Question

Ques. Describe Hubble’s Classification scheme for galaxies.

Answer 1

Classification is based on our perspective from Earth. There are four types of galaxies [1,2]:
1. elliptical (doesn’t have spiral structure, vary from round E0 to elongated ellipse E7),
2. spiral (type depend on the tightness of the arms and the size of the nucleus, S0 have a disc and no visible arms, Sa is the tightest wound with large nucleus, Sc is the most open with little nucleus, Sb is in the middle),
3. barred spiral (the same as the spiral but has a prominent bar through the nucleus),
4. irregular (any other).
[1] https://btc.montana.edu/ceres/html/galaxy/galhubble.html
[2] https://astronomy.swin.edu.au/cosmos/H/Hubble+Classification