Personal tools
Log in
You are here: Home Research Activities Galaxies and Cosmology

The study of galaxies

A galaxy is a massive, gravitationally bound system consisting of stars, gas and dust (that make up the interstellar medium), and a dynamically important but poorly understood component commonly called dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way galaxy. Galaxies range from dwarfs with as few as ten million stars to giants with a hundred trillion stars, each orbiting the centre of mass of their host galaxy.

Galaxies vary in their relative amounts of stars and interstellar medium but the mass of most galaxies is dominated by dark matter, 90% being typical. It is now thought that most, if not all, galaxies contain a super-massive black hole at their centre.

As far as their optical appearance goes, galaxies have historically been classified into one of three broad classes. Elliptical galaxies have a smooth, elliptical light distribution that shows little or no detail and are characterised by having relatively little interstellar medium. Spiral galaxies are flattened disk-like systems showing beautiful, curving spiral arms, and have much more interstellar medium than the ellipticals. The third class is the irregular galaxies, generally less massive than either the ellipticals or spirals, and having an amorphous appearance. These galaxies are normally very rich in interstellar medium.

Galaxies sometimes collide and interact with each other and this can both induce very rapid bursts of star formation and transform galaxies from one type into another. Mergers of galaxies in the distant past are thought to have been fundamental in shaping their present-day appearance.

Most galaxies are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations known as groups and clusters, which, in turn usually form larger superclusters. At the largest scale, these associations are generally arranged into sheets and filaments, which are surrounded by immense voids.

CALVERA EXPLODED WHERE IT SHOULDN’T HAVE: A “RUNAWAY” PULSAR DEFIES THE RULES OF THE MILKY WAY

Aug 29, 2025

CALVERA EXPLODED WHERE IT SHOULDN’T HAVE: A “RUNAWAY” PULSAR DEFIES THE RULES OF THE MILKY WAY A stellar explosion, a pulsar, and a supernova remnant - that’s the story of Calvera. Positioned more than 6,500 light-years above the Galactic plane, this system is rewriting what we know about stellar evolution in our galaxy. The research originates from a team at the Italian National Institute for Astrophysics (INAF), in collaboration with the University of Palermo, and is detailed in a study published in Astronomy & Astrophysics

Lake Como Schools on Astrophysics, Cosmology and Gravitation

Jul 08, 2025

Lake Como Schools on Astrophysics, Cosmology and Gravitation It has been held at Como – Villa del Grumello, 23-27 June, the 2025 edition of the “Lake Como Schools on Astrophysics, Cosmology and Gravitation” on the theme “Dark Matter, Dark Energy and the Cosmological Tensions”

The Lucchin Schools Return

Jun 01, 2025

The Lucchin Schools Return First Edition of the New INAF PhD School Series Concludes in Asiago