A galaxy is a vast collection of stars, gas, dust, and dark matter, held together by gravity. These immense structures vary significantly in size, from dwarfs containing a few thousand stars to supergiants with hundreds of trillions. Our solar system, including Earth, is a small component of the Milky Way Galaxy.
Understanding Galaxy Classification
Astronomers categorize galaxies by their visual appearance using the Hubble Sequence. Developed by Edwin Hubble in 1926, this system categorizes galaxies by their shape and structure. While the “tuning fork” diagram of the Hubble sequence does not imply an evolutionary path, it helps astronomers understand galaxy formation and evolution. This system provides a fundamental framework for studying the diverse forms galaxies can take.
Spiral Galaxies
Spiral galaxies have a flattened, rotating disk, a central bulge, and prominent spiral arms. The arms are regions where gas, dust, and young, hot stars are concentrated, making them appear brighter. Our Milky Way galaxy is a barred spiral, with arms originating from a bar-shaped structure at its center.
The Milky Way’s disk spans over 100,000 light-years, with Earth located along one of its spiral arms. Roughly two-thirds of all spiral galaxies observed are thought to be barred spirals. These galaxies generally contain a mixture of old and young stars, along with abundant gas and dust that fuels ongoing star formation.
Elliptical Galaxies
Elliptical galaxies have an oval or spherical shape, from nearly round to highly elongated. Unlike spiral galaxies, they appear smooth and largely featureless, lacking distinct spiral arms or a prominent disk. They contain very little interstellar gas and dust, resulting in minimal or no ongoing star formation. Consequently, elliptical galaxies are composed predominantly of older, redder stars.
M87, a supergiant elliptical galaxy, is a prominent example. It has an almost featureless, ellipsoidal shape and contains far less dust. Elliptical galaxies vary widely in size, from dwarf ellipticals with millions of stars to supergiants with trillions of stars.
Irregular Galaxies
Irregular galaxies lack a distinct, regular shape, often appearing chaotic. Their unusual forms can result from gravitational interactions or collisions with other galaxies. They often have abundant gas and dust, supporting active star formation.
The Magellanic Clouds, two dwarf galaxies orbiting the Milky Way, are well-known examples. Irregular galaxies are diverse, ranging from small dwarf irregulars to larger, more massive ones.
Lenticular Galaxies
Lenticular galaxies are an intermediate class, sharing characteristics with both spiral and elliptical galaxies. They have a prominent central bulge and a disk, similar to spirals, but lack distinct spiral arms. They contain very little interstellar gas and dust, resembling elliptical galaxies. As a result, ongoing star formation is minimal or absent in lenticular galaxies. Their stellar populations are generally older, much like those found in elliptical galaxies.
NGC 5866, also known as the Spindle Galaxy, is a lenticular galaxy. It has a flat stellar disk and a large, ellipsoidal bulge.