Astronomical Spectroscopy and Spectra

Overview

This lecture covers how astronomers use light and spectra to understand the properties and composition of astronomical objects, introducing key concepts like spectroscopy, types of spectra, blackbody radiation, and the Doppler effect.

Spectroscopy and Spectra

• Matter leaves unique "fingerprints" in light via absorption and emission at specific wavelengths.
• Spectroscopy is the process of obtaining and analyzing spectra to determine properties of distant objects.
• Spectra are often displayed as graphs showing light intensity versus wavelength.

Types of Spectra

• Continuous Spectrum: Produced by dense objects (like light bulbs or stars); shows an unbroken rainbow of colors.
• Emission Line Spectrum: Produced by hot, low-density gas; shows bright lines at specific wavelengths.
• Absorption Line Spectrum: Produced when light passes through a cooler gas; shows dark lines on a continuous background.

Spectral Fingerprints & Element Identification

• Each element produces a unique pattern of spectral lines, acting as a "fingerprint" for identification.
• Spectral analysis allows identification of chemical composition in both laboratory samples and distant celestial objects.

Blackbody (Thermal) Radiation

• Dense objects (stars, planets, people) emit blackbody radiation, a continuous spectrum depending only on temperature.
• Blackbody spectra follow two key laws:
  • Stefan-Boltzmann Law: Hotter objects emit more light per unit area at all wavelengths (E = σT⁴).
  • Wien's Law: Hotter objects emit light with a shorter peak wavelength (λ_max = constant / T).
• The color of an object relates to its temperature: cool objects peak in infrared/red, hotter ones in visible/blue, and very hot objects in ultraviolet/X-rays.

Practical Applications of Spectra

• The Sun's spectrum is close to a blackbody curve with absorption lines, indicating a temperature of ~5,800 K and absorption by the solar atmosphere.
• The difference between an object's spectrum and a pure blackbody curve reveals chemical composition.

The Doppler Effect in Astronomy

• The Doppler effect shifts spectral lines based on motion:
  • Moving toward us: blue-shift (shorter wavelengths).
  • Moving away: red-shift (longer wavelengths).
• Rotating objects broaden their spectral lines; the amount of broadening reveals rotation speed.

Example: Analyzing Mars’ Spectrum

• Mars' spectrum shows little blue (absorbed), much red (reflected), thermal radiation peak at ~225 K, ultraviolet emission lines from thin atmosphere, absorption lines indicating carbon dioxide, and wavelength shifts showing its motion.

Key Terms & Definitions

• Spectroscopy — Analysis of light spectra to determine object properties.
• Spectrum — Distribution of light by wavelength/intensity.
• Continuous Spectrum — Unbroken sequence of colors/wavelengths.
• Emission Line Spectrum — Bright lines at specific wavelengths from a hot gas.
• Absorption Line Spectrum — Dark lines where light is absorbed by cooler gas.
• Blackbody Radiation — Radiation emitted by an object that absorbs all incident light.
• Doppler Effect — Shift in spectral lines due to relative motion.
• Stefan-Boltzmann Law — Energy emitted per area ∝ T⁴.
• Wien's Law — Peak wavelength ∝ 1/T.

Action Items / Next Steps

• Review and become familiar with types of spectra and their causes.
• Understand and be able to apply Stefan-Boltzmann and Wien’s Laws.
• Prepare for upcoming lectures on the Sun and broader stellar properties.
• Optional: Watch the linked video on the Doppler effect, if available.