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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.

The long journey of human missions to Mars and back to Earth

Aug 09, 2021

The long journey of human missions to Mars and back to Earth The Embassy of Italy, in collaboration with the Italian Cultural Institute in Sydney and the Australian Academy of Science, invites you to an Italian Night with Five Southern Stars!

Gaia draws a new map of the galactic spirals

Jul 28, 2021

Gaia draws a new map of the galactic spirals An international team led by researchers of the the Italian National Institute for Astrophysics (INAF) has compiled the most detailed map yet of the Milky Way’s spiral structure within about 16,000 light years from the Sun, by making use of the position and brightness of around 600,000 young stars measured by Gaia, the European Space Agency’s star surveyor

The morphology of the X-ray afterglows and of the jetted GeV emission in long gamma-ray bursts

May 12, 2021

The morphology of the X-ray afterglows and of the jetted GeV emission in long gamma-ray bursts In a new article published in the Monthly Notices of the Royal Astronomical Society, an ICRA-ICRANet research team (some of them INAF associates) sheds light on the mass and spin of stellar-mass BHs from an extensive analysis of long-duration GRBs