Stardust and eternity – 3.4.2

Andromeda and the other galaxies

The Milky Way, our galaxy, is only one of billions or even trillions of galaxies in our Universe. Galaxies are immense gravitationally bound systems, composed of up to trillions of stars and interspersed with dust and gas. Unlike stars, only a few galaxies can be observed in the Northern Hemisphere without a telescope. Among these galaxies, the most important is Andromeda, also known as M31 in the Messier Catalogue. Similarly to the Milky Way, Andromeda is a barred “spiral galaxy”. However, it is also about twice its size and it is located in the constellation of Andromeda. Being our nearest large neighbour – as far as about 2.5 million light-years from Earth – it can be visible to the naked eye from Earth on moonless nights, just below the W-shaped constellation of Cassiopeia, from areas with moderate light pollution. Around 4.5 billion years from now, the Andromeda galaxy is expected to collide and eventually merge with the Milky Way due to its proper motion. As a result, a new giant elliptical galaxy will form, aptly named “Milkomeda”.

Other peculiar galaxies that can be easily observed from Earth are the Large and Small Magellanic Clouds. They are visible with the naked eye in the Southern Hemisphere at night. These two small – or “dwarf” – galaxies are actually “satellite galaxies” orbiting the Milky Way. Like Andromeda, they belong to the so-called “Local group” – a cluster of galaxy formed at least by 80 members, most of which are dwarf galaxies. Clusters of galaxies such as the Local Group are organized into larger groups called “superclusters”, the largest structures in the universe (hundreds of millions of light-years in size). The Local Group is part of the Virgo Supercluster.

In past centuries, astronomers observed galaxies as faint, nebulous patches of light. They did not know they were actually separated from the Milky Way. Some of them were also listed by Charles Messier (1730–1817) in his catalogue and were unknowingly categorized as nebulae or star clusters within our Galaxy. Only in 1924 Edwin Hubble (1889-1953) – with a recently assembled 2.5 m telescope on Mount Wilson, near Los Angeles – was able to determine the real nature of these celestial objects. He measured the actual distance of some variable stars (known as Cepheids) in the “spiral nebula” Andromeda, also showing that they were too far away to be located in our galaxy.

Having noticed differences in the morphology of galaxies, Hubble further classified them into three broad classes, according to their appearance: spirals, ellipticals, and irregulars. Spiral galaxies, like the Milky Way and Andromeda, are characterized by the presence of bright spiral arms that extend from a central bulge (sometimes barred) into a flattened, rotating disk. The spiral arms are sites where most of young stars are produced. Elliptical galaxies have nearly featureless ellipsoidal shapes, with a degree of ellipticity varying from from nearly spherical to elongated. They contain little gas and dust and do not actively create new stars, as they are usually composed of very old stars. Finally, irregular galaxies have no distinct shape. Most of the them were probably spirals or ellipticals that became distorted and disrupted by the gravity of neighboring galaxies.

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Andromeda galaxy – Image credit: NASA/JPL/California Institute of Technology

Large Magellanic Cloud – Image credit: ESA/NASA/Hubble

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Images

Hubble Ultra Deep Field (NASA, ESA, and S. Beckwith (STScI) and the HUDF Team)