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The Stars in the Universe & Their Spacing

Stars are massive balls of plasma that are luminous and held together by gravity. They are suspended in space by gravity as well, which normally prevents them from crashing into each other or other celestial bodies. People on Earth can see the stars nearest to the Earth at night when the sun is not outshining them. Viewers see more stars when they are away from cities because the lights from the city block out all but the brightest stars.

Structure
- Stars in the Milky Way galaxy are composed of approximately 71 percent hydrogen and 17 percent helium. Inside the core, which can reach temperatures as high as 15 million degrees Celsius, nuclear fusion occurs converting hydrogen to helium. Energy released during this reaction travels toward the surface of the star. The surface is much cooler than the star's interior at about 5,500 degrees Celsius. Just beyond the surface is the chromosphere and corona, which is the outer layer of gas in a star's atmosphere. While cooler than the star's core, the corona is hotter than the star's surface, 2 million degrees Celsius, most likely due to energy from the star's magnetism.
Classification
- The modern method of classifying stars is based on their temperatures using a lettered scale from A to Q. Originally it was based on another criteria, so now the letters are no longer in alphabetical order. Class O stars are the hottest and M stars are the coolest. Within each class, stars are further broken down into 10 subdivisions from zero to nine indicating decreased temperatures. Stars are also categorized based on their luminosity using roman numerals ranging from zero to seven. The sun, for example, is classified as a G2V yellow dwarf.
Distribution
- Stars occur by themselves or as part of multi-star systems in which the stars are bound to each other by gravity and orbit around each other. The majority of larger stars exist in multiple star systems, but smaller stars, such as red dwarfs, exist singly. While stars are found throughout the universe, they are usually found grouped within galaxies, which contain hundreds of billions of stars.
Constellations
- Scientists on earth classify some stars into constellations. While this is convenient for identification and navigation purposes, it is important to remember that these stars may not be located anywhere near one another in space. They may appear to be side by side from Earth, but one could be farther away than the other. One quite recognizable constellation is Orion. Sometimes referred to as The Hunter, Orion's distinguishable features include a belt made of three stars, four stars to indicate the outline of his body and three smaller stars that form a sword hanging off the belt.
Spacing of the Stars
- The closest star to the Earth is, of course, the sun, at a distance of 0.0000158 light years. The closest star to our sun is Proxima Centauri, which is 4.22 light years away. Stars are generally quite far apart; for example, in a 12.5 light year radius from the Earth, there are only 33 stars. If that's not amazing enough, consider that the center of the Milky Way is 30,000 light years from the Earth's sun, and there are about 100 billion stars in the Milky Way.

Astronomy