![]() They belong to different luminosity classes within that spectral type, as determined from their spectra. Two stars with similar effective temperatures but greatly different luminosities must therefore differ in size. Luminosity is proportional to the fourth power of the temperature (T 4). Luminosity (L) is related to the absolute magnitude (M V) of a star, and is the total amount of energy radiated per second. The differences in spectral lines among stars having the same spectral type are a function of the radius of the star, which results in different luminosities. ![]() Spectral lines can show different characteristics within the same spectral type or temperature (T), and so a second type of classification system for stars was devised using luminosity. DQ white dwarfs have a carbon-rich atmosphere as indicated by atomic or molecular carbon spectral lines. The major classifications have subclasses – Class D is divided into 7 different subtypes of white dwarfs based upon variations in the composition of their atmospheres, e.g. Class D (degenerate) is the modern classification for white dwarfs. The carbon stars (C) include stars that were originally classified as R and N stars. These include Wolf-Rayet stars (W), cool dwarfs (L), brown dwarfs (T), carbon stars (C), and stars with zirconium oxide lines that are between M and C stars (S). Though O B A F G K and M are the stellar classifications commonly shown on H-R diagrams, a number of new and extended spectral classes have been designated. Each major spectral classification is characterized by its own unique spectra. The hottest B stars are B0 and the coolest are B9, followed by spectral type A0. These categories are further subdivided into subclasses from hottest (0) to coolest (9). Most stars are classified by temperature (spectral type) from hottest to coolest as follows: O B A F G K M. Harvard Classification System Spectral Images One class of stars – the pulsating variables which include Cepheids, RR Lyraes, Semiregulars and Miras – occupy regions of instability on the H-R diagram and represent transitional periods between stages of evolution. Stages of stellar evolution occupy specific regions on the H-R diagram and exhibit similar properties. The H-R diagram is a scatter graph of stars – a plot of stellar absolute magnitude or luminosity versus surface temperature or stellar classification. Astrophysicists observe numerous stars at various stages in their evolutionary history to determine their changing properties and probable evolutionary tracks across the H-R diagram. Stellar evolution can not be studied by observing individual stars as most changes occur over several millions and billions of years. In the early 1900’s Ejnar Hertzsprung and Henry Norris Russell developed the Hertzsprung - Russell diagram (H-R diagram) – an important astronomical tool that represented a major step towards understanding how stars evolve over time. The Small Magellanic Cloud (Chandra Image)
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