Overview
The lecture discusses how stars are classified by their spectra, the historical context of this classification, and details about spectral types and their characteristics.
Star Classification by Spectrum
- Stars can be classified by the pattern of light (spectrum) they emit.
- Despite billions of stars, most fall into a few types based on spectral similarities.
- This classification system is called “spectral type.”
History of Spectral Classification
- Spectral types were identified by women astronomers in the early 1900s, notably Annie Jump Cannon.
- The system is sometimes called the Henry Draper classification, but Annie Jump Cannon did much of the key work.
- Annie Jump Cannon developed the categories and organized stars by their spectra and temperature.
The Seven Spectral Types
- Stars are divided into seven spectral types: O, B, A, F, G, K, M, from hottest to coldest.
- O stars are the hottest, M stars are the coolest.
- The types are ordered by temperature, but the letters are non-sequential due to historical changes.
- A mnemonic to remember the order is: “Oh Be A Fine Girl/Guy Kiss Me.”
Spectral Types, Colors, and Temperatures
- O and B stars are blue and very hot.
- A, F, and G stars are yellow to orange, with G stars (like our Sun) being yellow.
- K and M stars are cooler and appear orange to red.
- The arrangement of spectral types reflects increasing absorption lines and decreasing temperature.
Key Terms & Definitions
- Spectrum — The pattern of light emitted by a star, displaying various lines and colors.
- Spectral Type — Classification category for a star based on its spectrum.
- Annie Jump Cannon — Astronomer who established the modern spectral classification system.
- Mnemonic — A memory aid phrase; here, “Oh Be A Fine Girl/Guy Kiss Me” for spectral type order.
Action Items / Next Steps
- Memorize the order of spectral types: O, B, A, F, G, K, M.
- Practice using the mnemonic to recall spectral type order for exams.
- Review the color/temperature associations for each spectral type.