In some pictures galaxies resemble coiled shells; in others they look like smooth pale ovals or ragged patches of light. If gravity acts everywhere, why did it not gather them into identical spheres?
A galaxy’s shape depends on the motion of its matter, its gas supply and its history of interactions; our viewing direction also changes its apparent outline. A galaxy is a large gravitationally bound system of stars, gas, dust and other components. It is not a solid object: its parts follow different orbits. Gravity alone therefore does not specify one shape. NASA: types of galaxies.
First, turn the same galaxy around
Imagine a round plate. Seen straight on, its surface looks circular. Tilt it and it becomes an oval; almost edge-on, it is a narrow strip. Nothing has happened to the plate. Its projection, the two-dimensional image we see, has changed.
A spiral galaxy likewise has a flattened disk in which arms may be visible. Viewed edge-on, their pattern is often hidden: starlight overlaps along the line of sight, and dust may create a dark lane. A narrow galaxy in a photograph is therefore not automatically a separate “needle-shaped” kind. NASA: arms and viewing angle.
Only the inclination of one schematic disk changes below. Its matter is not rearranged, and no panel represents a later stage of its life.
Face-on
Inclination i: 0°
Axis ratio b/a: 1
Inclined disk
Inclination i: 60°
Axis ratio b/a: 0.50
Nearly edge-on
Inclination i: 80°
Axis ratio b/a: 0.17
Three projections of the same ideal thin circular disk at i = 0°, 60° and 80°. The angle is measured from the direction perpendicular to the disk. This is geometry, not three ages or three galaxy types. Bulge, dust and real thickness are omitted; arms are schematic.
Yet viewing angle cannot explain everything. A genuine elliptical galaxy is not simply a spiral seen from an awkward direction. We need to know how the stars are distributed in space and how they move.
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How flat a circle becomes in an image
For an ideal, infinitely thin circular disk, the ratio of apparent minor axis b to major axis a is
(b)/(a) = cosi.
The angle i is measured from the direction perpendicular to the disk: 0° means face-on and 90° means edge-on. At i = 60°, b/a = 0.5; at 80°, it is about 0.17. This is our geometric example, not a measurement of a real galaxy.
Real systems have thickness, a central bulge, dust and uneven light; their disks need not be circular. This formula alone therefore neither identifies a galaxy’s type nor gives a reliable inclination for an arbitrary oval image.
Gas can lose energy and settle into a disk
Particles interact in gas, shock waves occur, and heated matter can release energy as radiation. As gas cools, some disordered motions weaken and it can gather more densely. That does not mean all the gas can fall unhindered into a single point.
If a system has an overall rotational motion, cooling does not simply erase it. This property is described by angular momentum. Gas can exchange angular momentum between different regions and with its surroundings; organized rotation nevertheless helps maintain an extended disk. Stars born from disk gas inherit its overall motion. Barnes: the structure and evolution of disk galaxies.
Stars themselves generally do not collide with each other nearly as often as gas particles do. They cannot simply lose orbital energy in the same dissipative way and all “settle” into a thin plane. What matters is therefore not only how much matter there is, but what state it was in when the stars formed.
The plate helped us picture projection, but that analogy ends here. A galaxy has no solid surface mechanically holding its stars, and different radii do not rotate as parts of a rigid wheel.
Spiral arms are a pattern in a moving disk
In a spiral galaxy, disk rotation combines with an uneven distribution of matter. Gravity amplifies certain concentrations, gas flows and compresses, and bright young stars highlight parts of the pattern. Older stars also inhabit the disk: the spaces between arms are not empty gaps. NASA: stars, gas and arms.
An arm should not be pictured as a rigid ribbon with stars permanently attached. In different models, arms may be wave-like concentrations or structures that change, break into segments and form again. A central bar, an elongated stellar structure, and a neighboring galaxy’s gravity can also affect them. No single simple explanation is established for every spiral. Dobbs and Baba: a review of spiral-structure mechanisms.
The familiar traffic-jam comparison is useful only for some models: a pattern may move differently from its individual participants. It does not mean that all arms are the same unchanging wave or that stars literally brake like cars.
Less organized motions make a more three-dimensional stellar system
In elliptical galaxies, stars occupy a three-dimensional, often oval system and follow orbits in different directions. Such a system does not require a thin disk where nearly all stars turn together. It may still rotate overall; “different directions” does not mean no rotation at all. Its smooth light usually comes mainly from older stellar populations, and cold gas for new star formation is often scarce. This does not mean it contains no gas whatsoever. NASA: elliptical galaxies.
Interactions can change stellar orbits. In a major merger, the gravitational field changes, disks can be disrupted, and stellar motions become less aligned. Meanwhile, gas may compress and form new stars. But the outcome depends on masses, orbits and gas: not every encounter destroys a disk, and not every elliptical system has the same history. Barnes: mergers and the subsequent fate of disks.
Lenticular galaxies, or S0s, have a disk and a central bulge but lack prominent spiral arms. Their supply of cold gas and level of star formation are often low. They offer a useful counterexample: a disk and a spiral pattern are different things. Different processes, including changes in gas supply and interactions, may lead to a similar appearance. NASA: lenticular systems.
An irregular shape does not tell the whole story either
Irregular galaxies lack clear spiral or smooth elliptical structure. In some cases, a neighbor’s gravity stretches matter into tails and disrupts symmetry. In others, uneven patches of gas and star formation stand out. The term describes appearance; it does not prove that a system has just collided or has only just formed.
For example, the dwarf irregular galaxy UGC 4459 contains both younger and older stars. Its untidy outline does not make all its stars the same age. ESA/Hubble: the irregular galaxy UGC 4459.
A map of light must also be distinguished from a map of mass. A small number of bright young stars can stand out strongly in a photograph, while dust conceals some of the light. Astronomers therefore supplement images with measurements of motions and observations at other wavelengths. A single outline cannot uniquely reconstruct spatial structure and past history.
Classification helps us compare, not guess an age
The famous “Hubble tuning fork” places galaxy types alongside one another according to morphological features: their visible structure. It is not a single proven sequence that every galaxy follows from elliptical to spiral or the reverse. A system can change, but shape alone specifies neither its route nor its clock. ESO: what the Hubble tuning fork means.
Make a prediction: if we turn a gas-rich spiral disk edge-on, does it become an elliptical galaxy? No. Its image and the visibility of its arms change, rather than its stellar orbits and three-dimensional structure. A genuine transformation requires changes in the system itself.
Different forms are traces of different motions, gas conditions and interactions, seen from different angles. When comparing your next galaxy images, first ask what you see: arms, a disk, a bulge, dust or only an overall oval? Then ask which motion and gas measurements are missing from the explanation.