Lecture Notes: Redshift in Astrophysics (AQA A Level Physics)

Introduction

• Topic: Redshift in astrophysics
• Objective: Use redshift to understand astronomical distances.
• Goals:
  • Understand the Doppler effect
  • Detail examples of redshift in astrophysics
  • Apply formulae for redshift from observations

AQA A-Level Physics Specification 3.9.3.1: The Doppler Effect

• All electromagnetic waves have frequency and wavelength.
• Doppler Effect: Change in observed wavelength/frequency due to motion of observer or emitter.
  • Illustrated with the example of a siren on a passing vehicle.
  • It affects observed properties, not actual properties of the wave.

The Doppler Effect

• Mechanism: Relative motion between observer and emitter changes detected wave properties.
  • Moving closer decreases space between emitter and observer, increasing frequency and decreasing wavelength.
  • Moving apart increases space, decreasing frequency and increasing wavelength.
• Applications: Applicable to all waves, including visible light.

Redshift and Blueshift in Light

• Color Perception:
  • Long wavelength -> red
  • Short wavelength -> blue
• Redshift: Light appears redder when source moves away.
• Blueshift: Light appears bluer when source moves closer.

Redshift in Astronomy

• Observing Distant Objects:
  • Compare spectral lines from emission with known sources (labs or sun) to detect red/blue shifts.
  • Common reference: Balmer lines of hydrogen.

Calculating Redshift

• Formula:
  • z = (Δλ/λ) = (Δf/f) = v/c
  • Condition: v << c (speed of galaxy << speed of light)
• Interpreting z:
  • Positive z -> redshift
  • Negative z -> blueshift
  • z = 0 -> present time
• Examples: Highest redshift from Cosmic Microwave Background (z=1089), highest for a galaxy (z=11.1).
• Sample Problem: Determining velocity and direction of a star from spectral line difference.

Types of Redshift

• Cosmological Redshift:
  • Detected in distant galaxies, evidence for universe expansion and Big Bang model.
• Stellar Redshift:
  • Detected in nearby stars, helps determine star properties.

Stellar Redshift and Rotation

• Binary Star Systems:
  • Two stars orbiting a common center of mass.
  • Redshift and blueshift help confirm true binary systems.
• Observing Rotational Motion:
  • Blue shift when star rotates toward us, red shift when it rotates away.
• Calculating Orbital Speed and Diameter:
  • Use Doppler equation and circular motion equation.

Summary

• Key Equations and Concepts:
  • Δf/f = v/c, z = Δλ/λ = -v/c
  • Mainly applicable when v << c
• Skills:
  • Calculate redshift and velocities for binary stars and rotational motion.

Conclusion

• Learning Outcomes:
  • Understand and explain the Doppler effect and redshift
  • Apply redshift formulae in astrophysical contexts
• Next Steps:
  • Explore cosmological redshift in the next lesson.

Thank you for attending the lecture on redshift in astrophysics.