The cosmic genealogy of galaxies is written in their shapes

Space · Published 25 October 2026 Swipe more like this →

In short: Galaxies are not just random clusters of stars; they are structured systems whose forms reveal their life stories. From the rotating pinwheels of spirals to the smooth ovals of ellipticals, these shapes act as a visual record of collisions, mergers, and the slow aging of stellar populations.

Most modern galaxy classifications stem from Edwin Hubble’s 1926 scheme, which categorized systems based on their optical appearance. In 1961, Allan R. Sandage refined this system in 'The Hubble Atlas of Galaxies,' creating a framework that remains the standard for identifying morphology today. These categories include ellipticals, spirals, and irregulars, with further subdivisions based on features like central bars or specific structural proportions.

Spiral galaxies, such as our Milky Way and Andromeda, are characterized by pancake-like disks and central bulges. They feature spiral arms where young stars form in gas-rich environments, while older stars reside in the disk, bulge, and surrounding halos of dark matter and old star clusters. Notably, two-thirds of these are 'barred spirals,' featuring a ribbon of gas and dust across the center—a feature scientists believe indicates a galaxy has reached full maturity.

In contrast, elliptical galaxies range from round to oval and typically lack the gas and dust necessary for new star formation, resulting in older stellar populations. These systems often exhibit complete rotational symmetry with surface brightness decreasing predictably from the center. Scientists believe these shapes are frequently the result of collisions and mergers between spiral galaxies. Lenticular galaxies serve as a middle ground; they possess the bulge and disk of spirals but lack arms, potentially representing 'faded' spirals or products of galaxy mergers.

Irregular galaxies represent the outliers, appearing as rings, toothpicks, or small groupings. Their chaotic shapes are often caused by gravitational interactions with neighboring systems. These range significantly in scale, from dwarf irregulars with 100 million times the Sun’s mass to massive giants weighing up to 10 billion solar masses.

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