Chapter 15 Summary.docx

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Astronomy & Astrophysics
Michael Reid

Chapter 15 Summary: 15.1 PROPERTIES OF STARS  How do we measure stellar luminosities:  The apparent brightness of a star in our sky depends on both its luminosity -- the total amount of light it emits into space – and its distance from earth, as expressed by the inverse square law for light  We can therefore calculate a stars luminosity from its apparent brightness and distance  We can measure the latter through stellar parallax  How do we measure stellar temperatures:  We measure a stars surface temperature from its color or spectrum, and we classify spectra according to the sequence of spectral types OBAFGKM, which runs from hottest to coolest  Cool, red stars of spectral type M are much more common than hot, blue stars of spectral type O  How do we measure stellar masses:  We can measure the masses of stars in binary star systems using Newton’s version of Kepler’s third law if we can measure the orbital period and separation of the two stars 15.2 PATTERNS AMONG STARS  What is the Hertzsprung-Russell Diagram:  An H-R diagram plots stars according to their surface temperatures (or spectral types) and luminosities  Stars spend most of their lives fusing hydrogen into helium in their cores, and stars in this stage of life are found in the H-R diagram in a narrow band known as the main sequence  Giants and supergiants are to the upper right of the main sequence and white dwarfs are to the lower left  What is the significance of the main sequence:  Stars on the main sequence are all fusing hydrogen into helium in their cores, and a star’s position along the main sequence depends on its mass  High-mass stars are at the upper left end of the main sequence, and the masses of stars become progressively smaller as we move toward the lower right end  Lifetimes vary in the opposite way, because higher-mass stars live shorter lives  What are giants, supergiants, and white dwarfs:  Giants and supergiants are stars that have exhausted their core supplies of hydrogen for fusion and are undergoing other forms of fusion at a more rapid rate as they near the ends of their lives  White dwarfs are the exposed cores of st
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