Astronomers have long sorted galaxies into types based on their visible structure. Spiral galaxies, like our Milky Way, have a flat disk, winding spiral arms and ongoing star formation. Ellipticals tend toward smooth, featureless ellipsoids with older stellar populations and little cold gas or dust.
Some galaxies, known as irregulars, don't fit neatly into either mold. And some have aspects of both — like the galaxy NGC 2775, which we see in this Hubble Space Telescope image.
