The following table summarizes the properties of the three main types of galaxies in the Hubble classification scheme.
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Thus ellipticals comprise both the most massive and least massive galaxies, but bright
spirals are as luminous as
the brightest ellipticals. Generally, irregular galaxies are less massive and less bright than either of the
other two categories. Ellipticals have neither spiral arms nor extensive star-forming regions. Irregulars
often have star-forming regions (recall the bright clusters of new stars
that we saw in Sextans A)
but no clearly-defined
spiral arms. Spiral galaxies have both star-forming regions and
definite spiral arms.