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Lecture 7

AS 101 Lecture 7: AS101PART7


Department
Astronomy
Course Code
AS101
Professor
Shohini Ghose
Lecture
7

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StarSizes,luminosity,temperatureandDiameter
The Hertzsprung–Russell (H–R) diagram is a plot of luminosity versus
surfacetemperature.
It is an important graph in astronomy because it sorts the stars into
categoriesbysize.
• Approximately90%ofstarscanbefoundonthemainsequence.
The giants and supergiants are very large and lie above the main
sequenceontheHRdiagram.
Red dwarfs (faint, cool, lowmass stars) lie at the lower end of the main    
sequence.
White dwarfs are dying stars (which collapsed to the size of the Earth)
atthelowerleftoftheH–Rdiagram.
LuminositySpectralClassification
• Thewidthsofspectrallinescanbeusedtoclassifystars.
Smaller stars have denser atmospheres → atoms collide more often and
thus produce broader hydrogen Balmer lines (e.g., a main sequence
star).
Larger stars have more rarified atmospheres → narrower hydrogen
Balmerlines(e.g.,giantsandsupergiants).
Luminosity Class: a category of stars of similar luminosity, determined by the
widthsoflinesintheirspectra
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