Stellar Luminosity Classes
● Astar’s full classification includes spectral type (line identities) and luminosity class (line
shapes, related to the size of the star):
I - Supergiant
II - Bright Giant
III - Giant
IV - Subgiant
V - Main-Sequence
Example: Sun - G2 V
Sirius - A1 V
Proxima Centauri - M5.5 V
Betelguese - M2 I
H-R Diagram depicts:
● Spectral Type
Significance of the Main-Sequence
● Main-sequence stars are fusing hydrogen into helium in their cores like the Sun.
● Luminous main-sequence stars are hot (blue).
● Less luminous ones are cooler (yellow or red)
● Mass measurements of the main-sequence stars show that hot, blue stars are much more
massive than the cool, red stars.
● The mass of a normal hydrogen burning star determines its luminosity and its spectral
● Core pressure and temperature of a higher-mass star need to be larger in order to balance
● Higher core temperature boosts fusion rate, leading to larger luminosity.
Stellar properties review
● Luminosity: from brightness and distance.
● Temperature: from color and spectral type.
● Mass: from period (p) and average separation (a) of binary star orbit.
Mass and Lifetime
● Sun’s life expectancy: 10 billion years (until core hydrogen is used up)
● Life expectancy of 10Msun star: 10 million years
○ 10 times as much fuel, uses it 10^4 times as fast.
○ 10 million years = 10 billion years x 10/10^4.
● Life expectancy of a 0.1Msun star: 100 billion years
○ 0.1 times as much fuel, uses it 0.01 times as fast.
○ 100 billion years = 10 billion years x 0.1/0.01 Main-Sequence Star Summary
● High-Mass Star:
○ High luminosity
○ Larger Radius
● Low-Mass Star:
○ Low Luminosity
○ Small Radius
Giants, Supergiants, and White Dwarfs
Off the Main-Sequence
● Stellar properties depend in both mass and age: stars that have finished fusing H and He
in their cores are no longer on the main sequence.
● All stars become larger and redder after exhaus