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Chapter 15

Chapter 15 Cosmic Perspective

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Astronomy 1021
Chris Racknor

CHAPTER FIFTEEN Surveying the Stars All stars are made of primarily hydrogen and helium at the time they form the differences between stars are primarily due to differences in mass and stage of life Stars spend most of their lives as mainsequence stars that fuse hydrogen into helium in their cores the most massive stars which are also the hottest and most luminous live only a few million years the least massive stars which are coolest and dimmest will survive until the universe is many times its present ageThe key to recognizing the patterns among stars was the HR diagram which shows stellar surface temperatures on the horizontal axis and luminosities on the vertical axis the HR diagram is one of the most important tools of modern astronomyMuch of what we know about the universe comes from studies of star clusters we can measure star clusters age by plotting its stars on an HR diagram and determining the hydrogen burning life time of the brightest and most massive stars still on the main sequenceMeasuring Stellar Luminosities The apparent brightness of a star in our sky depends on both its luminosity which is 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 parallaxMeasuring Stellar Temperatures We measure a stars surface temperature from its colour or spectrum and we classify spectra according to the sequence of spectral types OBAFGKM which runs from the hottest to coolest cool red stars of spectral type M are much more common that hot blue stars of spectral type O Measuring Stellar Masses We can measure the masses of stars in binary star systems using Newtons version of Keplers third law if we can measure the orbital period and separation of the two stars HertzsprungRussell Diagrams An HR diagram plots stars according t
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