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Chapter 15

Astronomy Chapter 15 Notes.pdf

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Western University
Astronomy 1021
Trevor Kirsh

Chapter 15: Surveying the Stars • The apparent brightness of a star in our Classify the diverse families of sky depends on both its luminosity and its distance from Earth expressed by stars visible in the night sky the inverse square law for light • Luminosity is the total amount of light it • All stars are made primarily of emits into space hydrogen and helium at the time they • It is measured in power form • We can therefore calculate a star’s • The difference between stars are due luminosity from its apparent brightness to differences in mass and stage of life and distance, we can measure the • Stars spend most of their lives as main latter with stellar parallax sequence stars that fuse hydrogen into Most distance stars have smaller helium in their cores • parallax angles • The most massive stars, which are also • We can calculate a star’s distance if we the hottest and most luminous, live only know the precise amount of the star’s a few million years annual shift due to parallax • The least massive stars are the coolest • Luminosity is irrelevant to their distance and dimmest, will survive until the to Earth universe is many times its present age Apparent brightness is how bright it The key to recognizing the patterns • • appears to our eyes among stars was the H-R diagram, • Stars have a wide range of luminosities which shows stellar surface and our Sun is somewhere in the temperatures on the horizontal axis and middle luminosities on the vertical axis • Dim stars are far more common than • The H-R diagram is one of the most bright stars important tools of modern astronomy • Much of what we know about the How do we measure stellar universe comes from studies of star temperature? clusters • We measure a star’s surface • We can measure a star cluster’s age by temperature from its colour or plotting its stars on an H-R diagram spectrum, and we classic it according and determining the hydrogen burning to the sequence of spectral types, lifetime of the brightest and most OBAGFKM massive stars still on the main • Spectra types are lines that represent a sequence star’s spectrum that measures the • We measure the apparent brightness of surface temperature a star with energy per second reaching • This runs from the hottest to coolest. us per uni area • Cool, red stars of spectral type M are • The technical term for this is flux much more common than hot, blue stars of the spectral type O 15.1 How do we measure stellar masses? • We can measure the masses of stars in How do we measure stellar binary star systems using Newton’s luminosities? version of Kepler’s third law if we can measure the orbital period and separation of the 2 stars What is the significance of the main • binary star system is when 2 stars sequence? continually orbit one another •Our Sun falls along the main sequence, • A visual binary is a pair of stars that we a line of stars on the HR diagram can see distinctly as they orbit each •Stars on the main sequence are all other fusing hydrogen into helium in their • An eclipsing binary are a pair of stars cores, and a star’s position along the that orbit in the plane of our line of sight main sequence depends on its mass • We can find the binary nature of a star •High mass stars are at the upper left by using a system called spectroscopic end of the main sequence and the binary system
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