The Hubble Tuning Fork diagram puts galaxies into categories, and each of the categories corresponds to a different type of galaxy.
Elliptical galaxies, such as M87 (left), have very little gas and dust. Because gas and dust are found in the clouds that are the birthplaces of stars, we should expect to see very few young stars in elliptical galaxies. In fact, elliptical galaxies contain primarily old, red stars (also known as Population II stars).
Elliptical galaxies vary widely in size. Both the largest and the smallest known galaxies are elliptical. Very large elliptical galaxies can reach 300 million light years in diameter. Dwarf ellipticals, which are very common, may contain only 1/100,000th as many stars as the Milky Way!
Spiral galaxies like NGC 3310 (right) have two distinct regions. The disk of the galaxy contains the spiral arms. The disk is a region of star formation and has a great deal of gas and dust. It is dominated by young, blue Population I stars. The central bulge is devoid of gas and dust. As you might expect, the bulge is composed primarily of Population II stars. Type c spiral galaxies have the most gas and dust.
Lenticular galaxies like M85 (left) have a central bulges and disks, but no spiral arms. If the disk is faint, it is easy to mistake a lenticular galaxy for an elliptical galaxy – S0 galaxies look very much like E0 galaxies. Lenticular galaxies are sometimes called armless spirals.
Galaxy M85 is an S0 galaxy. There is a second type of lenticular galaxy called a barred lenticular galaxy. Barred lenticular galaxies have bars, much like the barred spirals, and so they are denoted SB0.
Galaxies that do not fit into either the spiral or elliptical classes are called irregular galaxies. Irregular galaxies, such as M82 (right), have a wide variety of shapes and characteristics. They are frequently the result of collisions between galaxies or gravitational interactions between galaxies.
M82 has also been called “The Exploding Galaxy.” It is believed to have recently passed close to another galaxy called M81. M81’s gravity warped M82, and the warping caused a burst of new stars to form. Today, M81 and M82 make a beautiful sight through binoculars.