Anatomy of the Universe

The Universe contains everything that exists, from the tiniest subatomic particles to galactic superclusters (the largest structures known). Nobody knows how big the Universe is, but astronomers estimate that it contains about 100 billion galaxies, each comprising an average of 100 billion stars. The most widely accepted theory about the origin of the Universe is the Big Bang theory, which states that the Universe came into being in a huge explosion – the Big Bang – that took place between 10 and 20 billion years ago. The Universe initially consisted of a very hot, dense fireball of expanding, cooling gas. After about one million years, the gas probably began to condense into localized clumps called protogalaxies. During the next five billion years, the protogalaxies continued condensing, forming galaxies in which stars were being born. Today, billions of years later, the Universe as a whole is still expanding, although there are localized areas in which objects are held together by gravity; for example, many galaxies are found in clusters. The Big Bang theory is supported by the discovery of faint, cool background radiation coming evenly from all directions. This radiation is believed to be the remnant of the radiation produced by the Big Bang. Small “ripples” in the temperature of the cosmic background radiation are thought to be evidence of slight fluctuations in the density of the early Universe, which resulted in the formation of galaxies. Astronomers do not yet know if the Universe is “closed”, which means it will eventually stop expanding and begin to contract, or if it is “open”, which means it will continue expanding forever. Galaxies
A galaxy is a huge mass of stars, nebulae, and interstellar material. The smallest galaxies contain about 100,000 stars, while the largest contain up to 3,000 billion stars. There are three main types of galaxy, classified according to their shape: elliptical, which are oval shaped; spiral, which have arms spiralling outwards from a central bulge; and irregular, which have no obvious shape. Sometimes, the shape of a galaxy is distorted by a collision with another galaxy. Quasars (quasi – stellar objects) are thought to be galactic nuclei but are so far away that their exact nature is still uncertain.

They are compact, highly luminous objects in the outer reaches of the known Universe: while the furthest known “ordinary” galaxies are about 10 billion light years away, the furthest known quasar is about 15 billion light years away. Active galaxies, such as Seyfert galaxies and radio galaxies, emit intense radiation. In a Seyfert galaxy, this radiation comes from the galactic nucleus; in a radio galaxy, it also comes from huge lobes on either side of the galaxy. The radiation from active galaxies and quasars is thought to be caused by black holes. Nebulae and star clusters
A nebula is a cloud of dust and gas inside a galaxy. Nebulae become visible if the gas glows, or if the cloud reflects starlight or obscures light from more distant objects. Emission nebulae shine because their gas emits light when it is stimulated by radiation from hot young stars. Reflection nebulae shine because their dust reflects light from stars in or around the nebula. Dark nebulae appear as silhouettes because they block out light from shining nebulae or stars behind them. Two types of nebula are associated with dying stars: planetary nebulae and supernova remnants. Both consist of expanding shells of gas that were once the outer layers of a star. A planetary nebula is a gas shell drifting away from a dying stellar core. A supernova remnant is a gas shell moving away from a stellar core at great speed following a violent explosion called a supernova. Stars are often found in groups known as clusters. Open clusters are loose group of a few thousand young stars that were born in the same cloud and are drifting apart. Globular clusters are densely packed, roughly spherical groups of hundreds of thousands of older stars. Stars
Stars are bodies of hot, glowing gas that are born in nebulae. They vary enormously in size, mass, and temperature: diameters range from about 450 times smaller to over 1,000 times bigger than that of the Sun; masses range from about a twentieth to over 50 solar masses; and surface temperatures range from about 3,000 °C to over 50,000 °C. The colour of a star is determined by its temperature: the hottest stars are blue and the coolest are red. The Sun, with a surface temperature of 5,500 °C, is between these extremes and appears yellow. The energy emitted by a shining star is produced by nuclear fusion in the stars core. The brightness of a star is measured in magnitudes – the brighter the star, the low is the magnitude. There are two types of magnitude: apparent magnitude, which is the brightness seen from the Earth, and absolute magnitude, which is the brightness that would be seen from a standard distance of 10 parsecs (32.6 light years).

The light emitted by a star may be split to form a spectrum containing a series of dark lines. The patterns of lines indicate the presence of particular chemical elements, enabling astronomers to deduce the composition of the stars atmosphere. The magnitude and spectral type of stars may be plotted on a graph called a Hertzsprung – Russell diagram, which shows that stars tend to fall into several well – defined groups. The principal groups are main sequence stars (those which are fusing hydrogen to form helium), giants, supergiants, and white dwarfs.

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