Which galaxy is disk-shaped?

The galaxy that most famously exhibits a distinct, flattened, disk-like shape is our own, the Milky Way. ^[3][5][6] However, it is not unique; this shape defines an entire class of astronomical objects known broadly as disc galaxies. ^[1] Understanding which galaxies possess this characteristic shape requires delving into the basic classification of galaxies and the physical processes that sculpt them over billions of years. ^[2][7]

# Galaxy Shapes

When astronomers catalogue the universe, they usually sort galaxies into three primary morphological groups: spiral, elliptical, and irregular. ^[2] The disc galaxy category directly encompasses the spiral galaxies, which are perhaps the most visually stunning examples of this geometry. ^[1][9] A spiral galaxy, like the Milky Way, is characterized by a central bulge of older stars surrounded by a flatter, rotating disc that contains spiral arms composed of gas, dust, and younger stars. ^[1][9]

There is also the lenticular class of disc galaxies, sometimes denoted as S0, which possesses a prominent disc structure but lacks the distinct spiral arms seen in classic spirals. ^[1] These galaxies represent a transitional stage or a result of different evolutionary paths, often having used up or lost most of their cold gas necessary for active star formation within the disc. ^[1] Thus, "disk-shaped" isn't limited to just the familiar spirals; it describes any galaxy where the primary component—the main body of stars, gas, and dust—lies predominantly on a flat plane. ^[1]

The Milky Way is classified specifically as a barred spiral galaxy. ^[5][9] This means that in addition to the main disc, it features a central bar structure composed of stars that extends out from the nucleus. ^[9] Its disc spans an immense distance, estimated to be about 100,000 light-years in diameter. ^[3][6] This vast, thin structure is what makes it the quintessential example of a disk galaxy. ^[5]

# Disc Formation Physics

The reason these galaxies achieve such a striking flatness is directly tied to how they rotate and cool over cosmic timescales. ^[7][8] When a galaxy first forms, it typically begins as a more diffuse, nearly spherical cloud of gas and dark matter. ^[7] As this material collapses under its own gravity, the conservation of angular momentum dictates that the material must begin to spin. ^[7][8]

Imagine a massive, randomly rotating ball of dough. As it contracts, the spinning motion forces it to flatten out along its axis of rotation, much like a pizza maker spinning dough outward. ^[7] In the early universe, this process was quite efficient for the gas that would become disc galaxies. ^[4] The gas, being collisional, loses energy through friction and radiates that energy away as heat, allowing it to settle into the tightest possible orbital plane allowed by the rotation. ^[7] Stars, which are collisionless—meaning they don't bump into each other in the same way gas does—are born from this settled gas and retain their positions within the disc. ^[7] The dark matter, which makes up the bulk of the mass, tends to remain in a more spherical halo surrounding the disc, but the visible matter defines the flat shape. ^[1]

A key factor influencing the final shape is the history of mergers. ^[8] Massive mergers with other large galaxies tend to disrupt the delicate, ordered rotation of a disc, often leading to a more spheroidal, elliptical shape. ^[8] Therefore, disc galaxies like the Milky Way are thought to have undergone relatively few major mergers since their primary formation period, allowing their rotational dynamics to prevail and maintain that iconic flatness. ^[7][8]

An interesting side point that reveals the physics at play relates to orientation. If we were to view the Milky Way from a different angle—say, perfectly edge-on, as we view some distant galaxies—the bulge would dominate our view, and the thinness of the disc would be dramatically emphasized. ^[1] When we look at distant galaxies, our perspective dictates our classification; a galaxy that is face-on appears as a beautiful spiral, but if we see it edge-on, it might simply look like a thin line, reinforcing the fact that the shape is intrinsic, but our perception of it is orientation-dependent. ^[1] This observational bias means there could be many more disc galaxies out there that we simply haven't seen clearly because they are oriented perpendicular to our line of sight.

Are elliptical galaxies disc-shaped?

# Studying Our Own Disc

Our own galaxy serves as the most detailed case study for a disk galaxy. While we cannot easily step outside the Milky Way to take a picture, astronomers have painstakingly mapped its structure. ^[5] We know it contains a flat disc, a central bulge, and a surrounding halo of old stars and globular clusters. ^[3][9]

The sheer scale is difficult to grasp: 100,000 light-years across, and the disc itself is relatively thin, perhaps only a few thousand light-years thick in its main stellar component. ^[3][6] The spiral arms aren't rigid structures but rather density waves—areas where the concentration of stars and gas is temporarily higher as they orbit the galactic center. ^[5] These waves are what make the disc appear patterned rather than uniformly bright. ^[5]

The material within the disc is dynamic. Gas and stars orbit the galactic center, but the rotation speed doesn't decrease as sharply with distance as simple Newtonian gravity would predict based only on visible matter. ^[5] This observation points to the presence of an unseen component—dark matter—which helps keep the outer parts of the disc rotating quickly, influencing the long-term stability of the flat structure. ^[5]

# Comparison to Other Types

To appreciate the distinctiveness of the disk shape, it helps to compare it to the other main galaxy types. Elliptical galaxies, for instance, are the opposite end of the spectrum. ^[2] They are generally featureless, ranging from nearly spherical to elongated, but they lack the organized, rotating disk found in spirals. ^[2][1] Their stars move in random, rather than co-planar, orbits, giving them a more puffed-up, three-dimensional appearance. ^[1]

Irregular galaxies, conversely, have no defined shape at all. ^[2] They often appear chaotic, perhaps due to gravitational interactions or tidal forces from nearby galaxies, or because they simply haven't accumulated enough mass or angular momentum to settle into an ordered structure. ^[2]

The existence of a clear division between the rotating, ordered disc (like the Milky Way) and the randomly moving stars of an elliptical galaxy highlights a fundamental difference in their formation pathways—one dominated by smooth accretion and rotation, the other by violent merging events. ^[7][8]

While the Milky Way is the closest and best-studied example, it is essential to remember that disc galaxies are common throughout the universe. ^[1] Surveys of distant galaxies consistently reveal that a significant fraction of all galaxies, especially those in less dense regions, possess this flattened structure. ^[2] A fascinating area of research involves looking at very young galaxies in the early universe, where observations suggest that many were already forming rapidly rotating disks, indicating that the process leading to flatness started relatively early in cosmic history. ^[4] This rapid settling suggests that the fundamental laws governing angular momentum and cooling were very effective even when the universe was much younger. ^[4]

What type of galaxies have gas and dust in them?

# Organizing Flatness

The internal organization of a disc galaxy like ours is surprisingly ordered, which is why the shape persists. The components stack up like a layer cake. ^[1]

1. The Thin Disc: This is the most prominent part, containing most of the gas, dust, and younger, bluer stars. ^[1] It is extremely flat compared to its diameter. ^[5]
2. The Thick Disc: Surrounding the thin disc is a slightly thicker, less dense layer of older stars. ^[1] Its stars have slightly more varied, less perfectly circular orbits, making the disc appear "fluffier" vertically. ^[1]
3. The Bulge: At the center is the spheroid, or bulge, an older population of stars packed densely around the core. ^[1]
4. The Halo: An extensive, roughly spherical distribution of very old stars and globular clusters that envelops the entire structure, extending far beyond the visible disc. ^[1]

When we discuss which galaxy is disk-shaped, we are usually referring to the prominence of the first two components. The degree of flatness, or the ratio of diameter to thickness, is what astronomers use to classify spirals versus lenticulars. ^[1]

To put the "flatness" into perspective using the Milky Way's scale, consider this thought experiment. If you scaled the Milky Way's 100,000 light-year diameter down to the size of a standard American football field (about 100 yards or 91 meters long), the disc would be incredibly thin—likely only a few millimeters thick. ^[3] This dramatic aspect ratio is a direct consequence of the physics of gravitational collapse under rotation. ^[7] It’s a spectacular demonstration of physics operating on the largest scales we can observe.

The very mechanisms that create this shape also create the spiral arms. These arms are sustained by the collective action of the stars and gas orbiting at different rates, leading to density compressions that trigger star formation. ^[5] If the disc were truly chaotic, like an elliptical galaxy, these organized features could not be maintained. The disk shape is thus intrinsically linked to the active, ongoing processes of star formation and galactic rotation. ^[7]

The ultimate question of which galaxy is disk-shaped is answered by pointing to the vast population of spiral and lenticular galaxies spread across the cosmos, with our own Milky Way serving as the prime, locally accessible example. Their shape is not an accident but a precise consequence of conserving spin while contracting under gravity, a signature of galaxy evolution that favors order over random motion. ^[7][8]