How Do Scientists Classify Galaxies? | The Hubble Scheme

The universe contains billions of galaxies, each holding millions or billions of stars. When astronomers first began mapping the deep sky, they noticed that these massive structures didn’t all look the same. Some appeared as glowing, round blobs, while others displayed intricate, swirling arms. This diversity led to a fundamental question in astronomy: how do scientists classify galaxies?

Classifying these celestial bodies is not just about sorting photos. The shape of a galaxy reveals its history, its age, and the complex physics governing its stars. By grouping them, researchers can piece together the life cycle of the universe itself.

Edwin Hubble established the primary system we use today in the 1920s. While technology has advanced, his “tuning fork” diagram remains the foundation. Modern astronomy has added layers of complexity, such as spectral analysis and luminosity, but morphology remains the first step in understanding these island universes.

How Do Scientists Classify Galaxies Using Morphology

Morphology refers to the study of the form and structure of things. When astronomers ask how do scientists classify galaxies?, they are usually referring to this visual inspection. Hubble categorized galaxies into three broad classes based on their appearance in optical light.

These classes are Elliptical, Spiral, and Irregular. Each class has specific subdivisions that describe the galaxy’s shape in more detail. This system does not just describe what a galaxy looks like right now. It also hints at how the galaxy formed and whether it is still creating new stars.

The classification process usually starts with a simple image. Astronomers look for a central bulge, the presence of a flat disk, and any visible spiral arms. They also check for a “bar” of stars cutting through the center. These visual cues place the galaxy onto the Hubble sequence.

The Hubble Tuning Fork Diagram Explained

The Hubble Tuning Fork is a diagram that splits galaxies into two main prongs. On the left side of the diagram, you have elliptical galaxies. These are round or oval-shaped. As the diagram branches out, it splits into two parallel tracks: normal spirals and barred spirals.

Between the ellipticals and the spirals sits a transition type called the Lenticular galaxy. This diagram was once thought to represent an evolutionary track, where galaxies started as ellipticals and spun out into spirals. We now know this is incorrect, but the classification structure remains valid for sorting data.

Elliptical galaxies occupy the “handle” of the fork. They range from perfect spheres to elongated cigar shapes. Spiral galaxies occupy the “tines” of the fork. The top tine holds normal spirals, while the bottom tine holds spirals with a distinct central bar structure.

Below is a detailed breakdown of the main categories found in this system.

Galaxy Class	Visual Features	Star Population & Gas Content
Elliptical (E0 – E7)	Smooth, featureless light distribution. Ranges from round (E0) to flattened (E7).	Old, red stars. Very little cold gas or dust. No active star formation.
Lenticular (S0)	Central bulge with a flat disk but no spiral arms. Looks like a lens.	Mostly old stars like ellipticals, but with significant disk dust.
Normal Spiral (Sa)	Large, bright central bulge. Tightly wound, smooth arms.	Mix of old and new stars. Lower gas content than Sc types.
Normal Spiral (Sc)	Small central bulge. Loosely wound, clumpy arms showing distinct structure.	High gas and dust content. Many young, hot blue stars forming.
Barred Spiral (SB)	Features a rectangular “bar” of stars crossing the center. Arms trail from bar ends.	Similar to normal spirals. The bar may funnel gas inward to fuel starbursts.
Irregular (Irr I)	No symmetry or clear shape. Chaotic mix of gas and stars.	Abundant gas and dust. Very active regions of new star birth.
Peculiar (Pec)	Distorted shapes, tails, or rings, often due to collisions or mergers.	Highly active. Mergers trigger massive waves of star formation.
Dwarf Spheroidal (dSph)	Small, faint, and round. Low surface brightness. Hard to detect.	Old stars, very little gas. The most common galaxy type in the universe.

Classifying Spiral Galaxies: Arms And Bulges

Spiral galaxies are the most iconic type. They look like pinwheels floating in space. To classify a spiral, astronomers look at two things: the size of the central bulge and the tightness of the spiral arms.

A spiral galaxy labeled “Sa” has a large, bright center and arms that are wrapped very tight against the body. These galaxies often have fewer patches of star formation. On the other end of the spectrum, an “Sc” spiral has a tiny core and very loose, open arms. These open arms are usually speckled with bright blue clusters, indicating young, hot stars.

Many spirals also contain a straight bar of stars running through the middle. Our own Milky Way is classified as a Barred Spiral. Specifically, it is likely an SBbc, meaning it falls between the “b” and “c” subcategories. The bar structure helps drive gas toward the center, feeding the black hole or fueling new star birth.

Elliptical Galaxies Classification: From Round To Stretched

Elliptical galaxies look like fuzzy cotton balls of light. They lack the dramatic arms and dust lanes of spirals. Classification here relies on geometry. Astronomers measure the shape of the ellipse against a simple scale.

The scale runs from E0 to E7. An E0 galaxy is perfectly circular. An E7 galaxy is highly elongated, looking like a flattened cigar or an American football. The number is calculated based on the ratio of the major axis to the minor axis.

Ellipticals are often called “red and dead.” They used up their gas billions of years ago. The stars inside are old, red giants moving in random orbits rather than a flat disk. Because they lack cold gas, they cannot form new stars, leaving them with a smooth, golden glow.

The Criteria Scientists Use To Classify Galaxies Without Shape

While shape is the primary method, it has limits. Distant galaxies often look like mere smudges. Sometimes, the visible light is blocked by dust, hiding the true structure. In these cases, researchers turn to other physical properties to categorize the object.

One major alternative is color. Galaxies generally fall into two color categories: the “Red Sequence” and the “Blue Cloud.” Ellipticals dominate the Red Sequence because their stars are old and cool. Spirals and Irregulars populate the Blue Cloud because massive, short-lived blue stars dominate their light.

This color method is less subjective than human visual inspection. A computer can measure the exact color index of a galaxy and sort it automatically. This helps link the visual shape of a galaxy to its actual stellar population and age.

Luminosity Classes And The Yerkes System

Another layer of classification involves brightness, or luminosity. This is known as the Yerkes system. It accounts for the fact that a giant spiral and a dwarf spiral might look the same shape-wise but are physically very different.

Luminosity classes range from I (Supergiants) to V (Dwarfs). This distinction is vital for understanding the mass of the galaxy. A supergiant elliptical galaxy (cD type) sits at the center of galaxy clusters and is the most massive type of galaxy known, often formed by swallowing its neighbors.

Understanding The Classification Of Irregular Galaxies

Irregular galaxies do not fit neatly into the spiral or elliptical categories. They lack a defined center or a symmetric structure. These are the chaotic rebels of the cosmos. To classify them, scientists look at exactly how disorganized they are.

An Irr I galaxy has some structure but not enough to be a spiral. It might have a hint of a bar or a single arm. The Large Magellanic Cloud, a satellite of the Milky Way, is a classic example. It shows signs of a bar but is too distorted to be a true barred spiral.

An Irr II galaxy is truly chaotic. These systems often result from gravitational interactions. When two galaxies pass close to one another, gravity rips stars and gas away from their main bodies. This leaves a messy, undefined shape that defies standard classification.

The De Vaucouleurs System Extension

The original Hubble system was simple, but nature is messy. Astronomer Gérard de Vaucouleurs expanded the system to capture more detail. He added notation for rings and varying degrees of “bar” structures.

In this system, a galaxy might be labeled (r) for having a ring or (s) for having no ring. This system is essentially a 3D version of Hubble’s 2D tuning fork. It allows for a more precise description of the complex structures found in spiral disks.

For example, a galaxy might have a weak bar, an inner ring, and loose arms. The Hubble system would just call it a “Barred Spiral.” The de Vaucouleurs system provides a code that describes all three features. This precision is necessary when studying the dynamics of how gas moves through a galaxy.

Modern Techniques In Galaxy Classification

Today, classification often involves analyzing the light spectrum. Spectroscopy splits light into its component colors, revealing the chemical composition and speed of the stars inside.

Using data from surveys like the Sloan Digital Sky Survey (SDSS), scientists can determine a galaxy’s type without ever looking at a picture. The presence of strong hydrogen emission lines indicates active star formation (likely a spiral or irregular). A spectrum dominated by metal absorption lines suggests an old, passive population (likely an elliptical).

This data-driven approach removes human bias. It also allows astronomers to classify millions of galaxies at once, far more than any single human could verify by eye.

Criteria	Measurement Tool	What It Reveals
Morphology	Optical Telescopes	Structural history and dynamic stability.
Spectral Type	Spectroscopy	Current star formation rate and chemical abundance.
Gas Content	Radio Astronomy	Fuel available for future stars (High in spirals, low in ellipticals).
Active Nuclei	X-Ray / Radio	Presence of a feeding supermassive black hole.

How Do Scientists Classify Galaxies With Computers?

We have entered the era of big data in astronomy. With telescopes capturing billions of objects, asking how do scientists classify galaxies? now has a new answer: Artificial Intelligence.

Machine learning algorithms are trained on datasets categorized by humans. These AIs identify patterns in the pixel data that match the Hubble classes. They can process terabytes of data overnight, providing a catalog of morphology for the entire observable universe.

However, human eyes are still valuable. Projects like Galaxy Zoo ask volunteers to look at images and vote on the shape. This “citizen science” helps train the AI. Humans are surprisingly good at spotting weird or peculiar galaxies that computers might miss or mislabel.

Why Classification Matters

You might wonder why we spend so much effort putting these objects into boxes. The reason is that classification is the first step to understanding evolution. If we know that nearly all spiral galaxies have blue stars and gas, and all ellipticals have red stars and no gas, we can infer a process.

Current theories suggest that galaxies change over time. Two spiral galaxies might crash into each other. The collision destroys the delicate spiral arms and scrambles the orbits. The gas is used up in a massive burst of star formation. The result is often a featureless elliptical galaxy.

Without a rigorous classification system, we would miss these connections. The labels allow us to see the story of the universe unfolding over billions of years.

Peculiar Galaxies And Interactions

Peculiar galaxies are the most visually stunning class. They defy standard descriptions. They often look like rings, letters, or chaotic splashes of light. The Antennae Galaxies are a famous example, looking like two insect feelers extending into space.

These shapes are almost always the result of gravity. When galaxies interact, tidal forces pull long streams of stars out into space. These “tidal tails” are temporary features. Over hundreds of millions of years, the system will settle down into a more standard shape, likely an elliptical.

Study of these peculiar types helps physicists refine their models of dark matter. The way the tails stretch depends on the mass of the galaxies, most of which is invisible dark matter. By classifying the type of distortion, we can weigh the invisible parts of the universe.

The Future Of Galactic Sorting

As telescopes like the James Webb Space Telescope (JWST) peer deeper into the past, our classification systems will face new tests. JWST shows us galaxies from the very early universe. These early galaxies often do not fit the Hubble sequence. They are clumpy, irregular, and small.

The “Hubble Sequence” appears to be something that emerged later in cosmic history. In the early days, the universe was too chaotic for neat spirals to form. Scientists are now working on new classification schemes to describe these primordial objects.

This evolution of methods ensures that as our vision gets sharper, our understanding gets deeper. The simple question of sorting shapes leads directly to the biggest questions of cosmic origins.