Newswise — For astronomers, stars are as individual as people, each exhibiting different characteristics and qualities.

"As we study stars, we are understanding that they are as individual as human beings," said Richard O. Gray, a professor of astronomy at Appalachian State University.

"While the traditional classification system of stars was limited to temperature and luminosity, we are realizing that there are so many other ways that stars can vary and be individuals."

For instance, astronomers have discovered that some stars have enhanced abundances of various chemical elements, such as carbon, barium or silicon. "There are many other sorts of peculiarities. Stellar astronomers are interested in studying peculiar stars as a way to understand normal stars," Gray said.

Gray uses a spectrograph to analyze the spectra of stars. He specializes in stellar spectral classification, and is the lead author of the newly released "Stellar Spectral Classification" published by Princeton University Press. The text, coauthored with Christopher J. Corbally, replaces a decades-old book on spectral classification still in use in many college classrooms and observatories.

The new book describes how stars are classified by temperature, luminosity and chemical composition. It also includes detailed descriptions of spectral classes that did not exist 10 years ago, such as those for brown dwarfs. "There have been many advances in the field since the last book was published more than 20 years ago," Gray said.

"All sciences engage in classification," he said. "Biologists classify plants and animals. Geologists classify rocks. Astronomers classify stars by examining their spectra. We can tell a star's temperature, its age, if it's a dwarf, giant or supergiant by analyzing the spectrum."

Once stars are classified, they can be sorted into like groups. "Are these groups of stars related, are they different? Making those connections is the beginning of understanding," Gray said.

Gray is particularly interested in barium dwarfs, which show large overabundances of the elements barium and strontium. All appear to be wobbling in space, apparently due to the gravitational effects of an unseen stellar companion that astronomers theorize is a white dwarf. Gray is looking for direct evidence of the white dwarfs by analyzing images of barium stars taken by GALEX (Galaxy Evolution Explorer), an orbiting space ultraviolet telescope.

"These stars are similar to the sun, but our sun doesn't have similar abundances of barium. The question is why," Gray said. Some astronomers theorize that the barium stars' chemical composition is the result of mass transferred from a dying companion star that is now a white dwarf.

"When we study and drill a little deeper, we learn interesting facts about how stars evolve, how they are born, how they interact with other stars," Gray said. "There are so many things that we can learn by looking at these peculiar objects."

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Stellar Spectral Classification