Stellar Luminosity Classes

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University of Massachusetts Amherst
Alexandra Pope

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: ● Temperature ● Color ● Spectral Type ● Luminosity ● Radius ● (Mass) 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 type. ● Core pressure and temperature of a higher-mass star need to be larger in order to balance gravity. ● 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 ○ Short-Lived ○ Larger Radius ○ Blue ● Low-Mass Star: ○ Low Luminosity ○ Long-Lived ○ Small Radius ○ Red 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
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