Blog Post 20, Hubble Tuning Fork

My Hubble Tuning Fork:

(Click on image for larger version)

The "Hubble Tuning Fork" is the classification system for galaxies developed by Edwin Hubble that groups galaxies into categories based on their appearance - how round or flat they are, whether or not they have a central bar structure, if they have spiral arms, and how tightly wound those arms are.

Elliptical galaxies (those that begin with an "E" in the diagram) are ellipsoidal with no central structure, and are classified by their eccentricity, or how long/thin they are. E0 galaxies (far left) are essentially spherical, whereas E7 galaxies are extremely long and thin. Elliptical galaxies generally have many older stars, and tend to have many globular clusters surrounding them.

Spiral galaxies have a central bulge surrounded by a flattened disk structure with arms spiraling outwards from the center. A faint halo surrounds the disk, in which there are some globular clusters. Spiral galaxies are split into two groups - barred spirals and non-barred spirals - and then into a further three groups, types a, b, and c. Barred spiral galaxies (SBa, SBb, SBc) have a central bar structure from which the spiral arms begin, whereas non-barred spirals (Sa, Sb, Sc) do not. Type a spiral galaxies have loosely-wound arms, whereas type c has very tightly wound arms.

Lenticular galaxies (S0) is an intermediate between an elliptical galaxy and a spiral galaxy. They have no spiral structure, but have a rotating disk and central bulge. They do sometimes have a central bar structure.

Irregular galaxies are those that do not fall under any of the above categories - they have a unique shape, and thus do not appear on the tuning fork. They often form from the collision of two galaxies. Here is an example of an irregular galaxy:

Image sources:
http://burro.case.edu/Academics/Astr222/Galaxies/Intro/M87.jpg
http://freestarcharts.com/images/Articles/Messier/M86_NASA_AURA_STScI.jpg
https://www.le.ac.uk/ph/faulkes/web/images/m59.jpg
http://cdn.iopscience.com/images/0067-0049/190/1/147/Full/apjs351534f1_lr.jpg
http://chandra.as.utexas.edu/bh/NGC3115.jpg
http://www.noao.edu/image_gallery/images/d4/m81y.jpg
http://cs.astronomy.com/cfs-file.ashx/__key/telligent-evolution-components-attachments/13-58-00-00-00-43-91-97/NGC1365_5F00_master_5F00_LRGB_5F00_n_5F00_blue_5F00_sb.jpg
http://www2.lowell.edu/rsch/LMI/gallery/n6946_2.jpg
https://upload.wikimedia.org/wikipedia/commons/1/15/NGC3486-hst-R814GB450.jpg
http://cse.ssl.berkeley.edu/SegwayEd/lessons/classifying_galaxies/SBa.htm
http://photojournal.jpl.nasa.gov/catalog/PIA07901
http://www.starcorral.com/html/observing_at_oil_capital.html
http://hubblesite.org/newscenter/archive/releases/2005/09/image/a/