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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.

MeerKAT+: the MeerKAT Extension

Feb 21, 2024

MeerKAT+: the MeerKAT Extension The handover of the first dish of the MeerKAT extension signals an important milestone for the SKA-MID construction

The first discoveries of the Webb space telescope in Rome: public lecture on 29 February

Feb 21, 2024

The first discoveries of the Webb space telescope in Rome: public lecture on 29 February On Thursday 29 February at 6 pm, Prof. Roberto Maiolino of the University of Cambridge (UK) will hold a public lecture on the theme "The invisible Universe revealed by the James Webb Space Telescope" at the Department of Physics of Sapienza University of Rome

The AGILE satellite re-entered the atmosphere

Feb 14, 2024

The AGILE satellite re-entered the atmosphere After 17 years of thriving operations, the AGILE Italian scientific satellite re-entered the atmosphere, thus ending its intense activity as a hunter of some of the most energetic cosmic sources in the Universe that emit gamma and X-rays