Galaxies

Galaxies, cities of stars in the cosmos

In the vast theater of the cosmos, galaxies are the great cities of light. Each contains billions of stars, planets, gas, and dark matter. Their brilliance, visible millions of light-years away, forms the very fabric of the observable universe.

The center of the Milky Way, seen from Cerro Paranal in Chile

Our cosmic home, the Milky Way, is one of its jewels. More than 100,000 light-years wide, it stretches like a river of stars around an invisible core: the supermassive black hole Sagittarius A*. Around it, spiral arms wind, sheltering countless nebulae where new suns are constantly being born. Thus, each star tells a fragment of cosmic history.

Multiple shapes and ages

Galaxies come in an infinite variety of shapes. Some, like the Andromeda galaxy, have vast spiral arms, bathed in blue light and stardust. Others, elliptical in shape, appear as compact golden spheres, remnants of ancient times when stars were born at a frantic pace.

There are also irregular galaxies, distorted by cosmic collisions or the gravitational forces of neighboring stars. These titanic encounters shape their evolution: when they cross paths, galaxies merge, distorting their structures and triggering new waves of star formation. In a few billion years, the Milky Way and Andromeda will thus unite in a slow galactic embrace, forming a single world of stars.

Simulation of the future collision between the Milky Way and Andromeda

The light of the past

Observing a galaxy is like traveling back in time. Lightfrom Andromeda takes more than 2.5 million years to reach our eyes. Light from distant galaxies, observed by the James Webb Telescope, left its point of origin when the Universe was only a few hundred million years old.

These ancient glimmers tell the story of the cosmos' youth: the birth of the first stars, the formation of giant black holes, and the gradual organization of matter into filaments. Each galaxy thus becomes a luminous witness to the evolution of the Universe, a time capsule revealing what the sky was like even before the Earth appeared.

The different types of galaxies and their formation

If the Universe is a canvas, galaxies are its patterns. Their diversity tells the story of cosmic time: how matter came together, how light burst forth from primordial chaos.

Astronomers distinguish three major families of galaxies: spiral, elliptical, and irregular. In addition to these main forms, there are rarer variants, testimony to the infinite creativity of the cosmos.

Spiral galaxies

Elegant and luminous, spiral galaxies —such as the Milky Way or Andromeda —have a dense core of ancient stars surrounded by vast coiled arms. These arms contain gas, dust, and countless nebulae where new stars are constantly being born. Their orderly shape reflects a delicate balance between gravity, rotation, and interstellar matter.

The famous Andromeda Galaxy — The famous Andromeda galaxy

Elliptical galaxies

Elliptical galaxies are older, often the result of past galactic collisions. Their light is soft and golden, a sign of ancient stars and low star formation. They can contain hundreds of billions of stars, grouped together in a huge sphere with no visible structure. Their apparent silence hides a venerable age: that of the earliest epochs of the Universe.

Irregular galaxies

As for irregular galaxies, they appear broken, deformed by external gravitational forces. Often, these are smaller galaxies, satellites of larger neighbors, torn away and reshaped by their influence. Some, still in formation, retain the luminous chaos of the early cosmic era.

According to observations made by the James Webb Telescope, the first galaxies appeared just a few hundred million years after the Big Bang. Formed from clouds of gas that collapsed in on themselves, they lit up the first lights in the sky. Since then, mergers and interactions have shaped their forms, slowly weaving the gigantic cosmic web that we observe today.

Did you know?

It is estimated that there are more than 200 billion galaxies in the observable universe.
The Milky Way and Andromeda will collide in about 4 billion years.
The most distant galaxy known, GN-z11, formed only 400 million years after the Big Bang.
The central black hole of some galaxies can contain the mass of several billion Suns.
The James Webb Space Telescope observes galaxies so ancient that their light has traveled for 13 billion years before reaching us.