Overview
This lecture covers the classification of stars by their spectral type and the use of the Hertzsprung-Russell (HR) diagram to organize and understand stellar properties, including temperature, luminosity, radius, and evolutionary stages.
Spectral Classification of Stars
- A star's spectrum shows absorption lines due to atoms or molecules in its cooler, outer layers.
- Types of spectral lines indicate a star's surface temperature: ionized elements for hot stars, molecules for cool stars.
- Stars are classified by spectral types in order of decreasing temperature: O, B, A, F, G, K, M.
- Each spectral type is divided into numbered subcategories (e.g., B0-B9); higher numbers are cooler.
- The Sun is a G2 star, slightly hotter than G3 but cooler than G1.
Properties of Spectral Types
- O-type stars are hottest and bluest; M-type stars are coolest and reddest.
- Spectral lines change gradually between types; hot stars have strong hydrogen lines, cool stars show molecules.
- Most stars are cool, red M-type; hot blue O-type stars are rare.
- Almost all stars have similar chemical composition: ~75% hydrogen, ~25% helium, ~1% heavier elements.
Determining Stellar Properties
- The luminosity (L), radius (R), and temperature (T) of a star are related: L = 4πR²σT⁴.
- Cool stars can be luminous if they have large radii; small, hot stars can have low luminosity.
- Stellar radius increases from lower left (small, hot) to upper right (large, cool) on the HR diagram.
The Hertzsprung-Russell Diagram (HR Diagram)
- Plots stellar luminosity (vertical axis) vs. surface temperature or spectral type (horizontal axis).
- Main Sequence: diagonal band with stars fusing hydrogen in their cores (90% of stars).
- Giants & Supergiants: luminous, cool stars in the upper right; very large radii.
- White Dwarfs: hot, dim, small stars in lower left; remnants of dead stars.
Stellar Classification: Luminosity Classes
- Luminosity class (I to V) describes a star’s region on the HR diagram, closely related to its size.
- Main sequence (V), giants (III), and supergiants (I); intermediate classes (II, IV).
- Complete classification combines spectral type and luminosity class (e.g., Sun = G2 V).
Spectroscopic Parallax and Stellar Distances
- Spectroscopic parallax uses spectral type and luminosity class (from HR diagram) with apparent brightness to estimate stellar distance.
- Useful for stars too far for direct parallax measurements.
Stellar Mass and Its Effects
- Masses of stars in binary systems are determined using Kepler’s third law with their orbital period and separation.
- For main sequence stars, mass determines luminosity, temperature, size, and lifetime: more massive stars are brighter, hotter, larger, and have shorter lifespans.
Key Terms & Definitions
- Spectral Type — Classification based on absorption lines, linked to temperature (OBAFGKM).
- Main Sequence — Stars fusing hydrogen in their cores, forming a diagonal band on the HR diagram.
- HR Diagram — Plot of luminosity vs. temperature/spectral type; shows star groups and evolution.
- Luminosity Class — Roman numeral indicating a star's size and luminosity grouping.
- Spectroscopic Parallax — Method to estimate distance to a star using its spectrum and apparent brightness.
- Binary Stars — Two stars orbiting each other, allowing mass determination.
- Mass-Luminosity Relation — More massive main sequence stars are more luminous.
Action Items / Next Steps
- Review the HR diagram and practice locating stars of different classifications.
- Study the relationships between luminosity, temperature, and radius.
- Complete any assigned readings on the nature and classification of stars.
- Prepare questions for clarification on spectral types and HR diagrams.