Galaxy Fundamentals

How are galaxies classified or organized?

In the 1920s the pioneering astronomer Edwin Hubble (1889–1953), who devoted his life to studying galaxies, proposed a way to classify galaxies based on their shapes. He proposed a “sequence” of galaxy types: from E0 (sphere-shaped elliptical galaxies) to E7 (cigar-shaped ellipticals), S0 (lenticular galaxies) to Sa and SBa (spiral galaxies with large bulges and bars), Sb and SBb (spirals with medium-sized bulges and bars), and Sc and SBc (spirals with small bulges and bars). The sequence is known as the Hubble sequence, and it is often shown visually as a Hubble “tuning fork diagram.”

Spiral galaxy NGC 4414. (NASA, The Hubble Heritage Team, STScI, AURA)


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