Overview
This lecture explains how to determine a star’s mass and its impact on other star properties, especially focusing on main sequence stars and their classification.
Determining the Mass of a Star
- Mass is the amount of material in a star and is determined indirectly.
- We infer a star’s mass by observing how its gravity affects nearby objects.
- Increased mass means increased gravity.
Mass and Star Properties (Main Sequence Stars)
- Higher mass in a star leads to a larger size.
- Greater mass causes stronger gravitational compression, resulting in a hotter core temperature.
- A hotter star core increases fusion rates, which raises the star’s luminosity (brightness).
- For main sequence stars, higher mass means all major properties (size, temperature, luminosity) increase together.
- Main sequence stars are stars that are still fusing hydrogen in their cores and have not yet started dying.
Spectral Types and Property Trends
- Spectral types classify stars from hottest to coolest: O, B, A, F, G, K, M.
- For main sequence stars:
- O and B types are hot, high mass, large, and very luminous.
- M types are cool, low mass, small, and dim.
- Stars like the Sun are G types and are medium in all properties.
- If comparing stars: the closer the spectral type to O, the larger and brighter the star is; the closer to M, the smaller and dimmer.
Frequency of Star Types
- M stars (cool, small, dim) are far more common than O stars (hot, big, bright).
- Most stars in the universe are small and faint rather than large and bright.
Key Terms & Definitions
- Mass — The total amount of material in a star.
- Luminosity — The brightness of a star.
- Main Sequence Star — A star in the stable, hydrogen-fusing stage of its life.
- Spectral Type — Classification of stars by temperature and color (O, B, A, F, G, K, M).
Action Items / Next Steps
- Review the order and properties of spectral types (O, B, A, F, G, K, M).
- Practice identifying which star properties are higher or lower based on spectral type for main sequence stars.
- Prepare for test questions comparing star masses, sizes, and luminosities.
- Stay tuned for upcoming lecture on dark matter.