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).