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 1000times bigger than that of the sun, masses range from about a twentieth to over 50 solar masses; and surface temperatures range from about 3000 degree Celsius top over 50000degree Celsius. The color 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 5500 degree Celsius, is between these extremes and appears yellow.The energy emitted by shining star is produced by nuclear fusion in the star's core. The brightness of a star is measured in magnitudes-the brighter the star,the lower its magnitude . There are two types of magnitude: apparent magnitude, which is the brightness seen from 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 the stars may be split to form  a spectrum containing a series of dark lines (absorption lines).The patterns of lines indicate the presence of particular chemical elements , enabling astronomers to deduce  the composition of the atmosphere.The magnitude and spectral type (color) 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 from helium),giants, super giants,and white dwarfs.