AST-1A Lecture Notes - Lecture 30: Roman Numerals, Low Mass, Fusion Power
Introduction to the Solar System
Notes
Joseph Yang
Patterns among Stars
● Most of the very brightest stars are reddish colour
● There’s a general trend to luminosities and colours: brighter ones are white with a bit of
blue ting, modest are yellow and dimmest are red
● Colours tell us about surface temperature
● H-R diagrams are one of the most important tools
○ Horizontal axis rep surface temperature: decreases from left to right
○ Vertical axis rep stellar luminosity
○ Stars near the upper left are hot and luminous, and stars near upper right are cool
and luminous
○ Also provides info on stellar radii
■ More luminous = larger in size
● Patterns in HR Diagrams
○ Most stars fall under main sequence: upper left to lower right
○ Supergiants are located in the upper right
○ Giants are just below super giants
○ Dwarf stars are located in the lower left
● Luminosity Classes
○ Classified with a roman numeral
○ I for supergiants
○ III for giants
○ V for main sequence
○ II and IV for others
● Complete Stellar Classification
○ A stars spectral type (OBAFGKM)
○ Luminosity class (roman numeral)
○ Our sun is G2 V
● Main Sequence Stars
○ Stellar masses decrease downward
○ Low mass stars are more common
○ Mass is important attribute of a hydrogen fusing star
○ Mass determines balance point
○ Very luminous star must be very large or have a vary high surface temperature
○ Not only main sequence stars follow this simple relationship between mass, temp,
and luminosity
■ Doesn’t hold for giants, super giants, or white dwarfs
● Lifetime of Main Sequence Stars
○ Stars are born with limited amounts of hydrogen
○ Main sequence lifetime: when a star is hydrogen fusing
○ Massive stars have shorter lives
○ Dependent on both its mass and luminosity
■ Mass determines how much hydrogen fuel is initially in the core
Document Summary
Most of the very brightest stars are reddish colour. There"s a general trend to luminosities and colours: brighter ones are white with a bit of blue ting, modest are yellow and dimmest are red. H-r diagrams are one of the most important tools. Horizontal axis rep surface temperature: decreases from left to right. Stars near the upper left are hot and luminous, and stars near upper right are cool. Also provides info on stellar radii and luminous. Most stars fall under main sequence: upper left to lower right. Supergiants are located in the upper right. Dwarf stars are located in the lower left. Mass is important attribute of a hydrogen fusing star. Very luminous star must be very large or have a vary high surface temperature. Not only main sequence stars follow this simple relationship between mass, temp, and luminosity. Doesn"t hold for giants, super giants, or white dwarfs. Stars are born with limited amounts of hydrogen.
