Craig C. Freudenrich
A close look at the night sky with a pair of binoculars reveals that all stars are not the same. First, you may notice that some stars are brighter than others. Brightness depends upon the star's distance from Earth and its energy output, or luminosity. Second, stars have different colors; they run from the blue-violet end of the color spectrum through red. A star's color (the wavelengths of light it emits, or spectrum) is related to its temperature (blue-white stars are about 10 times hotter than red stars). Third, as you scan the skies with the binoculars, you'll see that many stars are loners, some come in pairs and others are parts of large clusters containing thousands of stars. Astronomers use luminosity and temperature to classify stars.
In the early 1900s, astronomers Annie Jump Cannon and Celia Payne classified stars based on their absorption spectra, or surface temperatures. They divided them into categories -- ranging from O for blue-violet through B, A, F, G, K and M for red, or coolest. In 1912, astronomers Ejnar Hertzsprung and Henry Norris Russell graphically plotted the luminosity versus spectral types of thousands of stars. Their graph is called a Hertzsprung-Russell diagram; it reveals that most stars exist along a smooth curve called the main sequence. The sun is a main sequence G star. Other stars are off the main sequence in various clusters or groups. Because luminosity also is related to a star's size and mass (highly luminous stars are large, massive stars), stars could also be classified based on their sizes: supergiants, bright giants, giants, subgiants, main sequence and white dwarfs. These classifications also can reveal something about the lifetimes of stars.
All stars fuse hydrogen into helium to produce radiation. Massive, more luminous stars have more hydrogen fuel and tend to use it up faster than lesser stars. As a result, they have shorter lifetimes. For example, the stars in Orion's belt are O-type stars that have lifetimes of 3 to 6 million years. In contrast, the sun is a less massive, less luminous G-type star that will last about 10 billion years [source: Encyclopedia of Science]. Cooler M-type stars like Proxima Centauri (one of the closest stars to the sun) will last thousands of billions of years (much like how the glowing embers of a dying campfire last long after the flames are gone).
All stars are massive balls of gases that release energy by fusing hydrogen into helium. Stars differ in size, temperature and brightness. Some stars stand alone, some group in pairs (binary stars) and some exist in clusters. Because temperature is linked to the spectrum, the star's color can reflect its temperature. For example, red is the first color on the visible spectrum or the weakest wavelength; it's also a cooler star. Blue has a stronger wavelength and is hotter. Because different chemical elements can only exist at different wavelengths, the color of the star also reveals which elements are present in the star.
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