Under a truly dark sky, few sights rival the Milky Way. A hazy, silvery band stretches across the heavens, crowded with stars in some stretches and torn by dark rifts in others. It grows brighter and denser in places, then fades again, giving the impression of an immense, uneven structure reaching across the night sky.
It’s easy to look at that band and imagine something distant, like a vast bank of cloud far above us. But the Milky Way isn’t simply something out there — it’s where we already live. The Sun is just one of hundreds of billions of stars that make up this galaxy, and that river of light across the sky is our own view of home, seen from the inside.
To understand what we are really looking at, we need to explore the Galaxy that surrounds us.
What Are We Seeing When We Look at the Milky Way?
The hazy light of the Milky Way is actually the combined glow of vast numbers of distant stars in our own galaxy. Most are too distant or too faint to distinguish individually with the unaided eye, so together their light merges into the milky glow that gave the Milky Way its name.
The band does not look the same all the way around the sky. In some directions it is relatively narrow and faint; in others it becomes broader, brighter and much richer in stars. Toward Sagittarius and Scorpius, we see some of its richest star fields as we look toward the crowded inner regions of the Galaxy. Elsewhere, as our view follows the Milky Way toward the outer Galaxy, the band becomes much fainter and less conspicuous.
Dark lanes and patches interrupt some of the brighter stretches too, appearing almost like gaps where the stars suddenly thin out or disappear.
At any one time, Earth itself hides part of the Milky Way below the horizon. But the band has no true beginning or end: it forms a continuous circle around the sky, although we can never see the whole circle at once from the ground.
That band already contains clues about the much larger structure surrounding us. But it looks nothing like the enormous spiral galaxy shown in maps and illustrations.
How do the two fit together?
What Does Our Galaxy Really Look Like?
If we could somehow rise far above the Milky Way and look back, the band from our night sky would be transformed into something extraordinary: a vast, flattened galaxy, with enormous spiral arms winding around a bright, crowded centre.
This is what astronomers call a barred spiral galaxy. In a typical spiral galaxy, the great curving arms appear to wind outward from the central region. In a barred spiral like the Milky Way, a long, dense bar of stars crosses the centre, with the main spiral arms extending outward from around its two ends. The bar is not a solid structure, but a vast concentration of stars moving within the Galaxy.
Most of the Milky Way’s stars are spread across this broad disc, which is roughly 100,000 light-years across. From above, its spiral structure dominates the view. Turn the Galaxy on its side, however, and that huge spiral would collapse visually into a comparatively thin line, thickening around the crowded centre.
That change of viewpoint explains the band we started with. We live inside this thin Galactic disc rather than looking down on it from above. When we look along the disc, our view passes through far more stars than when we look above or below it, concentrating their combined light into the Milky Way across our sky.
And the scale is almost impossible to picture. Light travels about 186,000 miles (300,000 kilometres) every second, yet even light would take roughly 100,000 years to cross the Milky Way’s stellar disc from one side to the other.
Somewhere within that immense disc is the Sun, carrying Earth and the rest of the Solar System with it.
Where Is Earth in the Milky Way?
Earth travels around the Sun, but the Solar System itself is also moving through the Milky Way. We lie within the Galaxy’s disc, about 26,000 light-years from the Galactic Centre — a little over halfway from the centre toward the edge of the main stellar disc.
Pinpoint our location more closely and we find ourselves in a region of spiral structure called the Local Arm, also known as the Orion Arm. It lies between two larger spiral arms, named the Sagittarius Arm and Perseus Arm after the constellations in whose directions we see them from Earth.
Older maps often called our region the Orion Spur, treating it as a short branch between those larger arms. Measurements from ESA’s Gaia mission indicate that it is longer and more substantial than that older picture suggested.
So our location can be narrowed down step by step:
Milky Way → stellar disc → Local/Orion Arm → Solar System → Earth.
We now know where we are. But those sweeping spiral arms on our Galactic map are not solid structures or boundaries. What would we actually encounter if we travelled through one?
What Would We Find Across the Milky Way?
If we could leave our part of the Local Arm and travel across the Milky Way’s disc, we would not pass through neat alternating bands of “spiral arm” and empty space. Stars are found throughout the disc, including in the regions between the arms.
What changes is the Galactic landscape. As we approached a prominent spiral arm, we would encounter greater concentrations of gas and dust, including enormous cold clouds from which new stars can form. These clouds also explain the dark lanes we noticed in the Milky Way from Earth: interstellar dust blocks the light of more distant stars behind it, making some regions appear almost empty. Some of those newborn stars gather in clusters, while massive young stars light up and reshape nearby gas, producing the glowing nebulae that mark many active star-forming regions.
Spiral structure is especially rich in gas and dust, active star-forming regions and brilliant young stars, which helps the arms stand out so clearly in images of other galaxies.
Older stars, meanwhile, are spread much more widely across the Galactic disc rather than being confined to the arms.
The spiral arms are not rigid structures made from permanent groups of stars. Stars throughout the disc continue to orbit the Galactic Centre, while the spiral structure itself also changes over time. The arms are better thought of as large-scale patterns within the Galaxy’s disc than as fixed objects made from the same stars forever.
Seen this way, the spiral arms on our map stop looking like giant objects in their own right. They are regions within a continuous Galactic disc where particular kinds of material and stellar activity become especially conspicuous.
We now know where the Solar System sits and what those great spiral structures around us actually represent.
From our position in the Local Arm, let’s now head deeper into the Galaxy. As we travel inward across the disc, the surroundings begin to change. Stars become increasingly crowded, dust hides more of what lies ahead, and eventually we approach one of the most extreme environments in the Milky Way: the Galactic Centre.
What Happens as We Travel Toward the Galactic Centre?
The farther we move toward the Galaxy’s centre, the more crowded the star fields become. Enormous amounts of gas and dust also lie along our line of sight, hiding much of the inner Milky Way from view in visible light.
This is why the centre of our own Galaxy remained so difficult to explore. Astronomers had to look beyond visible light, particularly at infrared and radio wavelengths, which can reveal objects and structures hidden behind the intervening dust.
Beyond that obscuring material lies the Galactic Centre: a crowded region where stars are packed far more densely than in our part of the Galaxy and huge clouds of gas fill the space between them.
And at its very heart is something extraordinary.
Sagittarius A* is a supermassive black hole containing about four million times the mass of the Sun. It lies at the dynamical centre of the Milky Way — essentially the central point around which stars and other material in the Galaxy move. Sagittarius A* does not itself orbit some larger object at the Galactic Centre; it sits at the heart of that much larger central region.
This can be slightly confusing in images like the one above, because Sagittarius A* does not necessarily coincide with the brightest-looking part of the image. That surrounding light and emission comes from the extraordinary concentration of stars, gas and energetic material across the much larger Galactic Centre region.
We cannot see Sagittarius A* itself in the ordinary sense. Instead, astronomers have tracked individual stars orbiting extremely close to this unseen object. Their rapid, tightly curved orbits reveal an enormous mass concentrated into an extraordinarily small region, providing compelling evidence for the supermassive black hole.
We have now travelled from our relatively quiet position in the Local Arm to the crowded heart of the Milky Way. But the bright disc and its centre are not the whole Galaxy.
What happens if, instead of travelling through the disc, we leave it altogether?
What Lies Beyond the Milky Way’s Disc?
So far, almost our entire journey has taken place within the Milky Way’s broad, flattened disc. But the Galaxy does not end at the edge of that bright spiral structure.
Imagine now travelling above or below the disc. The dense star fields would gradually thin out until we entered the stellar halo, a vast, sparse region surrounding the Galaxy. Here, stars are much more widely scattered, and many belong to some of the oldest stellar populations in the Milky Way.
Scattered through this halo are globular clusters — enormous, tightly packed groups that can contain hundreds of thousands of stars. Many are extremely old, preserving stars that formed during the early history of our Galaxy.
But even these distant stars and clusters do not mark the full extent of the Milky Way. Surrounding the visible Galaxy is a much larger dark matter halo. We cannot see dark matter directly because it does not produce light, but astronomers can detect its gravitational influence on the motion of stars and other matter throughout the Galaxy.
Our exploration has now taken us across the Milky Way’s stellar disc, inward to the Galactic Centre and outward into the halo. But there is an obvious problem: no spacecraft has ever travelled outside the Galaxy and looked back at it.
So how can we possibly know what the Milky Way really looks like?
How Can We Map a Galaxy We’re Trapped Inside?
There is no photograph showing the entire Milky Way from above, below or outside it. Even the most distant spacecraft ever sent from Earth have travelled only a tiny fraction of the distance needed to escape the Galaxy and look back.
So the familiar face-on images of the Milky Way are scientific reconstructions or artists’ impressions, built from observations rather than photographs taken from outside.
Astronomers have to construct the map from within.
One of their most important tools is distance. If we can determine how far away stars are and where they lie in the sky, we can begin placing them within a three-dimensional map of the Galaxy. ESA’s Gaia mission has measured the positions and motions of nearly two billion stars, transforming our picture of the Milky Way and revealing structure that is impossible to see simply by looking across the night sky.
But stars alone cannot reveal everything. The same interstellar dust we encountered earlier hides large parts of the Galaxy at visible wavelengths. Infrared observations can penetrate some of that dust, while radio observations reveal gas and other structures across obscured regions of the Milky Way.
Young stars, star-forming regions and great clouds of gas provide further clues to the locations of spiral structure. By combining these different observations, astronomers gradually build up the Galactic map we have been exploring.
It is still not a finished map. Our position inside the Milky Way makes some structures particularly difficult to determine, which is why scientific maps do not always agree on the exact shape, number or prominence of every spiral arm.
In a strange way, living inside the Milky Way gives us an extraordinary close-up view while denying us the one view that would make everything easiest to understand: the view from outside.
And these same measurements reveal something else about the Galaxy we live in — something no single snapshot, from any angle, could show on its own: the entire Galaxy is moving.
Is the Milky Way Standing Still?
From Earth, the Milky Way can appear almost timeless, its band of stars seemingly fixed across the night sky. On Galactic timescales, however, almost everything we have explored is in motion.
The Milky Way rotates, but not like a solid wheel turning as one piece. Stars and clouds of gas orbit through the Galaxy, and objects at different distances from the centre move at different speeds and take different lengths of time to complete an orbit. The spiral structure itself also changes over immense periods of time.
It might seem natural to assume all of this is simply orbiting the enormous black hole at the centre. It isn’t. Sagittarius A*, despite containing about four million times the mass of the Sun, accounts for only a tiny fraction of the Milky Way’s total mass, which is thought to be well over a trillion times the mass of the sun when its enormous dark matter halo is included. The motions of stars, gas and other material throughout the Galaxy are governed by the combined gravitational influence of all that mass, not the black hole alone.
The Sun is a good example of just how large-scale that motion really is. Travelling around the Galactic Centre along with Earth and the rest of the Solar System, it moves at hundreds of thousands of kilometres per hour — and even at that speed, the Milky Way is so enormous that a single circuit takes roughly 230 million years, a period sometimes called a galactic year. In the Sun’s roughly 4.6-billion-year lifetime, it has completed only about 20 of these journeys.
What looks frozen in a dark night sky is anything but. The Milky Way is an enormous, evolving system in continual motion.
We have spent this journey discovering just how enormous the Milky Way really is. But even a galaxy 100,000 light-years across is only one part of a much larger cosmic landscape. So what lies beyond it?
What Lies Beyond the Milky Way?
Leave the Milky Way behind and something remarkable happens to our sense of scale. The enormous galaxy we have spent this journey exploring becomes just one galaxy among many.
Beyond the Milky Way, our nearest galactic neighbours include numerous smaller galaxies, among them the Large and Small Magellanic Clouds. These are galaxies in their own right, but they are also satellites of the Milky Way, held within its gravitational influence.
Farther away, about 2.5 million light-years from us, lies Andromeda (M31) — another great spiral galaxy and the nearest large galaxy to the Milky Way.
Andromeda is not alone. Along with the Triangulum Galaxy (M33), the Milky Way and numerous smaller dwarf galaxies, it belongs to a collection of nearby galaxies known as the Local Group. The name is quite literal: this is the group of galaxies occupying our local region of the universe. The group is held together by gravity rather than simply being a chance gathering of nearby galaxies, although its members do not all orbit a single central object. The Milky Way and Andromeda are its two dominant large members.
Pull back farther still and even the Local Group becomes just one small part of a much larger pattern. Galaxies gather into groups and clusters, while on the greatest scales they form an enormous cosmic web, with galaxies concentrated along vast filaments separated by immense, comparatively empty voids.
We began beneath a band of light crossing Earth’s night sky. We now know that band is our inside view of a galaxy roughly 100,000 light-years across — and that even this enormous structure is only one part of something vastly larger.
Our place in that structure can be followed step by step:
Earth → Solar System → Local/Orion Arm → Milky Way → Local Group → wider universe.
The next time the Milky Way stretches across a dark sky, remember what that hazy band really represents: our view from inside an immense galaxy, seen from one small world travelling through it.