What Are Spiral and Elliptical Galaxies?

Galaxies are the fundamental building blocks of the universe—vast, gravitationally bound systems composed of stars, stellar remnants, interstellar gas, dust, and dark matter. Among the myriad of galactic morphologies, spiral and elliptical galaxies dominate the observable universe. Understanding their formation is crucial for unraveling the cosmic history from the Big Bang to the present day.

Spiral galaxies, like our own Milky Way, are characterized by a flat, rotating disk with prominent spiral arms that wind outward from a central bulge. The disk contains young, blue stars and abundant gas and dust, fueling ongoing star formation. The central bulge, in contrast, is composed mostly of older, redder stars. A supermassive black hole often resides at the nucleus. The Milky Way, the Andromeda Galaxy (M31), and the Whirlpool Galaxy (M51) are classic examples. Elliptical galaxies, on the other hand, have a smooth, nearly featureless ellipsoidal shape with no discernible disk or spiral arms. They range from near-spherical to highly elongated. Their stellar populations are predominantly old and metal-rich, and they contain very little cool gas or dust, resulting in negligible new star formation. Examples include M87 in the Virgo Cluster and the giant elliptical IC 1101, one of the largest known galaxies.

The Formation of Spiral Galaxies

Hierarchical Merging and Angular Momentum

The leading model for spiral galaxy formation is hierarchical merging within the framework of cold dark matter cosmology. Small primordial density fluctuations in the early universe grew under gravity, collapsing into dark matter halos. These halos captured primordial gas, which cooled and fragmented into the first stars and protogalactic clumps. Over billions of years, these clumps merged, building larger and larger galaxies. The key to forming a disk is angular momentum. As gas and small structures fall into a growing galaxy, collisions and tidal torques transfer angular momentum, causing the infalling material to settle into a rotating disk rather than a spherical cloud. The disk is a direct consequence of the net angular momentum of the accreting gas.

Density Waves and Spiral Arms

The spiral arms themselves are not permanent structures but rather density waves—regions of slightly higher density that rotate more slowly than the stars and gas in the disk. As material moves through these waves, it gets compressed, triggering star formation. The bright young stars light up the arms, but the pattern itself is maintained by gravitational interactions between the disk and the galaxy's dark matter halo or by the self-gravity of the disk. This leads to the beautiful, long-lived spiral patterns seen in galaxies like NGC 1232.

Gas Accretion and Star Formation

For a spiral galaxy to maintain its blue color and ongoing star formation, it must continuously accrete fresh gas from the intergalactic medium. This gas flows along cosmic filaments and is funneled through the dark matter halo to the disk. Once in the disk, gas clouds collapse under gravity, forming molecular clouds and then new stars. The rate of star formation in spirals is typically a few solar masses per year, with the gas reservoir lasting many billions of years. However, if a spiral galaxy exhausts its gas supply or merges with another massive galaxy, its star formation can cease, leading to a red, passive spiral—a transitional stage.

Key Factors in Spiral Galaxy Formation

  • Initial angular momentum of the protogalactic gas cloud
  • Gradual accretion of smaller galaxies and gas clouds under dark matter halo guidance
  • Density wave instabilities in the disk that organize star formation into spiral arms
  • Continuous inflow of cool gas from the cosmic web to sustain star formation over cosmic time
  • Weak or absent major mergers, which would disrupt the delicate disk structure

The Formation of Elliptical Galaxies

Major Mergers and Dynamical Violent Relaxation

Elliptical galaxies are believed to form primarily through major mergers—collisions between two roughly equal-mass disk galaxies. When two spiral galaxies merge, the stars and dark matter interact gravitationally but hardly ever collide directly because of the vast empty spaces between stars. However, the gravitational potential fluctuates violently, causing the orbits of individual stars to randomize. This process, called violent relaxation, erases any memory of the original disk structure and produces a smooth, spheroidal distribution. The merger also triggers a starburst that rapidly consumes any remaining gas, leaving the resulting elliptical galaxy gas-poor and dominated by old stars. The central supermassive black holes from the progenitor galaxies coalesce, often producing powerful active galactic nucleus (AGN) activity.

The Role of Gas and Star Formation History

During the merger, gas is violently compressed, leading to a burst of star formation that can be thousands of solar masses per year. This starburst uses up nearly all the available cold gas within a few hundred million years. After the burst, the galaxy becomes red and dead, with no further star formation unless minor mergers later bring in fresh gas. The stellar population of ellipticals is thus uniformly old, with ages typically exceeding 5–10 billion years. The metal content is high because the starburst enriched the gas with heavy elements before star formation ceased.

Minor Mergers and Accretion

Not all ellipticals form from two equal-mass mergers. Some grow by minor mergers—accreting many smaller satellite galaxies over time. These events can thicken the disk of a spiral galaxy, gradually building a spheroidal component without fully destroying the disk. In fact, the Milky Way's own bulge may have formed partly through minor mergers and secular processes. However, the largest ellipticals in galaxy clusters (cD galaxies) are built by both major and minor mergers, cannibalizing dozens of galaxies over cosmic time.

Key Factors in Elliptical Galaxy Formation

  • Major galaxy mergers between two disk-dominated systems
  • Violent relaxation that randomizes stellar orbits and erases disk structure
  • Depletion of gas through starbursts and AGN feedback, halting new star formation
  • Merger-induced starbursts that produce a homogeneous old stellar population
  • Multiple sequential mergers in dense cluster environments that build the most massive ellipticals

Observational Evidence and Simulations

Astronomers use both observations and computer simulations to test these formation models. The Hubble Space Telescope has captured detailed images of merging galaxies at various stages, such as the Antennae Galaxies and the Mice Galaxies, which show tidal tails and bridges of stars torn loose during the interaction. Observations of distant, young galaxies with the James Webb Space Telescope reveal many clumpy, irregular galaxies that are likely the progenitors of modern spirals and ellipticals. Large cosmological simulations like the IllustrisTNG and EAGLE projects can follow the formation of galaxies from the early universe to the present, reproducing the observed distribution of spiral and elliptical morphologies. These simulations show that the merger history and the amount of gas accretion are the primary determinants of a galaxy's final shape and color.

The Role of Dark Matter

Dark matter halos are the invisible scaffolding within which galaxies form. The mass and concentration of the dark matter halo influence the spin of the disk and the stability of spiral arms. In ellipticals, the dark matter distribution is more extended and less concentrated, reflecting the violent merger history that mixed both ordinary and dark matter. Observations of gravitational lensing and the motion of stars and globular clusters in elliptical galaxies confirm that dark matter is present and dominates the total mass, especially in the outer regions.

Environmental Effects

Galaxies do not evolve in isolation. In dense galaxy clusters, interactions with the hot intracluster medium can strip the cold gas from a spiral galaxy, transforming it into a gas-poor, red sequence galaxy without changing its morphology much—this is called ram-pressure stripping. Furthermore, high-speed encounters can tidally distort spiral disks, or even merge them into ellipticals. Cluster ellipticals are much more common than field ellipticals, consistent with the idea that mergers and harassment are more frequent in crowded environments.

Open Questions and Future Research

Despite significant progress, several puzzles remain. Why do some galaxies with clear spiral disks have very little star formation (anemic spirals)? How do the most massive elliptical galaxies form in such a short time? What is the role of feedback from supermassive black holes in quenching star formation after a merger? Upcoming facilities like the Extremely Large Telescope and the Nancy Grace Roman Space Telescope will provide higher-resolution observations of galaxies at cosmic noon—the peak epoch of star formation and mergers—helping to answer these questions. Astronomers are also combining gravitational wave detections of merging black holes with galaxy merger histories to understand the co-evolution of galaxies and their central black holes.

Understanding the formation of spiral and elliptical galaxies is not merely a taxonomic exercise—it reveals the fundamental processes of cosmic evolution: gravity, thermodynamics, and dark matter dynamics that have shaped the universe over 13.8 billion years. Each galaxy tells a story of its past mergers, gas accretion, and stellar birth, and by reading these stories across cosmic time, we piece together the grand narrative of how we got here.