Charting the Stars
Astronomers classify stars to understand their properties and evolutionary stages.
Stellar Spectral Classification
Stars are classified based on their surface temperature by examining their absorption spectra. The modern classification system is the OBAFGKM sequence.
The sequence runs from hottest to coolest.
O: Hottest, blue-white stars (> 30,000 K).
B: Hot, blue-white stars.
A: White stars.
F: Yellow-white stars.
G: Yellow stars, like our Sun (a G2 star).
K: Cool, orange stars.
M: Coolest, red stars (< 3,500 K).
A common mnemonic is 'Oh Be A Fine Guy/Girl, Kiss Me!'
The Hertzsprung-Russell (H-R) Diagram
The H-R diagram is a scatter plot that is the single most important tool for understanding stellar evolution.
Vertical Axis (Y-axis): Luminosity (intrinsic brightness) or Absolute Magnitude. Bright stars at the top, dim stars at the bottom.
Horizontal Axis (X-axis): Surface Temperature (or spectral class). Uniquely, temperature decreases from left to right (hot O-type stars are on the left, cool M-type stars are on the right).
Major Regions of the H-R Diagram
1.Main Sequence: A prominent diagonal band from the upper-left (hot, luminous) to the lower-right (cool, dim). About 90% of stars, including the Sun, lie here. These are stable stars fusing hydrogen in their cores.
2.Red Giant Branch: A region in the upper-right. These stars are cool (red) but highly luminous, meaning they must have a very large radius. They are evolved stars that have exhausted their core hydrogen.
3.Supergiants: The most luminous stars, located at the very top of the diagram.
4.White Dwarfs: A region in the lower-left. These stars are very hot but have low luminosity, meaning they must be very small (about the size of Earth). They are the dense, remnant cores of dead low-mass stars.