Chapter 17 - 17.3

Most knowledge about stars comes from the light they emit.
Polaris (North Star) appears fixed in the sky due to Earth's spin axis.
Key questions include:
- What is a star's luminosity (total light output)?
- What is the star's composition and mass?
- How can we discover these characteristics with light data?

Luminosity: Energy output of a star per second.
Apparent Brightness: The light received from a star at a specific location; varies by distance and actual luminosity.
Magnitude Scale: Measures apparent brightness in units of "magnitudes."
- Originated from human eye sensitivity to light.
- Higher numbers indicate dimmer objects; negative numbers are brighter (e.g., Sun = brightest, is negative).
Magnitude System Breakdown:
- Stars classified from magnitude 1 (brightest) to magnitude 6 (dimmiest visible).
- Each step on this scale equates to a factor of 2.5 in light energy.
Tools for Measurement:
- Various telescopes (e.g., Hale Telescope, Hubble Space Telescope) measure greater apparent magnitudes; sensitivity increases with mirror size.

Star Color and Temperature:
- Color indicates surface temperature (e.g., Betelgeuse is reddish and Rigel is bluish).
- Surface temperature correlates with emitted light color; recall from Chapter 5.
Use of Wien’s Law: Allows calculation of surface temperature from light color, measuring surface rather than core temperatures.
Color Indices: Measure brightness in specific light wavelengths using filters to infer temperature without full spectrum analysis.

Stellar Spectra: Measurement of light spectrum to identify elements present in stars.
- Absorption Features: Specific to elements, can identify chemical composition of starlight.
- Temperatures influence visibility of certain absorption lines:
  - Neutral metals visible at 3,000 - 5,000 Kelvin.
  - Ionized metals and hydrogen lines apparent at higher temperatures (above 6,000 Kelvin).
Spectral Classification System:
- Class based on temperature:
  - O, B, A, F, G, K, M (from hottest to coolest).
- Subclasses (0-9) refine each category to specify temperatures.
Brown Dwarfs: Objects less massive than stars lacking nuclear fusion; fill the gap between stars and planets.

Apparent brightness is measured in magnitudes; it varies by distance and luminosity.
Colors of stars viewed at night correlate with their surface temperatures and are analyzed using spectra.
Understanding the spectra and magnitudes helps classify stars and identify various celestial phenomena.