These dwarf irregular galaxies are the Small and Large Magellanic Clouds discovered by the explorer Magellan. The simplest galaxy classification system, invented by Edwin P. Hubble, classifies galaxies as either spiral, elliptical, or irregular in shape. Spiral galaxies have unmistakable characteristic features. The arms of the spiral define a plane. A large concentration of stars at the center of the galaxy makes a bulge there.
Spiral galaxies are rich in the gas and dust needed to form new stars.
Their blue color tells astronomers that star formation is indeed ongoing in these galaxies. Our solar system lies about two-thirds of the distance from the nucleus in the Milky Way's spiral arm, called the Sagittarius Arm. The stars of the constellation Sagittarius all lie in this spiral arm of the Milky Way. Elliptical galaxies also have characteristic structure, but are quite different from spirals. These galaxies can range in shape from nearly spherical to cigar shaped.