At the end of the radiation period small irregularities in the clouds of gas caused some of their atoms to group together into regions of greater density. Because their gravitational attraction was greater, these regions, or ‘lumps’ pulled in atoms from adjacent regions. As the lumps grew, their attraction to nearby matter increased and the clouds eventually separated into large clumps or even collapsed into single clumps. This tendency is known as gravitational instability. At this time the Universe was almost exclusively composed of hydrogen, helium and dark matter, and the clouds of hydrogen and helium began to condense under gravitational attraction and the first stars and galaxies were formed.
Galaxies are found in groups of up to a few dozen galaxies or in clusters containing several thousands of galaxies. Virtually all groups and clusters are included in superclusters. When galaxies are shown on scales larger than 30×106 light years it is known as the large-scale structure. Galaxies do not appear randomly scattered in space but are bunched together in their groups, clusters and superclusters. They also lie in chains called filaments, and in roughly two-dimensional concentrations such as the Great Wall. There are vast areas where galaxies are scarce and these are known as voids, which are roughly spherical and about 300×106 light years across.
Gerard Henri de Vaucouleurs (1918-95) was one of the first to appreciate the appearance of the Universe as great sheets wrapped around regions devoid of galaxies, so that its overall appearance is like a froth of bubbles. If it is imagined that the air inside the bubbles represents the empty volumes of space (voids, or ‘cosmic bubbles’) then the bubble walls represent the locations of the groups and clusters, and the intersections between bubbles represent the locations of superclusters.
Galaxies are classified by their appearance: spiral (with arms), elliptical (without arms), and irregular (no obvious form). Our galaxy is located in the Local Group, which is ≈3×106 light-years across and ≈9×106 light-years from other prominent galaxies. The Andromeda Galaxy, the largest of the thirty-one galaxies in the group, is ≈2.5×106 light-years from Earth and is the most distant object visible to the naked eye. The Galaxy is the second largest galaxy in the group. It is a disc-shaped spiral ≈140,000 light-years across and consists of about 1011 stars. Our Sun is in one of its spiral arms ≈28,000 light-years from the galactic centre, which is in Sagittarius. The Sun orbits the disc centre once in ≈230×106 years at a speed of ≈250 km/s. Proxima Centauri C, the star closest to the Sun, is about 4.2 light years distant.
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