Spirals, Ellipticals, and Irregulars: The Types of Galaxies

In the 1920s, astronomer Edwin Hubble revolutionized our understanding of the universe by proving that the "spiral nebulae" seen through telescopes were actually entirely separate galaxies, far beyond the Milky Way.

He soon realized that galaxies come in a variety of distinct shapes and sizes. He organized them into a classification system known as the Hubble Tuning Fork.

1. Spiral Galaxies

Like our own Milky Way and the neighboring Andromeda galaxy, spiral galaxies are beautiful, spinning pinwheels of stars.

They consist of a flat, rotating disk containing stars, gas, and dust, and a central concentration of stars known as the bulge. The spiral arms are regions of active star formation, which is why they glow brightly with hot, young, blue stars.

Some spirals feature a straight "bar" of stars cutting across the center, with the spiral arms trailing from the ends of the bar. These are called Barred Spirals (the Milky Way is one of them).

2. Elliptical Galaxies

Elliptical galaxies look like smooth, featureless spheres or squashed footballs. Unlike spirals, they have no disk and no spiral arms.

These galaxies are essentially "retired." They contain very little gas and dust, meaning they can no longer form new stars. Consequently, they are populated by older, redder, lower-mass stars. Elliptical galaxies are often the result of massive galactic collisions and mergers. The largest galaxies in the universe, such as M87, are giant ellipticals found at the center of massive galaxy clusters.

3. Irregular Galaxies

As the name suggests, irregular galaxies lack any distinct, symmetrical shape. They are often chaotic, mangled blobs of stars and gas.

Their distorted shapes are usually the result of gravitational interactions with larger galaxies. For example, the Magellanic Clouds—two small satellite galaxies orbiting the Milky Way—are irregulars. Despite their messy appearance, irregular galaxies are often incredibly rich in gas and dust, making them hotbeds of intense star formation.

4. Lenticular Galaxies

Lenticulars sit at the "crossroads" of the Hubble tuning fork, bridging the gap between spirals and ellipticals. They have a central bulge and a disk, but like ellipticals, they have exhausted their star-forming gas and lack spiral arms.