![]() Because they don’t have much mass to hold up against gravity, they don’t have to work very hard and their fuel use rate is low. On the other end of our spectroscopic sequence, we see the letter M. These stars are so large that they are also sometimes called blue supergiants. This means at least six generations of O-type stars have been born and have died in our sky since the dinosaurs walked the Earth. An O-type star might only live about 10 million years or less. Because they are using their hydrogen fuel at a high rate, they don’t live as long as the other main sequence stars. These large, hot, blue stars are the O-type stars in our spectroscopic sequence. Think about a gas range – when the flame is really hot it’s blueish-white. This makes the star “white hot,” or blueish-white. To work hard against gravity, then, large stars must generate a lot of heat. The force that works against gravity in a star is the pressure from the heat energy released by its internal fusion. Now let’s explore stars with different spectral classes.Ī star that contains a lot of mass must work very hard against gravity. Each letter within the spectroscopic sequence is called a star’s spectral class. This order is called the spectroscopic sequence. As a result, stars today are classified using the following letters: O B A F G K M. At first, there was a system of categorizing stars in alphabetical order, but as scientists learned more about stars, the letters were rearranged and many were dropped. Stars were originally classified by the light they emit. If a collapsed gas cloud falls short of having enough mass to start hydrogen fusion, it is called a brown dwarf. A star that is fusing hydrogen into helium is called a main sequence star. This depends upon the size of the original collapsing gas cloud. The most important factor in determining different types of stars is the mass–or amount of matter–a star contains. ![]() While all stars begin this way, from this point stars can look very different from each other. When it heats up enough to begin nuclear fusion–the creation of helium from its own internal hydrogen–it’s called a star. Stars of all kinds start the same way–a cloud of gas in space begins to collapse on itself.
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