What are Galaxies?

The Anatomy of a Galaxy

A galaxy is a colossal, gravitationally bound system of stars, planets, the remnants of dead stars, and vast clouds of gas and dust. These immense structures are the fundamental building blocks of the universe, cosmic islands scattered across the vast ocean of space. They vary enormously in size and content. The smallest, known as dwarf galaxies, might contain only a few thousand or million stars and span just a few hundred light-years across. In contrast, the largest can host hundreds of trillions of stars and stretch for more than a million light-years in diameter. Their mass, often measured in units equivalent to the mass of our Sun (solar masses), can range from a few million to several hundred trillion times that of our star. When we gaze at the night sky, every star we see with the naked eye is a resident of our own galaxy, the Milky Way.

While the luminous glow of stars, gas, and dust defines a galaxy’s visible appearance, the vast majority of its substance is completely invisible. A galaxy is not merely what we see; it is fundamentally a gravitational structure, and its most massive components are hidden from view. The first of these is dark matter, an enigmatic substance that does not emit, reflect, or absorb light, making it undetectable by any conventional telescope. Its presence is inferred only through its gravitational effects on the matter we can see. Astronomers observe that stars in the outer regions of galaxies orbit the galactic center far faster than they should if only the visible matter were providing the gravitational pull. This discrepancy suggests that the visible part of a galaxy is embedded within a much larger, spherical “halo” of dark matter, which may account for up to 85% of the universe’s total mass. This unseen halo provides the gravitational scaffolding that holds the entire galaxy together.

At the very heart of this gravitational structure lies another unseen giant. Observational evidence indicates that nearly every large galaxy, including our own, hosts a supermassive black hole at its center. These are not the more common stellar-mass black holes formed from the collapse of a single star; they are behemoths with masses ranging from millions to tens of billions of times that of the Sun. This central engine plays a role in the life of its host galaxy, governing the dynamics of the core and, when active, capable of unleashing enough energy to influence the entire system. The visible galaxy, in all its splendor, is essentially luminous material that has settled within the deep gravitational well created by its dark matter halo and its central supermassive black hole.

A Universe of Shapes: Classifying the Galaxies

In the 1920s and 1930s, as the existence of these “island universes” beyond our own was confirmed, astronomer Edwin Hubble developed a system to organize them based on their visual appearance, or morphology. This method, known as the Hubble Sequence, remains the foundational classification scheme used today. Often depicted in a Y-shape, it is colloquially called the “Hubble tuning fork” diagram and divides galaxies into three broad classes: ellipticals, spirals, and irregulars, with a transitional type known as lenticulars.

• Elliptical Galaxies (E) appear as smooth, featureless spheres or ovals of light. They are classified on a scale from E0 for a nearly perfect circle to E7 for a highly elongated, flattened ellipse.
• Spiral Galaxies (S) are perhaps the most iconic type, characterized by a bright central bulge, a flattened rotating disk, and graceful spiral arms winding out from the center. They are sub-classified from Sa to Sc based on how tightly wound their arms are and the relative size of their central bulge.
• Barred Spiral Galaxies (SB) are a major subclass of spirals. Instead of arms emerging directly from the bulge, they begin at the ends of a straight, bar-like structure of stars that cuts through the galactic center. They follow a similar classification from SBa to SBc.
• Lenticular Galaxies (S0) are an intermediate form. They possess a central bulge and a disk like a spiral, but they have no visible spiral arms, giving them a smooth, lens-like appearance.
• Irregular Galaxies (Irr) are the catch-all category for galaxies that lack any distinct, symmetrical shape. They often appear chaotic and disorganized.

A common misconception arises from the terminology Hubble used, referring to ellipticals as “early-type” and spirals as “late-type” galaxies. This language implies an evolutionary sequence, suggesting that round ellipticals might flatten and grow arms over time to become spirals. However, extensive observation has shown this is incorrect. Hubble himself was clear that no such evolutionary path was intended; his system is purely descriptive, a morphological map of the galaxies we see today. The true story of galactic evolution is far more dynamic and, in many cases, flows in the opposite direction. Rather than being the starting point, the featureless elliptical galaxies are often the end product of cosmic evolution.

The Spiral Galaxies: Cosmic Pinwheels

Spiral galaxies are complex, dynamic systems with a distinct and elegant structure. They are composed of three main visible parts: a central bulge, a disk, and a halo, each with its own characteristics.

The disk is a flattened, rotating plane of stars, gas, and dust. The stars within the disk move in organized, nearly circular orbits around the galactic center, all traveling in roughly the same direction, much like planets orbiting a star. The disk is where the most dramatic action takes place.

At its heart lies the central bulge, a dense, spheroidal concentration of stars. This region is typically dominated by older, redder stars, and the stellar orbits here are more random and less organized than in the disk.

Surrounding the entire visible structure is the halo, a vast, nearly spherical region that is sparsely populated with very old stars and massive, ancient star clusters known as globular clusters. The halo is also where the majority of the galaxy’s invisible dark matter is thought to reside, forming a gravitational anchor for the entire system.

The most striking features of these galaxies are their spiral arms. These arms are not solid, rigid structures like the spokes of a wheel. If they were, the galaxy’s differential rotation—where the inner parts spin faster than the outer parts—would cause them to wind up and disappear in a relatively short amount of cosmic time, a dilemma known as the “winding problem”. Instead, the arms are understood to be density waves: slow-moving patterns of compression that sweep through the galactic disk. As stars and gas clouds orbit the galaxy, they pass through these high-density waves. Inside the wave, the material is squeezed together, much like cars bunching up in a traffic jam on a highway. The stars themselves continue their orbits, moving into and out of the arms, but the wave pattern persists. This compression of interstellar gas is what makes the arms so prominent. It triggers the collapse of molecular clouds, leading to furious bursts of new star formation. These newborn stars include massive, hot, brilliant blue stars that are extremely luminous but have very short lifespans. It is the light from these young stars that makes the spiral arms shine so brightly and gives them their characteristic blue-white color. The reddish-yellow bulge, by contrast, is populated by older stars, its hot blue stars having long since died out.

Spiral galaxies come in two main varieties. In normal spirals (S), the arms appear to unwind directly from the central bulge. In barred spirals (SB), a straight bar of stars, gas, and dust extends across the center, and the spiral arms begin from the ends of this bar. This is not a rare configuration; about two-thirds of all spiral galaxies, including our own Milky Way, are believed to be barred. The formation of a bar is thought to be a natural part of a spiral galaxy’s evolution, possibly indicating that it has reached a state of maturity.

Our own galactic home, the Milky Way, is a large barred spiral galaxy containing an estimated 100 to 400 billion stars. Its visible disk spans about 100,000 light-years in diameter but is remarkably thin, only about 1,000 light-years thick. Our Solar System does not occupy a privileged position. We are located about 27,000 light-years from the bustling galactic center, roughly halfway to the edge. We reside within a minor, partial arm known as the Orion Spur (or Orion-Cygnus Arm), situated between the larger Sagittarius and Perseus arms. From this vantage point, our Solar System travels at a speed of over 500,000 miles per hour, yet it still takes us about 230 million years to complete a single orbit around the Galactic Center—a period sometimes called a “cosmic year”.

The Elliptical Galaxies: Ancient Giants

In stark contrast to the structured beauty of spirals, elliptical galaxies present a smooth, almost uniform appearance. They are shaped like spheres or elongated ovals (ellipsoids) and are largely devoid of the features that define spirals, such as a disk or arms. The stars within them do not follow the ordered, circular paths seen in a spiral’s disk. Instead, they move on random, highly inclined orbits, swarming around the galactic center like bees around a hive.

These galaxies are often considered to be in a state of cosmic retirement. They are dominated by old, low-mass stars that glow with a reddish or yellowish hue. They contain very little of the cold gas and dust that serve as the raw material for making new stars. As a result, active star formation has largely ceased, leading astronomers to colloquially refer to them as “red and dead” galaxies. They represent a final, quiescent state of galactic evolution.

While they can come in dwarf sizes, elliptical galaxies also include the most massive galaxies known in the universe. These giant ellipticals are often found dominating the gravitational centers of large galaxy clusters, where the density of galaxies is highest.

The placid, uniform appearance of an elliptical galaxy is ly deceptive. It is not a sign of a simple or uneventful history. On the contrary, its key characteristics are the “fossil” evidence of a past filled with unimaginable violence. The leading theory for their formation is that they are the end products of a major merger between two or more large spiral galaxies. When spiral galaxies collide, their immense gravitational forces wreak havoc. The orderly, rotating disks are completely disrupted, and the stars are thrown onto new, randomized orbits—a process called “violent relaxation”. This is precisely what creates the disordered swarm of stellar orbits seen in ellipticals today. Furthermore, the collision compresses the vast reserves of gas from the parent spirals, triggering a final, spectacular burst of star formation that rapidly consumes nearly all the available fuel. Once this gas is used up, star formation is quenched, leaving behind a galaxy with no fuel for renewal. The “boring,” featureless ball of old, red stars is the settled scar of one of the most chaotic events the cosmos can produce.

The In-Between Systems: Lenticular and Irregular Galaxies

Beyond the two main classes of spirals and ellipticals lie galaxies that are either transitional forms or products of gravitational disruption. These are the lenticular and irregular galaxies.

Lenticular Galaxies: The Bridge Between Types

Lenticular galaxies, designated S0, occupy a fascinating middle ground in the Hubble classification scheme, appearing as a hybrid between an elliptical and a spiral. Like a spiral, a lenticular galaxy possesses a distinct central bulge and a flattened, rotating disk. However, this disk is completely featureless, lacking the signature spiral arms. Like an elliptical, it is composed mainly of older, redder stars and shows very little evidence of ongoing star formation, having exhausted most of its interstellar gas.

The origin of lenticular galaxies is still a subject of study. One possibility is that they are former spiral galaxies that have “faded” over time. In this scenario, a spiral galaxy might have used up all its gas for star formation, causing its bright blue arms to disappear, leaving behind only the bulge and a featureless disk. This process could be accelerated in dense environments like galaxy clusters, where interactions with other galaxies can strip away gas. Another theory suggests they can be formed by certain types of galaxy mergers that result in a disk-like remnant.

Irregular Galaxies: The Chaotic Members

Irregular galaxies are exactly what their name implies: they lack any regular or symmetrical structure. They are often chaotic in appearance, without a defined bulge or spiral arms. This jumbled morphology is not usually an intrinsic property but rather the result of external forces. The distorted shapes of irregular galaxies are frequently caused by gravitational interactions, close encounters, or outright collisions with neighboring galaxies.

This “irregularity” is often a temporary state, not a permanent identity. An irregular galaxy can be thought of as a snapshot of a regular galaxy, like a spiral, in the midst of a traumatic gravitational event. A prime example is the Large and Small Magellanic Clouds, two of the Milky Way’s most famous satellite galaxies. They are classified as irregulars, but they show evidence of a faint bar and spiral structure, suggesting they were once more organized dwarf spiral galaxies that have been distorted and disrupted by the powerful gravity of our much larger Milky Way.

Unlike the “retired” elliptical and lenticular galaxies, irregulars are often sites of intense activity. The gravitational disruptions that create their chaotic shapes can also compress their gas clouds, leading to vigorous, high rates of star formation. This makes them shine brightly with the blue light of many young, hot stars.

The Galactic Life Cycle: Formation and Evolution

Galaxies are not static entities; they are dynamic systems that form, grow, and change over billions of years. Our modern understanding of this process suggests that the universe builds its structures from the ground up, in a process of hierarchical assembly.

From the Beginning: The Bottom-Up Model

In the moments after the Big Bang, the universe was an almost perfectly smooth, hot, dense soup of particles and energy. “Almost” is the key word. Tiny, random quantum fluctuations created regions with infinitesimally higher density. Over millions of years, as the universe expanded and cooled, gravity began to work on these seeds. The slightly denser regions exerted a slightly stronger gravitational pull, attracting more and more matter—particularly the abundant, invisible dark matter. This gravitational amplification caused the initial dark matter to collapse into small, dense “halos.” These halos then acted as gravitational wells, pulling in the ordinary matter (hydrogen and helium gas) from their surroundings.

These first small structures—proto-galaxies or dwarf galaxies—were the building blocks. The prevailing theory of galaxy formation, known as the “bottom-up” or “hierarchical” model, posits that these small initial clumps repeatedly merged over cosmic time to form the larger galaxies we see today. This same principle of smaller things merging to make bigger things appears to be a universal rule of cosmic construction. Just as gas clumps merge to form stars, and stars group into clusters, these primordial galaxies merged to form giants like the Milky Way, which in turn gathered into the galaxy clusters and superclusters that define the universe’s large-scale structure. This reveals a single, elegant mechanism—gravitational attraction and merging—governing the formation of structure across all cosmic scales.

The Engine of Change: Galactic Mergers

Collisions and mergers are not rare, incidental events; they are a fundamental driver of how galaxies evolve. When two galaxies draw close, their mutual gravity can lock them into an intricate dance that can last for hundreds of millions of years, eventually leading to their unification. These interactions are broadly categorized by the relative size of the participants.

A major merger occurs when two galaxies of roughly similar mass collide. These are transformative events. As described earlier, the collision of two spiral galaxies can completely destroy their disks, randomize their stars’ orbits, and create a new, larger elliptical galaxy. The Antennae Galaxies and the Mice Galaxies are spectacular, well-documented examples of major mergers currently in progress, their shapes distorted and drawn out into long tidal tails of stars and gas.

When a much larger galaxy collides with a smaller one, the process is known as a minor merger or, more graphically, galactic cannibalism. The larger galaxy’s structure is mostly preserved, but it strips stars and gas from its smaller victim, absorbing them and growing more massive in the process. Our own Milky Way is an active cannibal, currently in the process of stripping material from several dwarf satellite galaxies, including the Sagittarius Dwarf Galaxy and the Magellanic Clouds.

A key consequence of mergers, both major and minor, is the triggering of starbursts. The collision violently compresses the vast clouds of interstellar gas within the galaxies, sparking a massive wave of star formation at rates that can be 100 times higher than normal. This intense burst of activity quickly uses up the available gas, often playing a key role in the transformation of gas-rich spirals into gas-poor ellipticals.

The Stellar Cycle: Fueling the Galaxy

Within all star-forming galaxies, a continuous cycle of birth, life, and death unfolds. Stars are born inside vast, cold, dense molecular clouds, sometimes called stellar nurseries. Gravity causes dense knots within these clouds to collapse under their own weight, heating up to form a protostar—a stellar embryo. When the core temperature and pressure become extreme enough, nuclear fusion ignites. In this process, hydrogen atoms are fused together to create helium, releasing an immense amount of energy. This outward pressure from fusion balances the inward pull of gravity, and a stable, main-sequence star is born.

The star will spend most of its life in this stable phase. However, when a massive star exhausts its fuel, it dies in a cataclysmic explosion known as a supernova. This explosion blasts the heavy elements—carbon, oxygen, iron, and more—that were forged in the star’s core back out into the interstellar medium. This material enriches the surrounding gas clouds, which will then collapse to form the next generation of stars and planets. Our own Sun and Earth are made from elements created in the hearts of long-dead stars. This is the grand cycle of cosmic recycling, where the death of one generation of stars provides the raw material for the next.

The Heart of the Matter: Supermassive Black Holes and Active Galaxies

At the gravitational center of nearly every large galaxy lurks a supermassive black hole (SMBH), a silent giant whose influence can shape the fate of its entire host galaxy. The galactic center is the bottom of a deep gravity well, and just as water flows downhill, massive objects in the galaxy tend to sink toward this point over cosmic time, making it the natural home for an SMBH. The origin of these behemoths is still an active area of research. They may have started as “seed” black holes—remnants of the very first generation of massive stars—that grew over billions of years by accreting matter and merging with other black holes. Another possibility is that they formed from the direct collapse of immense gas clouds in the dense environment of the early universe.

For most of its life, an SMBH is dormant and invisible. However, when a supply of fuel—such as a passing gas cloud or an unlucky star—is captured by its gravity, the black hole can “turn on.” The material doesn’t fall straight in but instead forms a swirling, flattened structure called an accretion disk. Intense friction and gravitational forces within this disk heat the material to millions of degrees, causing it to shine with incredible brilliance across the entire electromagnetic spectrum, from radio waves to X-rays. This ferociously luminous region is known as an Active Galactic Nucleus, or AGN. AGN are some of the most energetic and consistently luminous objects in the cosmos, capable of outshining the combined light of all the billions of stars in their host galaxy.

The bewildering variety of AGN observed by astronomers—with names like quasars, blazars, and Seyfert galaxies—can seem confusing, but a “unified model” brings a simplifying order to this cosmic zoo. Many of the differences between these objects are not due to fundamentally different engines but are simply an accident of our viewing perspective. The central engine is thought to be the same: an SMBH, its accretion disk, and often a surrounding dusty, doughnut-shaped structure called a torus. In some cases, powerful jets of particles are launched at near-light speed from the poles of the black hole.

• Quasars are the most powerful and distant AGN. We see them as they were in the early universe, when galaxies were rich in gas to fuel their central black holes. Their light is so intense it often washes out the host galaxy.
• Blazars are a special type of quasar where one of the relativistic jets happens to be pointed almost directly at Earth. This orientation dramatically amplifies its brightness, making it appear extremely luminous and variable.
• Seyfert Galaxies are lower-luminosity AGN found in nearby spiral galaxies. If our line of sight to the center is clear, we see the bright accretion disk and classify it as a Seyfert 1. If the dusty torus blocks our view of the disk, we only see light from regions farther out and classify it as a Seyfert 2. The different classifications are often just the same object viewed from different angles.

The energy unleashed by an AGN can have a dramatic effect on its host galaxy. Powerful winds and jets blasting out from the nucleus can heat up or even expel the cold gas throughout the galaxy. Since cold gas is the fuel for star formation, this process, known as galactic feedback, can effectively shut down a galaxy’s ability to create new stars, thus regulating its growth. This creates a delicate balance: the galaxy feeds its black hole, and the black hole, in turn, controls the galaxy’s evolution.

The Grand Design: The Cosmic Web

Zooming out from individual galaxies reveals that they are not scattered randomly throughout space. Instead, they are organized into a vast, intricate structure of staggering scale, known as the cosmic web. This web is the fundamental architecture of the universe.

Galaxies are social creatures, bound by gravity into congregations of increasing size. Small collections of up to a hundred or so galaxies are called galaxy groups. Our own Milky Way is a member of the Local Group, which includes the Andromeda galaxy and more than 50 smaller dwarf galaxies. Larger gatherings, containing hundreds or even thousands of galaxies immersed in vast clouds of hot gas and dark matter, are known as galaxy clusters. The nearby Coma Cluster is a dense metropolis of over 1,000 galaxies.

These groups and clusters are themselves gathered into even larger assemblies called superclusters, which can stretch for hundreds of millions of light-years. The Local Group is located on the outskirts of the Virgo Supercluster, which in turn is just one lobe of the immense Laniakea Supercluster.

On the grandest scales, this hierarchy of structures is woven into a breathtaking pattern. The superclusters are not isolated blobs but are connected in long, twisting filaments and flattened sheets, forming the threads of the cosmic web. Most of the matter in the universe resides in these filaments. Where filaments intersect, they form the densest nodes, which are the great galaxy clusters. In between these structures lie the enormous, astonishingly empty regions known as voids, which contain very few galaxies. The universe, therefore, resembles a three-dimensional spider’s web or a sponge, with matter concentrated along the web’s strands and vast empty spaces in between.

This cosmic web is not just a curious arrangement; it is the skeleton of the universe and a direct relic of the earliest moments of time. The web’s structure is a large-scale map of the tiny density fluctuations present in the primordial universe just after the Big Bang. Over 13.8 billion years, gravity has amplified these initial variations, pulling matter from the less dense regions (which became the voids) into the denser regions (which became the filaments). The visible galaxies we see are like bright lights strung along the invisible scaffolding of dark matter that forms the true backbone of this web. By studying the geometry and distribution of galaxies within the web, astronomers can probe the initial conditions of the universe, map the distribution of dark matter, and measure the influence of dark energy on the cosmos’s expansion. The grandest structure we can observe is a direct link to the universe’s origin and its most mysteries.

Observing the Cosmos: The Tools of Discovery

Our knowledge of galaxies is a testament to the power of telescopes to act as time machines and to grant us senses far beyond our own. The primary tool of an astronomer is light, and by studying the full range of the electromagnetic spectrum, we can piece together a complete picture of these distant systems.

A galaxy that appears calm and ordinary in visible light might be a hotbed of activity when viewed in other wavelengths. This is because different physical processes emit different kinds of light.

• Visible Light, the kind our eyes can see, reveals the general distribution of stars but is easily blocked by cosmic dust, hiding many features from view.
• Infrared Light has longer wavelengths that can penetrate these obscuring dust clouds. This allows astronomers to peer into the hearts of stellar nurseries to see newborn stars and to detect the warm glow of materials not hot enough to shine in visible light. Crucially, infrared is our window to the early universe. The light from the most distant galaxies has been traveling for so long that the expansion of the universe has stretched it from visible and ultraviolet light into infrared wavelengths, a phenomenon called cosmological redshift.
• Ultraviolet Light, with its shorter, higher-energy wavelengths, is emitted by the hottest, most massive, and youngest stars. Observing in UV allows astronomers to pinpoint which galaxies are actively forming stars and where within those galaxies the action is happening.
• X-ray and Radio Light reveal the most violent and energetic processes. X-rays can trace the multimillion-degree gas in galaxy clusters or the super-heated matter in the accretion disk just before it falls into a black hole. Radio waves are emitted by the immense jets of particles launched from active galactic nuclei, revealing structures that are completely invisible in other wavelengths and can be many times larger than the host galaxy itself.

By combining observations from different telescopes, each specializing in a different wavelength, astronomers can assemble a complete, multi-layered view. A composite image showing a galaxy in visible, infrared, and radio light is not just a colorful picture; it is a rich data map revealing the interplay between old stars, new star formation, and high-energy phenomena simultaneously.

The cosmological redshift is another indispensable tool. As the universe expands, it stretches the fabric of space itself. Light waves traveling through this expanding space are also stretched, shifting them toward the red end of the spectrum. Edwin Hubble discovered that the farther away a galaxy is, the greater its redshift. This relationship allows astronomers to determine a galaxy’s distance. Because light travels at a finite speed, looking at a galaxy with a large redshift means we are not seeing it as it is today, but as it was billions of years ago when the light began its journey. This effect turns telescopes into time machines, enabling us to observe the evolution of galaxies across cosmic history, from the chaotic, irregular forms of the early universe to the grand spirals and giant ellipticals of the present day.

Summary

Galaxies are the great cities of the cosmos, vast islands of stars, gas, dust, and planets held together by the immense and unseen grip of dark matter. They are not uniform but come in a rich variety of forms, from the elegant, star-forming disks of spiral galaxies to the placid, ancient swarms of elliptical galaxies, with a spectrum of transitional and disrupted systems in between. Our own Milky Way is a large barred spiral, a typical resident of the modern universe.

These structures are not static but are constantly evolving. They are built from the bottom up, through a hierarchical process of mergers where smaller galaxies combine over billions of years to create larger ones. These cosmic collisions are the primary engine of change, capable of transforming a galaxy’s shape, triggering furious bursts of star formation, and feeding the supermassive black hole that lurks at its core. This central engine, when awakened, can become an active galactic nucleus—one of the most luminous objects in the universe—whose powerful feedback can regulate the growth of the entire galaxy.

On the grandest scale, galaxies themselves are but components of a larger architecture. They are arranged in groups, clusters, and superclusters that trace the filaments of the cosmic web, a vast, interconnected structure that forms the very skeleton of the universe. This web is a relic of the universe’s earliest moments, and its study provides a direct window into the fundamental properties of our cosmos. Through telescopes that can see across the entire spectrum of light and peer back in time, we have pieced together this grand narrative, a story of formation, interaction, and evolution on the most magnificent scale imaginable.