8. The Lives of Stars

    • The Hertzsprung-Russell diagram, or H-R diagram for short, is a plot of the luminosity of stars against their spectral class or effective disk temperatures.

    • The main sequence of the H-R diagram extends diagonally from hot, luminous stars on the upper left to cool ones of low luminosity in the lower right.

    • The red giant stars are luminous stars with cool disk temperatures found in the upper right side of the H-R diagram.

    • Supergiant stars are very rare, extremely luminous, and very massive. They are located along the top of the H-R diagram.

    • The white dwarf stars are hot stars of low luminosity, found in the bottom left side of the H-R diagram.

    • The luminosity class and spectral class of a star can be combined to determine the starís luminosity and effective disk temperature.

    • A star begins to shine when it arrives on the main sequence of the H-R diagram. Very massive stars are found on the upper left side of the main sequence, and they have short lifetimes of about 10 million years. Low mass stars are found in the bottom right side of the main sequence, and they can have lifetimes as long as 10 billion to 100 billion years.

    • Main-sequence stars shine by hydrogen burning, or the nuclear fusion of protons into helium nuclei, in their hot centers.

    • The Sun and all other main-sequence stars with a mass less than about 1.5 times the mass of the Sun shine by the proton-proton chain of hydrogen-burning nuclear reactions.

    • Main-sequence stars of more than 1.5 times the Sunís mass shine by the CNO cycle of hydrogen-burning nuclear reactions in which carbon acts as a catalyst in the conversion of protons into helium nuclei.

    • When a main-sequence star has exhausted its supply of hydrogen fuel, the starís core collapses and heats up to a temperature that is high enough to burn helium nuclei into carbon nuclei. At the same time, the outer atmosphere of the star expands and cools to form a red giant star.

    • Stars within star clusters are all of about the same age and initial chemical composition.

    • The H-R diagram of globular star clusters can be used to delineate the course of stellar evolution. The most luminous and massive stars, found in the upper left part of the main sequence, are the first to leave the main sequence; their turnoff point in the H-R diagram can be used to clock the age of the star cluster.

    • There is an exponential decline in the observed abundance of the elements in the Sun with increasing atomic weight. The most abundant element is hydrogen, the lightest element.

    • The hydrogen and most of the helium in the Sun and other stars was formed in the first moments of the big bang that gave rise to the expanding universe.

    • Carbon, oxygen, and nitrogen are synthesized inside giant or supergiant stars. Heavier elements originate during the explosive death of very massive stars.

    • The first stars could only contain hydrogen and helium, for no heavier elements had yet been formed. The second generation of stars, including the Sun, originated from interstellar material that had been seeded with heavier elements synthesized within massive, first-generation stars.

    • The oldest first generation stars can be nearly 14 billion years old, the approximate age of the expanding universe. The Sun is 4.6 billion years old and a second generation star.

Copyright 2010, Professor Kenneth R. Lang, Tufts University