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.

Very little magnetism in this odd couple

Nov 27, 2014

The new observations of IGR J17544-2619 obtained by the NASA satellite NuSTAR have shed light onto some of the most obscure properties and behaviours of SFXTs. The unprecedented accuracy of these measurements has allowed the team led by Varun Bhalerao at the Inter University Center for Astronomy and Astrophysics (India), with the participation of INAF researchers Patrizia Romano and Lorenzo Natalucci, to see a cyclotron line in the X-ray spectrum of this source, the unambiguous signature of the interaction between the stellar matter and the magnetic field of the compact object that is attracting it.

4th "Paolo Farinella" Prize 2014

Apr 03, 2014

To honor the memory and the outstanding figure of Paolo Farinella (1953–2000), an extraordinary scientist and person, a prize has been established in recognition of significant contributions given in the fields of interest of Paolo, which span from planetary sciences to space geodesy, fundamental physics, science popularization, security in space, weapons control and disarmament.

First COSPAR Symposium - Abstract submission deadline extended

Jun 07, 2013

The First COSPAR Symposium will be held in Bangkok, Thailand, 11-15 November 2013. Abstract deadline has been extended to 30 June 2013.