Stars come in a variety of hues, including red, orange, yellow, green, white, and blue, with red being the coldest and blue being the hottest. The hue of a star determines its temperature, composition, and distance from Earth. For example, red stars are cooler and more distant than orange ones, which are cooler and farther away than yellow ones, and so on.
The color of a star is also important for understanding how it was made. For example, red stars like the one above are called iron stars because they contain large amounts of iron. Iron is a very heavy element and makes up 8 of every 100 grams of earth's crust. Blue stars, on the other hand, are called hydrogen stars because they consist mainly of this gas. Hydrogen is the lightest element and makes up over 99% of the sun's mass.
Stars are also classified by size. Small stars less than eight solar masses will burn through their fuel supply burning bright for a few hundred thousand years before collapsing into neutron stars or black holes. Large stars greater than about 20 solar masses will end their lives as supernovae explosions. These events can be seen from far away because they make heavy elements such as gold, silver, platinum, and zinc which are useful for making tools and weapons.
The temperature of a star's surface determines its hue. Blue stars are the hottest, white and yellow stars are the coolest, and red stars are the coolest of all. A star that mostly produces red light will have a surface temperature of around 3,500 kelvin... or 200 degrees Celsius.
Red stars are called "red" because they emit most of their light in the red part of the spectrum. This is due to the fact that iron is one of the most abundant elements in the universe and acts as a strong absorber for blue and ultraviolet light but allows red light through. Thus, red stars are the ones that we see when we look up into the night sky.
There are two types of red stars: those that are actually carbon stars and those that are actually metal-rich stars. Carbon stars are old stars that have used up their supply of hydrogen and then switched over to using helium instead. Because there's not enough carbon left on the star to make it shine with green light, it emits most of its energy in the red and infrared parts of the spectrum. Metal-rich stars are like ours except that they contain more iron than silicon, oxygen, magnesium, or calcium. These stars are called "metal-rich" because they have evolved away from the gas cloud that formed them and now possess abundances of these heavier elements beyond what would be expected based on how much iron they started out with.
Temperature is shown by the different hues of stars. The brightest stars are blue or blue-white, whereas the coldest stars are red. Also, some stars are orange or yellow, and these tend to be more massive than our sun. Mass determines how much pressure can reach a star's atmosphere, so more massive stars have more intense pressures at their surfaces.
Stars come in many different colors. This means that they have different temperatures. Blue stars are cool, while red stars are hot. Some stars are even gray or white, which means that they are neither hot nor cool enough to show color.
The hue of a star depends on its temperature. Stars less than about 5500 degrees Kelvin (blue) are too cold for their atoms to possess any electrons, so they appear completely free from all chemical elements. Such stars are called "undefined" because no element can be identified by its spectrum. As stars become hotter they begin to lose mass, forming a red giant or a supergiant. Elements can be identified only by using spectral analysis tools such as those used by astronomers to study distant galaxies.