK!
