At the Heart of Galaxies: When Shape Tells the Story of the Universe

Galaxies and the Milky Way — Galaxy morphology: **elliptical**, **spiral**, **lenticular**, and **irregular**.
Image source: astronoo.com (new window)

Scientific Summary

The page provides an overview of galaxies: their nature (stars, gas, dust, and dominant dark matter, revealed by rotation curves), their classification according to the Hubble sequence into four families—elliptical, spiral, lenticular, irregular—and the processes of merging and interaction that shape their evolution, illustrated by concrete cases (Cigar, Antennae, Cartwheel). A section is dedicated to quasars, an intense phase of galactic nucleus activity linked to the accretion of matter by a supermassive black hole, a privileged witness to the young Universe. The article finally situates these structures on a large scale (clusters, superclusters, cosmic web) and offers a table of twenty representative galaxies as well as references and images from Hubble and JWST.

What do all galaxies have in common?

The fundamental commonality among all galaxies is that they are gravitationally bound systems composed of stars, gas, interstellar dust, and, above all, dark matter, which constitutes the majority of their mass. Their diversity of shapes—spiral, elliptical, lenticular, or irregular—is only the visible manifestation of their evolutionary history and interactions, but their cohesion and dynamics are governed by the same universal force: gravity. Many also host, at their core, a supermassive black hole whose activity, when intensified, can give rise to a quasar, one of the most energetic manifestations in the Universe.

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What is a galaxy?

A galaxy is a gravitationally bound system composed of stars, interstellar matter (gas and dust), and dark matter, the latter generally representing the majority of the total mass of the system, although it neither emits nor absorbs light and is only detected by its gravitational effects.

A classification into four major families

Since the classification proposed by Edwin Hubble in 1926, galaxies are generally distinguished into four major morphological types, sometimes referred to as the "Hubble sequence":

Elliptical galaxies, with an oval shape that is more or less elongated, poor in gas, and composed mainly of old stars
Spiral galaxies, whose flattened disk features spiral arms rich in gas and young stars, organized around a central bulge; some, called barred spirals, have an elongated bar-shaped structure crossing the nucleus
Lenticular galaxies, an intermediate form with a disk like spirals but without visible spiral arms, with an older stellar population
Irregular galaxies, without a defined symmetrical structure, often the result of gravitational interactions or collisions with other galaxies

Permanently interacting structures

Galaxies are not isolated objects: under the effect of gravity, they interact with each other, mutually deform, and can eventually merge, a process that plays a major role in the evolution of their morphology over time. Our own galaxy, the Milky Way, is thus on a collision course with the Andromeda galaxy, a merger expected in several billion years.

A large-scale organization

On a larger scale, galaxies are not uniformly distributed in the Universe: they group together in clusters, gathering from a few dozen to several thousand gravitationally bound galaxies, themselves organized into superclusters. These superclusters are finally part of a gigantic cosmic web of matter filaments separated by vast voids, the largest known structure in the observable Universe.

Table of characteristic galaxies

20 representative galaxies
Common Name	Designation	Type	Constellation	Distance (light-years)	Approximate Diameter
Milky Way	—	Barred Spiral	(our galaxy)	—	≈ 100,000 ly
Andromeda Galaxy	M31 / NGC 224	Spiral	Andromeda	≈ 2.5 million	≈ 220,000 ly
Triangulum Galaxy	M33 / NGC 598	Spiral	Triangulum	≈ 2.7 million	≈ 60,000 ly
Large Magellanic Cloud	LMC	Irregular (dwarf)	Dorado / Mensa	≈ 160,000	≈ 14,000 ly
Small Magellanic Cloud	SMC / NGC 292	Irregular (dwarf)	Tucana	≈ 200,000	≈ 7,000 ly
Whirlpool Galaxy	M51 / NGC 5194	Spiral	Canes Venatici	≈ 23 million	≈ 60,000 ly
Sombrero Galaxy	M104 / NGC 4594	Spiral (lenticular seen edge-on)	Virgo	≈ 29 million	≈ 50,000 ly
Cigar Galaxy	M82 / NGC 3034	Irregular with starburst	Ursa Major	≈ 12 million	≈ 37,000 ly
Bode's Galaxy	M81 / NGC 3031	Spiral	Ursa Major	≈ 12 million	≈ 90,000 ly
Pinwheel Galaxy	M101 / NGC 5457	Spiral	Ursa Major	≈ 21 million	≈ 170,000 ly
Black Eye Galaxy	M64 / NGC 4826	Spiral	Coma Berenices	≈ 17 million	≈ 54,000 ly
Centaurus A	NGC 5128	Peculiar Elliptical (radio galaxy)	Centaurus	≈ 12 million	≈ 60,000 ly
Cartwheel Galaxy	Cartwheel Galaxy / ESO 350-40	Ring (result of a collision)	Sculptor	≈ 500 million	≈ 150,000 ly
Antennae Galaxies	NGC 4038 / NGC 4039	Merging Spirals	Corvus	≈ 45 million	≈ 100,000 ly (pair)
Messier 87	M87 / NGC 4486	Giant Elliptical	Virgo	≈ 53 million	≈ 240,000 ly
IC 1101	IC 1101	Giant Elliptical (cD galaxy)	Serpens	≈ 1.04 billion	≈ 4 million ly (one of the largest known)
Little Ghost Galaxy	NGC 6822	Irregular (dwarf, Local Group)	Sagittarius	≈ 1.6 million	≈ 7,000 ly
Spindle Galaxy	NGC 891	Spiral seen edge-on	Andromeda	≈ 30 million	≈ 100,000 ly
Mexican Hat Galaxy (barred)	NGC 1300	Barred Spiral	Eridanus	≈ 61 million	≈ 110,000 ly
Tadpole Galaxy	Tadpole Galaxy / UGC 10214	Disturbed Spiral (with tidal tail)	Draco	≈ 420 million	≈ 280,000 ly (with tidal tail)

Spiral Galaxies

The spiral galaxy: M74 — This image of the spiral galaxy M74, taken by NASA and ESA's Hubble Space Telescope, is emblematic of spiral galaxies. Bright clusters of glowing gas illuminate the spiral arms, testifying to an environment conducive to star formation.
Image source: ESA/Hubble & NASA (new window)

Elliptical Galaxies

The elliptical galaxy: NGC 2865 — Elliptical galaxies have shapes ranging from perfectly round to oval. They are less common than spiral galaxies. NGC 2865, visible here in an image captured by the Hubble Space Telescope, is an elliptical galaxy located 100 million light-years away.
Image source: ESA/Hubble & NASA (new window)

Lenticular Galaxies

The lenticular galaxy: NGC 4886 — Lenticular galaxies are a hybrid between spirals and ellipticals. They have the bulge and central disk characteristic of spiral galaxies but lack arms. However, like ellipticals, lenticular galaxies have older stellar populations and low star formation activity. The lenticular galaxy NGC 4886, photographed here by the Hubble Space Telescope, contains mainly old stars but no spiral arms.
Image source: ESA/Hubble & NASA (new window)

Irregular Galaxies

The irregular galaxy: NGC 5264 — Irregular galaxies have unusual shapes, such as toothpicks, rings, or even small clusters of stars. Their mass ranges from irregular dwarf galaxies, with a mass 100 million times that of the Sun, to large galaxies weighing up to 10 billion solar masses. The Hubble Space Telescope captured this image of NGC 5264, an irregular dwarf galaxy.
Image source: ESA/Hubble & NASA (new window)

The Special Case of Quasars: A Galaxy with an Erupting Core

A quasar (short for quasi-stellar radio source) is not a distinct object from the galaxies studied above, but a particularly luminous phase of their nucleus: when a central supermassive black hole actively accretes matter to the point of making the galaxy's core brighter than all its hundreds of billions of stars combined.

A central engine powered by a supermassive black hole

Matter (gas, dust, sometimes stellar debris) spiraling toward the central black hole forms an accretion disk heated to extreme temperatures by friction, emitting intense visible, ultraviolet, and X-ray light. Some of this energy can also be channeled into relativistic jets perpendicular to the disk, the source of the radio radiation that gave its name to the first quasars discovered in the 1960s, such as 3C 273.

A host galaxy drowned in the brilliance of its nucleus

A quasar belongs to the broader family of active galactic nuclei (AGN), of which it represents the most luminous form. Its luminosity, which can exceed a thousand times that of the entire Milky Way, almost completely eclipses the host galaxy, long difficult to observe directly; it is only with adaptive optics and telescopes like Hubble or JWST that astronomers today manage to distinguish the spiral or elliptical structure of these host galaxies, often marked by signs of recent mergers.

Privileged witnesses to the young Universe

The most distant quasars, observed as they were less than a billion years after the Big Bang, indicate that supermassive black holes of several billion solar masses had already formed very early in cosmic history, a strong constraint for models of primitive galaxy formation.

Quasar activity is only a transient episode, often triggered by the merger of two galaxies that abruptly channels gas toward the center: our own Milky Way hosts at its core a supermassive black hole (Sagittarius A*), currently quiescent, but which may have experienced a comparable phase of activity in the past.

References

FAQ: Everything You Need to Know About Galaxies

What is the difference between a spiral, elliptical, lenticular, and irregular galaxy?

Spiral galaxies (like the Milky Way) have a disk structure with spiral arms where stars are born, a central bulge, and a halo. They contain gas and dust, allowing for continuous star formation. Elliptical galaxies have a spherical or oval shape, with no arms or disk, and very little gas or dust; they mainly host old stars and are often the result of galactic mergers. Lenticular galaxies are an intermediate form: they have a disk like spirals but no visible spiral arms, with an older stellar population. Irregular galaxies, like the Magellanic Clouds, have no symmetrical structure and often exhibit regions of active star formation, due to gravitational interactions.

What are the Magellanic Clouds and why are they important?

The Large and Small Magellanic Clouds are two irregular dwarf galaxies, satellites of the Milky Way. They are located approximately 160,000 and 200,000 light-years away, respectively, and are connected by a bridge of gas and stars. Their importance lies in their proximity: they allow detailed study of star formation in a low-metallicity environment, similar to that of the early Universe. They are also laboratories for understanding the interaction between galaxies and their intergalactic medium, notably through the Magellanic Stream, a stream of gas extending over hundreds of thousands of light-years.

What happens when two galaxies collide?

Galactic collisions are major events that unfold over hundreds of millions of years. Although stars generally do not collide (interstellar space is immense), tidal forces deform the galaxies, creating bridges and tidal tails (as in the Antennae Galaxies, NGC 4038/4039). Gas clouds collide, causing intense starburst episodes. In the long term, the merger can transform two spirals into an elliptical galaxy, and if each galaxy had a supermassive black hole, the two black holes will merge into an even more massive one, as in NGC 6240.

What is the closest galaxy to the Milky Way and why is it important?

The closest galaxy to the Milky Way is the Canis Major Dwarf Galaxy, discovered in 2003, located about 25,000 light-years from the Sun (it is currently being cannibalized by the Milky Way). However, the Andromeda Galaxy (M31) is the closest large galaxy, at about 2.5 million light-years. Andromeda is a spiral galaxy similar to ours but more massive. It is important because it serves as a laboratory for understanding the structure and evolution of spirals, and it is on a future collision course with the Milky Way: the two galaxies are expected to merge in about 4.5 billion years.

What is the Cigar Galaxy (M82) and why is it so special?

The Cigar Galaxy (M82) is an irregular galaxy located about 12 million light-years away, famous for its exceptionally high star formation rate (starburst). It gravitationally interacts with the neighboring galaxy M81, and this interaction compresses its gas clouds, triggering the birth of thousands of massive stars. These stars, being very hot, emit an intense stellar wind and produce supernovae at a high rate. M82 is also an important source of infrared radiation and X-rays, making it a prime target for studying extreme star formation and the accompanying processes (such as gas outflows called superwinds).

What is dark matter and what is its role in galaxies?

Dark matter is a form of matter that does not emit, absorb, or reflect light, making it invisible. It is only detected by its gravitational effects on visible matter (stars, gas) and on light (through gravitational lensing). In galaxies, dark matter generally represents the majority of the total mass (about 85%). It forms a spherical halo that extends well beyond the visible disk and contributes to the cohesion of the galaxy by increasing its gravitational mass. The presence of dark matter was deduced, among other things, by studying the rotation curves of galaxies: the rotational speeds of stars on the periphery are too high to be explained by visible mass alone, indicating that additional mass (dark matter) is present.