Overview
This lecture explains how luminosity and apparent brightness are measured, focusing on the units used for each and the significance of the magnitude system in astronomy.
Units of Luminosity
- Luminosity is the total light energy a star produces per second.
- The standard unit for energy is the joule (J).
- Luminosity is measured in joules per second, also called a watt (W).
- Using watts for stars is impractical because stars are extremely bright; the Sun is about 10³³ watts.
- To simplify, astronomers use "solar luminosity," where the Sun's luminosity is defined as one solar luminosity (1 L☉).
- Other stars' luminosities are compared to the Sun as multiples of solar luminosity.
Measuring Apparent Brightness: Magnitude System
- Apparent brightness is measured using the "magnitude" system.
- Magnitude was created by the ancient Greek astronomer Hipparchus.
- Brightest stars were assigned magnitude 1; stars half as bright got magnitude 2, and so on up to magnitude 6.
- Magnitude numbers increased for dimmer stars; a higher magnitude means a dimmer star.
- The system extends past 6 with the use of telescopes, allowing modern astronomers to see much fainter objects.
- The magnitude system is "flipped": low magnitudes = bright stars, high magnitudes = dim stars.
- Test questions may ask which object has higher apparent brightness (brighter star) vs. higher magnitude (dimmer star).
Key Terms & Definitions
- Luminosity — Total light energy produced by an object per second.
- Joule (J) — Unit of energy.
- Watt (W) — A joule per second; measures power/luminosity.
- Solar Luminosity (L☉) — Luminosity of the Sun; standard unit for stellar luminosity.
- Apparent Brightness — How bright an object appears from Earth.
- Magnitude — Unit for measuring apparent brightness; higher magnitude means dimmer star.
- Apparent Magnitude — The specific term for magnitude as seen from Earth.
Action Items / Next Steps
- Be able to explain why higher magnitude means a dimmer star for upcoming tests.
- Record that there will be a test question on the flipped nature of the magnitude system.
- Optional: Review how the magnitude scale works for extremely dim objects.