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Abstract
We present \(UBVI\) and H\(\alpha\) images of a sample of Low Surface Brightness (LSB)
disk galaxies. These galaxies are generally late types, if they can be sensibly classified
at all. However, they are not dwarfs, being intrinsically large and luminous.
The morphology of LSB galaxies is discussed in terms of the physical interpretation
of the Hubble sequence. Galaxies with high contrast relative to the sky background
are subject to being more finely typed than those which appear merely as fuzzy blobs
on photographic plates. This causes the stages of the Hubble sequence to be nonlinear
in the sense that large morphological type distinctions are made between high surface
brightness spirals when only small physical differences exist, and small morphological
distinctions are made between low surface brightness galaxies even when large physical
differences exist.
Many LSB galaxies lack the old red disk conspicuous in higher surface brightness spirals.
Their morphology is strikingly similar in all bands from \(U\) to \(I\), suggesting farily
homogeneous stellar populations lacking a well developed giant branch. These properties,
together with their very blue colors, suggest that LSB galaxies are relatively younger
than their high surface brightness counterparts. A few of these LSB galaxies appear
to be very young (\(\simlt 1\)~Gyr), and as such may represent local examples of protogalaxies.
Comments 19 pages Plain TeX. Also available on the WWW via
http://www.ast.cam.ac.uk/preprint/PrePrint.html (warning: figs 2-23 ~1 Mb
each) Accepted for publication in the Astronomical Journal; scheduled for
April issue