The Hubble Tuning Fork is a classification system developed by Edwin Hubble in 1926 as a theory of galaxy evolution. Today we view it as outdated, as there is evidence that spiral galaxies do not evolve from ellipticals, but it is a useful visual for galaxy types.
Ellipticals (E0-E7):
Elliptical galaxies are spheroid or elongated spheres where stars have no uniform rotation around the center of the galaxy. Elliptical galaxies tend to have older, and therefore redder, stars. Elliptical galaxies are classified due to their shape. E0 galaxies appear almost perfectly spherical, at least from our perspective on Earth. E3 galaxies appear as slightly elongated ellipsoids, E5 slightly more, and E7 galaxies are extremely elongated.
Lenticular galaxies (S0):
Lenticular galaxies (charmingly named after the lentil bean) fall between elliptical and spiral galaxies. They have no spiral shape but do contain a bulge and thin disk, similar to a spiral galaxy.
Spiral Galaxies (Sa-Sc) and Barred Spiral Galaxies (SBa-SBc):
Spiral galaxies are galaxies like the Milky Way or Andromeda. They typically contain a disk around which stars revolve, a bulge, and spiral arms. Spiral galaxies contain younger, bluer, stars than ellipticals. Spirals are classified by how tightly their arms are wound with Sa spirals being the tightest wound and Sc being the loosest. Barred spirals have a bar of stars running across the nucleus out of which the arms extrude. Barred spirals are also classified by the distribution of their arms.
Irregular:
Irregular galaxies do not fit into any of these categories and are often the result of two galaxies colliding.
- E0: Image: M87
- E3: Image M86
- E5: Image: M59
- E7: Image
- S0: Image: Messier 102
- SA: Image: NGC 1302
- Sb: Image: M77
- Sc: Image: NGC 628
- Sba: Image: M83
- Sbb: Image: Ngc 1300
- Sbc: Image: NGC 1365
Great job Danielle! Good tuning fork :) 5/5
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