Which astronomer developed the system of classifying galaxies by shape into spirals, ellipticals, and irregulars?
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Earth & Space Science · Foundations
Galaxies
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In 30 seconds
Galaxies are vast systems of stars, planets, gas, and dust, all bound together by gravity. Most are between 10 and 13.6 billion years old, and they come in three broad shapes — spiral, elliptical, and irregular — a scheme astronomer Edwin Hubble developed. Spiral galaxies with a bar of stars through the center are called barred spirals. Galaxies gather into groups and clusters, which sit inside superclusters. We study galaxies by spreading their light into spectra: a Galaxy A system of stars, planets, and clouds of gas and dust, all held together by gravity. Full entry →'s Redshift The shift of spectral lines toward longer wavelengths, seen when a galaxy is moving away from us. Full entry → tells us how fast it is receding and helps us estimate its distance.
Why this matters
Every star — including the Sun — lives inside a galaxy, and so do the planets that orbit it. Galaxies are the arenas where stars form, evolve, and die, and the Milky Way is our own address within that layout. A shared classification system lets astronomers summarize what a galaxy is made of and how it behaves in a single word, and understanding how galaxies cluster frames the modern picture of a universe organized into groups, clusters, and superclusters. Later study of the universe's expansion, the life cycles of stars, and the search for planets beyond our solar system all build on this foundation. Reading astronomy news and sky images becomes far more meaningful when you can recognize a spiral arm, a redshifted spectrum, or an active galaxy's brilliant center.
The college version
Stars, gas, dust, and gravity
A galaxy is a vast system of stars, planets, and clouds of gas and dust held together by gravity. NASA describes galaxies as systems in which all these components are bound together, and they range enormously in size: the largest contain trillions of stars and span more than a million light-years, while the smallest hold only a few thousand stars and stretch a few hundred light-years. Most galaxies are old — between 10 and 13.6 billion years — and some formed when the universe itself was young. Gravity is the organizing force: it keeps stars orbiting the galactic center, holds gas and dust in place, and governs the slow dance of entire galaxies around one another. A galaxy is not empty space with a few lights in it; it is a structured city of matter on a scale that is hard to picture from inside. It is worth remembering that we do live inside one — the Milky Way — and that our home galaxy gets its own full treatment in a separate lesson.
Hubble's shapes: spirals, ellipticals, and irregulars
Astronomer Edwin Hubble developed a system for classifying galaxies by their appearance, and his three broad categories are still in use. Spiral galaxies have a central bulge, a flattened disk, and spiral arms winding outward. They hold a mixture of young and old stars, and their arms glow with the blue light of newly forming stars and the emission nebulae that surround them. About two-thirds of nearby spiral galaxies carry a bar — a boxy or peanut-shaped concentration of stars running through the center — and astronomers call these barred spirals; the arms of a Barred spiral A spiral galaxy with a boxy or peanut-shaped bar of stars through its center, from whose ends the spiral arms begin. Full entry → begin at the ends of the bar. Elliptical galaxies look like smooth spheres or squashed ellipsoids with no spiral arms at all; they are made almost entirely of old, reddish stars, with little gas and dust left for new star formation. Irregular galaxies are the catchall category: they lack the regular shapes of the other two types, tend to be smaller and less luminous, and many are churning with intense star formation. Hubble's scheme describes shapes, not life stages — it organizes what galaxies look like, while how galaxies form and change is left to later topics.
Groups, clusters, and the larger web
Galaxies rarely float alone. NASA describes a nested organization: groups contain roughly 100 or fewer galaxies held together by mutual gravity, and clusters are larger structures that may contain thousands. Groups and clusters, in turn, are arranged into superclusters — structures so vast that they are not gravitationally bound. Named examples anchor these ideas: our Milky Way belongs to the Local Group, a neighborhood of more than 50 galaxies that includes the Andromeda galaxy, our nearest large neighbor; the Local Group sits just off the edge of the Virgo cluster; and both are part of the Laniakea Supercluster A vast arrangement of galaxy groups and clusters that is not itself gravitationally bound. Full entry →. Add the empty voids between these structures and you get the cosmic web of matter that astronomers map across the sky.
Reading galaxies with light
Nearly everything we know about galaxies comes from their light. Telescopes collect that light, and spectrographs spread it into a rainbow of colors. The dark lines in a galaxy's spectrum act as fingerprints: they reveal which elements are present, and their positions reveal motion. When the lines are shifted toward longer wavelengths — a redshift — the galaxy is moving away from us, and the size of the shift tells how fast. Combined with distance measurements, redshift is how astronomers estimate the distances to faraway galaxies; the expansion of the universe behind this relationship belongs to the universe topic. Light also reveals what lurks at the centers of galaxies. Most large galaxies host supermassive black holes, and in some galaxies — called active galaxies — matter spiraling toward that black hole heats up and shines so brightly that the center can outshine the rest of the galaxy. Astronomers call such a brilliant core an Active galactic nucleus The brilliantly bright center of an active galaxy, powered by matter falling toward a supermassive black hole. Full entry →.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Galaxies are giant collections of stars, planets, gas, and dust held together by gravity. They come in three main shapes. Spirals look like spinning pinwheels with arms, and some have a bright bar through the middle — those are barred spirals. Ellipticals look like smooth, round blobs of old stars. Irregulars are the messy ones, with no particular shape. Galaxies rarely sit alone: they gather into groups, groups gather into clusters, and clusters sit inside giant superclusters, like cities grouped in a country. Our own galaxy, the Milky Way, is a spiral in the Local Group. Astronomers learn most of this from light. The pattern of lines in a galaxy's spectrum tells what it is made of and whether it is moving toward us or away from us. When the lines shift toward the red end, the galaxy is receding — that shift is called redshift. Some galaxies have amazingly bright centers, called active galactic nuclei, powered by matter falling into a giant black hole.
Picture it like this
Think of a galaxy as a city. The city's lights and buildings are its stars; the parks and open lots are the gas and dust between them; the roads that pull everything into a pattern are gravity. A spiral galaxy is a city laid out in sweeping avenues, an elliptical is an older downtown of tidy blocks, and an irregular is a sprawl that grew without a plan. Just as cities group into states and countries, galaxies group into clusters and superclusters.
Where the picture stops working
The analogy has limits. A city's streets are fixed, while a galaxy's arms are patterns of stars and gas that shift over hundreds of millions of years. Cities are built by people, but no one builds a galaxy — gravity assembles it from a cloud of gas, and stars form continuously inside it. And while cities stop growing at their borders, galaxies can collide and merge, reshaping each other completely.
Worked example
Imagine you are looking at two galaxies. The Large Magellanic Cloud, an irregular galaxy, is one of our nearest neighbors — astronomers measure its distance at a little more than 160,000 light-years. Because one light-year is exactly the distance light travels in a year, the light you see from it tonight left that galaxy about 160,000 years ago. Now consider the Milky Way's disk, which spans more than 100,000 light-years: light from a star on the far side of our own galaxy takes more than 100,000 years to reach us. Finally, NASA reports that the Sun completes one orbit of the Milky Way's center in about 240 million years — so if we ever observe a galaxy whose light has traveled for 240 million years, we are seeing it as it was a full galactic year ago.
Key takeaway
Galaxies are the gravitationally bound homes of stars and planets, sorted by shape through Hubble's scheme, organized into groups, clusters, and superclusters, and understood through the light they emit.
Quick check
3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.
What is the defining feature of an elliptical galaxy?
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An astronomer measures a galaxy's spectrum and finds its lines shifted toward longer wavelengths. What does this redshift tell her, in simple terms?
Choose an answer, then check it.
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related
You’ll learn to
- Define a galaxy and identify the material — stars, gas, and dust — that gravity binds together in one.
- Distinguish spiral, barred spiral, elliptical, and irregular galaxies, and attribute the classification scheme to Edwin Hubble.
- Explain how galaxies are organized into groups, clusters, and superclusters, naming the Local Group, the Virgo cluster, and the Laniakea supercluster.
- Apply the concept of redshift to interpret what a galaxy's spectrum reveals about its motion and distance.
- Analyze how studying a galaxy's light, including brilliant active galactic nuclei, reveals what powers its center.
Common mistakes
Every galaxy is a spiral like the Milky Way.
Spirals are one of three broad classes; ellipticals are smooth ellipsoids of old stars, and irregulars are smaller, shapeless galaxies — all of them common.
Redshift means the galaxy looks red.
Redshift refers to spectral lines shifted toward longer wavelengths, measured with a spectrograph; it signals motion away from us, not the galaxy's color.
Superclusters are the biggest gravitationally bound structures.
Groups and clusters are gravitationally bound; superclusters are arrangements of them that are not bound together by gravity.
A galaxy's brilliant center is powered by nuclear fusion like a star's core.
Active galactic nuclei are powered by matter spiraling into a supermassive black hole, a process far more efficient than fusion.
Galaxies are scattered evenly through space.
They gather into groups, clusters, and superclusters, with vast voids between the structures.
Easily confused
Spiral galaxy vs. Elliptical galaxy
Spirals have a disk and arms with young stars and ongoing star formation; ellipticals are smooth, armless ellipsoids of old stars with little gas.
Barred spiral vs. Unbarred spiral
A barred spiral has a bar of stars through its center and arms beginning at the bar's ends; an unbarred spiral's arms wind straight out from the central region.
Galaxy group vs. Galaxy cluster
A group holds roughly 100 or fewer galaxies; a cluster may contain thousands.
Redshift vs. Blueshift
Redshift is a shift of spectral lines toward longer wavelengths, meaning motion away from us; blueshift is the opposite shift, meaning motion toward us.
Key vocabulary
- Galaxy
- A system of stars, planets, and clouds of gas and dust, all held together by gravity.
- Spiral galaxy
- A galaxy with a central bulge, a flat disk, and spiral arms, containing a mix of young and old stars.
- Barred spiral
- A spiral galaxy with a boxy or peanut-shaped bar of stars through its center, from whose ends the spiral arms begin.
- Elliptical galaxy
- A smooth, ellipsoid-shaped galaxy made almost entirely of old stars, with no spiral arms.
- Irregular galaxy
- A galaxy without the regular shape of a spiral or elliptical, typically smaller and often undergoing intense star formation.
- Galaxy group
- A small collection of roughly 100 or fewer galaxies held together by their mutual gravity.
- Galaxy cluster
- A large structure that may contain thousands of galaxies held together by gravity.
- Supercluster
- A vast arrangement of galaxy groups and clusters that is not itself gravitationally bound.
- Redshift
- The shift of spectral lines toward longer wavelengths, seen when a galaxy is moving away from us.
- Active galactic nucleus
- The brilliantly bright center of an active galaxy, powered by matter falling toward a supermassive black hole.
Sources & references
- Galaxies (Universe) — NASA Science
- Astronomy 2e, 26.2 Types of Galaxies — OpenStax (Rice University)
- Astronomy 2e, 26.5 The Expanding Universe — OpenStax (Rice University)
- Astronomy 2e, 27.2 Supermassive Black Holes: What Quasars Really Are — OpenStax (Rice University)
EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.
Researched 2026-08-21
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