5.5 Formation of Spectral Lines

Spectral lines reveal the secrets of celestial objects. Emission lines form when excited atoms release energy, while absorption lines appear when cooler gases absorb specific wavelengths. These processes are key to understanding the composition and conditions of stars and galaxies.

Ions play a crucial role in astronomical spectra. The degree of ionization in gases depends on temperature and density. By analyzing the presence and strength of spectral lines from different ionization levels, astronomers can determine the temperature of distant celestial bodies.

Formation of Spectral Lines

Formation processes of emission and absorption line spectra

• Emission line spectra form when atoms in a low-density gas become excited by an energy source, causing electrons to jump to higher energy levels, then as electrons fall back to lower energy levels, they emit photons at specific wavelengths corresponding to the energy difference between the levels, creating bright emission lines in the spectrum (hydrogen gas excited by an electric current)
• Absorption line spectra form when a continuous spectrum from a hot, dense source (interior of a star) passes through a cooler, low-density gas (outer layers of a star), atoms in the cooler gas absorb photons at specific wavelengths, causing electrons to jump to higher energy levels, creating dark absorption lines in the spectrum at the same wavelengths as the emission lines would appear (Fraunhofer lines in the solar spectrum)
• These processes are fundamentally linked to atomic structure and energy transitions

Ions in astronomical contexts

• Ions are atoms that have lost or gained electrons, resulting in a net positive (cations) or negative (anions) charge
• Ionization occurs when an atom absorbs enough energy to overcome the binding energy of one or more electrons, which can be caused in astronomical contexts by:
  1. High-energy photons (ultraviolet, X-ray, or gamma-ray) from hot stars or other energetic sources (supernova remnants)
  2. Collisions with other particles in high-temperature environments (cores of stars, accretion disks around black holes)
• The degree of ionization depends on the temperature and density of the gas:
  • Higher temperatures lead to more ionization as atoms have more kinetic energy to overcome electron binding energies (hot stellar cores)
  • Lower densities favor ionization as there are fewer collisions between particles that can cause recombination or electrons rejoining with ions (interstellar medium)

Spectral indicators of gas temperature

• The presence and strength of spectral lines from different ionization levels indicate the temperature of a gas (planetary nebulae, H II regions)
• Lower ionization levels (neutral atoms, singly ionized atoms) are more prevalent in cooler gases, so spectral lines from these species will be stronger in the spectra of cooler objects (red giants, molecular clouds)
• Higher ionization levels (doubly ionized atoms, triply ionized atoms) are more prevalent in hotter gases, so spectral lines from these species will be stronger in the spectra of hotter objects (blue supergiants, active galactic nuclei)
• The relative strengths of spectral lines from different ionization levels estimate the gas temperature:
  1. If lines from both neutral and singly ionized atoms are present, the gas temperature is likely intermediate (solar photosphere)
  2. If lines from highly ionized atoms dominate, the gas temperature is likely very high (solar corona)
• The presence or absence of certain spectral lines can also indicate temperature ranges as some elements have ionization energies that correspond to specific temperature thresholds, so if lines from a particular ionization level are observed, the gas temperature must be at least high enough to produce that level of ionization (He II lines indicate temperatures above 50,000 K)

Fundamental concepts in spectral line formation

• The electromagnetic spectrum encompasses all types of electromagnetic radiation, from radio waves to gamma rays
• Blackbody radiation provides the continuous spectrum background against which spectral lines are observed
• Spectroscopy is the primary tool used to analyze the composition and properties of astronomical objects through their spectral lines
• Quantum mechanics explains the discrete energy levels in atoms and the specific wavelengths of spectral lines
• Atomic excitation occurs when an atom absorbs energy, causing electrons to move to higher energy levels, which is crucial for both emission and absorption processes