We live inside a galaxy we cannot step outside to see from afar. No spacecraft has travelled beyond the Milky Way to photograph it from above; its barred spiral shape has instead been pieced together from observations made inside it.
Other galaxies give us the outside view we cannot get of our own. Some have magnificent spiral arms, others are smooth and rounded, while still others appear twisted, stretched or barely organised at all. But appearance is only the beginning of the story. A galaxy is not simply an enormous collection of stars. Much of what shapes it cannot even be seen, and galaxies themselves grow and change over billions of years. Entire galaxies can even collide without most of their stars ever touching.
So what exactly are these enormous systems, and how did the universe come to be filled with them?
What Makes a Galaxy a Galaxy?
A photograph of a galaxy can give the impression of an enormous crowd of stars packed together in space. In reality, those stars are usually separated by immense distances. Even our nearest neighbouring star system, Alpha Centauri, is more than four light-years away.
Now imagine that kind of separation repeated across billions of stars. The Milky Way stretches roughly 100,000 light-years from one side to the other, yet all those widely scattered stars remain part of the same enormous system.
What makes that possible is gravity. A galaxy is a vast gravitationally bound system in which stars and other material move under their combined gravitational influence.
Its sheer scale also separates a galaxy from a star cluster. A cluster may contain thousands or even millions of gravitationally bound stars, but it is a much smaller structure that exists within a galaxy. Galaxies also contain much more than stars alone. The Milky Way, for example, contains many star clusters as just one part of its much larger structure.
So a galaxy is not simply a bigger and bigger crowd of stars. It is an enormous system held together across almost unimaginable distances — and stars are only part of what it contains. To discover the rest, we need to look beyond the starlight.
What’s a Galaxy Made Of?
Stars may dominate our view of a galaxy, but they are only part of what is there. The apparently empty spaces between them contain gas and tiny grains of dust known collectively as the interstellar medium.
This material matters because galaxies do not simply contain stars — they continually make them. In cold, dense clouds of gas and dust, gravity can bring material together until new stars begin to form. As stars age and die, they return some of their material to space, enriching the gas from which future stars and planets can form.
The contents of a galaxy are therefore constantly being recycled. Nebulae, stars and planets are all part of this much larger cycle of matter.
But even if we account for all the stars, gas and dust we can observe, something doesn’t add up.
Astronomers can measure how stars and gas move through galaxies, and the gravity produced by visible matter alone cannot explain those motions. Galaxies behave as though they contain far more mass than we can see.
We call that unseen material dark matter. It does not shine or reflect light, so astronomers detect its presence through its gravitational effects. In galaxies such as the Milky Way, it appears to extend far beyond the bright regions visible in photographs.
This turns our first impression of a galaxy almost on its head. The stars provide the spectacle, but much of the mass shaping the galaxy is invisible. And all this material — visible and invisible — is not spread around at random. It gives galaxies an internal structure that we can begin to map.
Inside a Galaxy: How the Milky Way Is Structured
Galaxies arrange their stars and other material in very different ways, so there is no single blueprint for what lies inside one. Our own Milky Way provides a useful place to start because it contains many of the structures found in spiral galaxies.
If we could travel far enough beyond the Milky Way to look back, those structures would become much easier to see. The most familiar part would be the broad, flattened disc. This contains much of the Milky Way’s gas and dust, along with huge numbers of stars. Patterns within the disc form the spiral arms, including the Orion Arm region where our Solar System is located.
Toward the middle, stars become more densely concentrated in the central bulge. Running through this region is an elongated bar of stars, which is why the Milky Way is classified as a barred spiral galaxy.
But the bright disc and bulge are not the galaxy’s outer limits. Surrounding them is a much larger halo, containing old stars and globular clusters. Extending farther still is the dark-matter halo whose gravity we encountered in the previous section.
At the very centre is Sagittarius A*, a supermassive black hole about four million times the mass of the Sun. It sounds powerful enough to control the entire Milky Way, but it doesn’t. Its gravity dominates the region around the Galactic Centre; the galaxy as a whole is held together by the combined gravity of all its matter.
So even the Milky Way is much larger and more structured than the bright spiral we usually picture. And this is only one way of arranging a galaxy. Elsewhere in the universe, the same basic ingredients have produced galaxies that look dramatically different.
The Main Types of Galaxies
Look across the universe and galaxies come in a remarkable variety of shapes. Some have graceful spiral arms, others resemble enormous glowing balls, while some appear so disordered that they barely seem to have a shape at all.
Astronomers use these differences to classify galaxies into several broad types of galaxies. But the shapes are more than convenient labels. They can also reveal differences in a galaxy’s stars, gas and history.
Spiral galaxies
Spiral galaxies have broad, flattened discs crossed by curving spiral arms. These arms often contain abundant gas and dust, together with bright young stars and regions where new stars are forming.
Some spirals also have a long bar of stars running through their centre. These are known as barred spiral galaxies, and our own Milky Way is one example.
The arms themselves are not rigid structures made from the same stars forever. Instead, they are patterns of enhanced density through which stars, gas and dust move as they orbit the galaxy.
Elliptical galaxies
Elliptical galaxies replace the familiar spiral pattern with a much smoother appearance. They range from almost spherical systems to elongated oval shapes and can include some of the largest galaxies known.
They generally contain less cold gas and dust than spirals and are dominated by older stars. With less raw material available for making stars, many ellipticals have relatively little new star formation.
A famous example is M87, a giant elliptical galaxy in the Virgo Cluster. Its centre contains the supermassive black hole that became the first ever to be directly imaged as a black-hole shadow.
Lenticular galaxies
Lenticular galaxies have a flattened disc and central bulge, rather like spiral galaxies, but lack prominent spiral arms.
The Sombrero Galaxy (M104) is often described as a lenticular galaxy, although its precise classification is more complicated than its familiar appearance might suggest. Its brilliant central region and prominent dark dust lane make it one of the most distinctive nearby galaxies. That ambiguity is useful in itself: galaxy classification does not always divide the universe into perfectly neat boxes.
Irregular galaxies
Irregular galaxies lack an obvious spiral or elliptical structure. Some are small and naturally irregular, while others have been pulled and distorted through encounters with neighbouring galaxies. Many contain substantial amounts of gas and active regions of star formation.
The Large Magellanic Cloud, one of the Milky Way’s nearest galactic neighbours, is a well-known example. Unlike most galaxies, it is also close and bright enough to be clearly visible to the naked eye from suitable southern skies.
The important point is that these types are not fixed identities that galaxies receive when they are born. A galaxy can use up or lose gas, form new stars, be disturbed by its neighbours or merge with another galaxy altogether.
The shapes we see today are therefore not just different designs. They contain clues to what a galaxy has experienced. To understand why galaxies ended up looking so different, we need to turn the clock back and ask how they formed in the first place.
How Galaxies Form and Evolve
The first galaxies did not appear as finished spirals and ellipticals. To understand where they came from, we have to rewind almost to the beginning of the universe.
After the Big Bang, matter was spread remarkably evenly through space, but not perfectly so. Slightly denser regions grew under gravity, with dark matter helping to provide the framework in which gas could gather. As that gas collected and cooled, the first generations of stars formed and early galaxies began to take shape.
Galaxy building did not stop there. Galaxies continued to attract gas and form new stars, while smaller galaxies could combine into larger ones. Encounters and mergers reshaped them further.
A galaxy is therefore less like a finished object and more like something built and remodelled over billions of years. The spiral, elliptical or irregular galaxy we see today is only its present state.
Remarkably, we can see some of this history for ourselves. Because light takes time to reach us, looking billions of light-years into space also means looking billions of years into the past. The James Webb Space Telescope is now revealing galaxies from within the first few hundred million years of cosmic history. One striking example, nicknamed the Firefly Sparkle, is seen as it existed about 600 million years after the Big Bang. Rather than resembling a finished modern galaxy, it is caught in the process of assembling and contains several distinct clusters where stars are forming.
Looking deep into space therefore allows astronomers to watch different chapters of galaxy history. But that history is not shaped by what happens inside a galaxy alone. Galaxies have neighbours — and those neighbours belong to structures far larger than any individual galaxy.
Where Galaxies Live
The Milky Way may be enormous, but it does not live alone. Our galactic neighbourhood contains the Andromeda Galaxy, the Triangulum Galaxy and dozens of smaller galaxies. Together with the Milky Way, they form the Local Group.
Now pull the view farther back. Across the universe, galaxies gather into groups like ours and into much larger galaxy clusters, some containing hundreds or even thousands of galaxies.
Despite the similar names, a galaxy cluster is very different from the star clusters we met earlier. A star cluster is a group of stars within a galaxy; a galaxy cluster is a collection of entire galaxies.
Zoom out farther still and even these clusters become part of a much bigger picture. Galaxies are not scattered randomly through space. They trace enormous filaments and concentrations separated by vast, comparatively empty regions called voids. Together, this network forms the cosmic web.
The change in scale is extraordinary. Our Solar System belongs to the Milky Way, the Milky Way belongs to the Local Group, and our galactic neighbourhood is itself part of a universe structured on scales vastly larger still. But galaxies are not fixed points in this web. Gravity keeps them moving, and when neighbouring galaxies pass close enough, the results can transform them both.
When Galaxies Collide
When two galaxies containing billions of stars collide, you might expect countless stars to smash into one another. In reality, almost all of them miss.
The reason takes us back to something we discovered earlier: a galaxy is mostly empty space. Its stars are separated by such enormous distances that two populations of stars can pass through each other with very few direct collisions.
The galaxies themselves, however, do not escape unchanged. Their gravity can distort their shapes, pull out enormous streams of stars and create long tidal tails stretching into space.
Gas clouds can also collide and become compressed, triggering bursts of new star formation. If the galaxies remain gravitationally bound, they may swing past each other repeatedly before eventually merging into a single larger galaxy.
A spectacular example is the Antennae Galaxies (NGC 4038 and NGC 4039), two spiral galaxies caught in a prolonged merger. Their mutual gravity has produced enormous tails of stars, gas and dust, while compressed gas within the interacting galaxies has triggered widespread new star formation.
This helps explain some of the strange and irregular galaxies we encountered earlier. What looks like a bizarre shape may actually be a galaxy caught during — or recovering from — an encounter with another. Our own Milky Way and Andromeda are also moving towards one another. Once expected to collide almost inevitably, the latest Hubble and Gaia measurements suggest only about a 50–50 chance of a collision within the next 10 billion years.
Galaxy collisions bring several parts of our journey together: the enormous spaces between stars explain why the stars usually survive, the gas between them explains why new stars can suddenly form, and the collision itself shows how dramatically a galaxy can change over time.
Such encounters have been happening throughout cosmic history. And when we pull back far enough to consider the sheer number of galaxies in the observable universe, even these immense events become part of a much larger picture.
How Many Galaxies Are Out There?
The numbers are staggering. The observable universe contains at least 100 billion galaxies, and estimates that attempt to include extremely faint galaxies beyond our current detection limits have reached into the trillions.
One of the most striking demonstrations came from the Hubble Deep Field. In 1995, Hubble stared for 10 days at a seemingly empty patch of sky only about 1/13 the diameter of the full Moon. The resulting image revealed roughly 3,000 galaxies, many so distant that their light had been travelling towards us for billions of years.
That tiny window into the universe also helps explain why there is no simple final count. As telescopes become sensitive enough to detect fainter and more distant objects, they reveal galaxies that earlier surveys missed. Astronomers can count what we can see and estimate populations below our detection limits, but there is no complete cosmic census.
There is another limit too. We can only count galaxies within the observable universe — the region from which light has had enough time to reach us. So the important takeaway is the scale rather than one supposedly exact number: at least 100 billion galaxies, and potentially far more.
What looks from Earth like mostly dark, empty space is anything but empty. Yet among that enormous population, a few galaxies are close and bright enough for us to see for ourselves.
Galaxies You Can See from Earth
The easiest galaxy to see is the one we are standing inside. From a dark location away from artificial light, the Milky Way appears as a pale, irregular band stretching across the night sky. We are looking through the crowded disc of our own galaxy from within it.
But you can also look beyond the Milky Way with your own eyes. From northern skies, the best place to start is the Andromeda Galaxy (M31). Under a clear, dark sky it appears as a small, misty patch in the constellation Andromeda, while binoculars make its elongated shape and bright central region easier to see.
That faint patch is only the most obvious part of something enormous. Andromeda’s full extent spans about six times the diameter of the full Moon, although our unaided eyes normally reveal only its much brighter central region.
Andromeda lies roughly 2.5 million light-years away. The light entering your eyes therefore began its journey about 2.5 million years ago, long before modern humans existed. Looking at Andromeda means looking not only across an extraordinary distance, but millions of years into the past.
Observers under southern skies have two other remarkable naked-eye targets. The Large and Small Magellanic Clouds, neighbouring galaxies of the Milky Way, appear as detached cloudy patches in the night sky.
Binoculars and telescopes bring many more galaxies within reach, but don’t expect them to resemble the brilliant, colourful objects seen in astronomical photographs. Most appear as subtle grey glows, fuzzy patches or faint elongated shapes. Looking slightly to one side of a particularly faint object — a technique called averted vision — can sometimes make it easier to see.
Yet that faintness is part of what makes observing a galaxy so remarkable. The tiny glow in the eyepiece may not look spectacular, but you are seeing the combined light of stars in an entirely different galaxy.
We began this journey standing inside a galaxy we cannot see from afar. Along the way, we discovered that galaxies are enormous gravitational systems containing visible and invisible matter, built and remodelled over billions of years and sometimes transformed through encounters with one another.
And now we can finish where we started: standing inside the Milky Way and looking out. That faint patch of Andromeda is not a cloud or a distant nebula. You are standing inside one galaxy and looking directly at another.
