Hubble sequence

The Hubble sequence classifies galaxies by visible morphology, arranging elliptical, lenticular, and spiral forms while treating irregular galaxies separately.

Galaxy: A gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The sequence sorts galaxies according to their observed forms.
Elliptical galaxy: A galaxy with a smooth, rounded appearance and little visible internal structure. Ellipticals occupy the sequence’s E branch, ordered by apparent elongation.
Edwin Hubble: An American astronomer who established that galaxies exist beyond the Milky Way and developed a galaxy classification scheme. He devised the scheme whose name and arrangement define this sequence.
Galaxy Zoo: A citizen-science project in which volunteers classify galaxies from astronomical images. Its large-scale visual classifications extend the observational practice underlying the sequence.
Galaxy morphology: The study and description of galaxies’ visible shapes and structural features. This broader descriptive framework supplies the traits used to classify galaxies.
Lenticular galaxy: A disk galaxy with a central bulge but no prominent spiral arms. Lenticulars form the intermediate S0 class between ellipticals and spirals.
The Realm of the Nebulae: Edwin Hubble’s 1936 book presenting observations and a classification of extragalactic nebulae. It set out the morphological framework that became known as the Hubble sequence.
Galaxy color–magnitude diagram: A plot relating galaxies’ colors to their luminosities, revealing populations such as the red sequence and blue cloud. It groups galaxies by observable properties other than morphology.
Galactic bulge: A dense, roughly spheroidal concentration of stars near the center of a galaxy. Bulge prominence helps distinguish spiral subtypes along the sequence.
Spiral galaxy: A disk galaxy with spiral arms extending from a central bulge or bar. Spirals occupy the sequence’s disk-galaxy branches, graded by structure.

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