The Galaxy Classification System

Astronomers classify galaxies primarily by their visual appearance, using a system first developed by Edwin Hubble in 1926. This classification scheme, often called the “Hubble tuning fork” due to its distinctive shape when diagrammed, divides galaxies into several major groups. While Hubble’s original system has been expanded over the years, it remains the foundation of how we understand galaxy morphology.

How Classification Works

Galaxy classification is primarily based on what galaxies look like through telescopes. Today, while visual observation is still important, scientists also use computational methods and analyze physical characteristics to classify galaxies more accurately. This classification helps astronomers understand how galaxies form and evolve over time.

Spiral Galaxies: The Cosmic Pinwheels

Spiral galaxies are among the most visually spectacular objects in the universe, and they’re also the most common type, making up approximately 77% of all known galaxies. Our own Milky Way falls into this category.

Structure and Characteristics

Spiral galaxies have three main components:

1. A central bulge: This spherical collection of stars at the galaxy’s center primarily contains older stars.
2. A disk: This flat, pancake-like structure surrounds the bulge and contains the spiral arms where new stars form.
3. A halo: This faint, roughly spherical region encompasses the disk and contains old stars, star clusters, and dark matter.

The striking spiral arms that give these galaxies their name are regions where gas and dust are concentrated, leading to active star formation. These arms appear brighter because they contain hot, young stars that shine brilliantly. As you move from the center to the outer regions of a spiral galaxy, you’ll find stars of decreasing age—the newest stars are born in the spiral arms, while the oldest reside in the bulge and halo.

Spiral Subtypes

Astronomers further classify spiral galaxies based on characteristics like how tightly their arms are wound and the size of their central bulge. These subtypes range from Sa (tightly wound arms, large bulge) to Sc (loosely wound arms, small bulge). The Sombrero Galaxy (M104) is a prime example of an Sa galaxy with its prominent bulge, while NGC 3949 displays the loose arms and small bulge typical of an Sc galaxy.

Barred Spiral Galaxies: Spirals with a Crossbar

Take a spiral galaxy and add a straight bar of stars running through its center, and you have a barred spiral galaxy. Our own Milky Way falls into this category.

The Bar Structure

The distinctive bar is composed of stars, gas, and dust that cut straight across the galaxy’s center. From the ends of this bar, the spiral arms typically begin their graceful outward sweep. Astronomers believe that the presence of a bar indicates that a galaxy has reached full maturity in its evolution.

Approximately two-thirds of all spiral galaxies feature these bar structures. Like regular spiral galaxies, barred spirals are subclassified based on their arm tightness and bulge size, with designations like SBa, SBb, and SBc.

Elliptical Galaxies: The Cosmic Spheroids

If spiral galaxies are cosmic pinwheels, elliptical galaxies are cosmic footballs or basketballs. These galaxies appear as smooth, featureless ellipses with no spiral arms or distinctive structures.

Characteristics and Composition

Elliptical galaxies:

• Have a smooth, amorphous appearance with brightness that gradually decreases from center to edge
• Contain predominantly old stars with very little gas or dust
• Show little to no ongoing star formation
• Have stars distributed throughout the galaxy in random orbital directions
• Can range from perfectly spherical to highly elongated shapes

The stars in elliptical galaxies are often packed closely together, especially near the center, where the density can be so high that if Earth were in such a galaxy, the night sky would be perpetually bright from the light of surrounding stars.

Classification of Ellipticals

Elliptical galaxies are classified by their apparent shape using the notation En, where n is a number from 0 to 7 representing how elongated the galaxy appears. E0 represents a perfectly circular appearance, while E7 indicates the most elongated form. However, this classification reflects only how the galaxy appears from our perspective, not necessarily its true three-dimensional shape.

More recent classification systems also consider the subtle shapes of the elliptical galaxies’ light distribution, dividing them into “boxy” or “disky” ellipticals based on whether their outer edges appear slightly square-like or more disk-like.

Lenticular Galaxies: The Bridge Between Types

Lenticular galaxies (designated S0) represent a fascinating transition between spiral and elliptical galaxies. They feature:

• A bright central bulge similar to elliptical galaxies
• A disk-like structure similar to spiral galaxies, but without spiral arms
• Very little ongoing star formation, like ellipticals
• Primarily older stellar populations

These galaxies maintain the general structure of spiral galaxies but lack the active star-forming regions that make spiral arms visible. When viewed edge-on, lenticular galaxies can resemble a lens shape, which gives them their name.

Irregular Galaxies: The Cosmic Oddballs

As their name suggests, irregular galaxies don’t conform to any of the patterns above. They have no definable shape or structure.

Types and Origins

Irregular galaxies come in two main varieties:

1. Irr-I galaxies: Normal, low-luminosity galaxies with active star formation but no clear structure
2. Irr-II galaxies: Peculiar systems with unusual appearances, often due to cosmic events like galaxy collisions

Many irregular galaxies are thought to have once been spiral or elliptical galaxies that were distorted by gravitational interactions with neighboring galaxies. The Large Magellanic Cloud, a satellite galaxy of our Milky Way, is classified as an irregular galaxy with some barred spiral characteristics (SBm).

IC 10: A Notable Example

IC 10 is a remarkable irregular galaxy that’s relatively close to us (about 2.2 million light-years away). It’s classified as a “starburst galaxy,” meaning it’s experiencing an intense period of star formation fueled by large amounts of hydrogen gas. Despite its relative proximity, IC 10 wasn’t discovered until 1887 and remains difficult to study because of cosmic dust that obscures our view.

Special Galaxy Types

Beyond the main morphological classifications, there are several special categories of galaxies defined by their activity rather than just their shape:

Active Galaxies

Some galaxies have extraordinarily energetic central regions powered by supermassive black holes. These active galactic nuclei (AGNs) can produce enormous amounts of energy across the electromagnetic spectrum. Seyfert galaxies and quasars are examples of galaxies with different levels of central activity.

Starburst Galaxies

Like IC 10 mentioned above, starburst galaxies are undergoing periods of exceptionally rapid star formation. This furious stellar creation is fueled by abundant supplies of cool hydrogen gas that condenses into molecular clouds and eventually forms new stars.

Conclusion

Galaxy morphology provides astronomers with important clues about a galaxy’s age, composition, and evolutionary history. From the elegant spiral arms of galaxies like Andromeda to the smooth spheroids of ellipticals and the chaotic structure of irregulars, each shape tells a unique cosmic story.

The classification systems created by Hubble and expanded by later astronomers have given us a framework for understanding these diverse cosmic structures. While these classifications are continuously refined as our observational and computational technologies improve, the fundamental categories remain a valuable tool for both professional astronomers and curious non-experts alike.

Understanding galaxy morphology is just one step in appreciating the vastness and diversity of our universe—a cosmos filled with billions of galaxies, each with its own unique shape, history, and future.