Lecture Notes on Star Brightness and Temperature

Key Concepts

• Brightness of Stars:

  • Fundamental properties include temperature and color.
  • Stars are closely related to perfect black bodies.
  • Color and temperature are interconnected with light's particle and wave nature.

• Davies Law and Black Bodies:

  • Black bodies change color with temperature.
  • Stars are the closest to perfect black bodies.
  • Stars' color changes are observable with good conditions and equipment.

Observations of Orion

• Orion Constellation:
  • Example of stars across the visible spectrum.
  • Betelgeuse (Alpha Orionis): Distinctly orange-red.
  • Rigel (Beta Orionis): Bluish-white.
• Orion's Belt Stars: Bluish-white in color.
• Orion Nebula: Appears reddish due to being a star-birth region.

Star Colors and Temperatures

• Night Sky Observations: Various colors visible with equipment.
• Colors & Temperatures:
  • Bluish-white Stars: Surface temperatures ≥ 10,000 K.
  • Yellow Stars (e.g., Sun): Surface temperatures 5,000 - 6,000 K.
  • Reddish Stars: Surface temperatures 3,000 - 4,000 K.
• Temperature to Fahrenheit Conversion: Approximate by doubling Kelvin value.

Black Body Curves

• Characteristics:
  • Stars like Rigel and Sirius peak in the ultraviolet spectrum.
  • Appear bluish white due to higher blue light emission.
• Sun & Capella:
  • Peak in yellow-green area.
  • Capella slightly cooler and more golden.
• Betelgeuse: Peaks in infrared; visible red flux is dominant.

Measuring Star Temperatures

• Challenges:
  • Earth’s atmosphere absorbs ultraviolet and infrared light.
  • Difficult to measure full spectrum from Earth.
• Solutions:
  • Use visible spectrum measurements (blue and red) to fit black body curves.
  • Stars approximate perfect black bodies; two measurements can estimate temperature.

Electromagnetic Spectrum and Stars

• Spectrum Range:
  • Stars emit across radio to ultraviolet.
  • Hottest stars may peak in x-ray.
• Measurement Limitations:
  • Atmospheric absorption limits peak emission observations.

Black Body Curve Fitting

• Process:
  • Measure at two wavelengths and fit to blackbody curve.
  • Utilize mathematical shape of blackbody curves for accurate fitting.

B-V Index in Astronomy

• B-V Index Explanation:
  • Measures difference between blue (B) and visible (V, yellow) light.
  • Cool Stars (3,000 K): Negative B-V index.
  • 10,000 K Stars: B-V index near zero.
  • Hot Stars: Positive B-V index.
• Astronomy Applications:
  • B-V index used to describe star characteristics and study curves.

Conclusion

• Summary:
  • Understanding star color and temperature through blackbody curves.
  • Importance of B-V index in studying star properties.
• Next Lecture: Will further explore these concepts.