OCR Specification focus:
‘Distinguish continuous, emission line, and absorption line spectra from astronomical sources.’
Spectral types reveal how astronomical objects emit and absorb electromagnetic radiation. Understanding continuous, emission line, and absorption line spectra allows physicists to infer stellar properties, compositions, and physical conditions.
Spectra Types in Astrophysics
Spectroscopy is a foundational tool in astrophysics because it enables astronomers to determine the composition, temperature, and behaviour of distant objects using the electromagnetic radiation they emit or absorb. In this subsubtopic, the OCR specification requires students to distinguish continuous, emission line, and absorption line spectra, each produced under different physical conditions.
These spectra act as diagnostic fingerprints, allowing astronomers to classify stars and identify materials present in celestial environments.
Continuous Spectra
A continuous spectrum is a smooth, unbroken spread of wavelengths across a broad range, showing no gaps or discrete features. It is characteristic of dense or opaque radiating bodies that emit thermal radiation. Stars, including the Sun, approximate this type of emission because they behave like blackbody radiators, producing a distribution of wavelengths dependent on temperature.
Continuous spectrum: A smooth and uninterrupted distribution of wavelengths emitted by a dense object, with no discrete lines or gaps.
Because continuous spectra originate from dense matter, their shape is determined primarily by the object's temperature, allowing astronomers to infer thermal characteristics. However, they do not directly provide information about elemental composition, since the radiation arises from a broad range of atomic and molecular interactions within the dense medium.
Between interacting electron transitions, collisions in high-density environments randomise emitted photon energies, thereby smearing out discrete features. As a result, continuous spectra are most strongly associated with stellar photospheres and incandescent solids or liquids.
Emission Line Spectra
An emission line spectrum consists of discrete bright lines on a dark background. It arises when atoms or ions in a low-density gas emit photons of specific energies as electrons transition from higher to lower energy levels.
Helium emission spectrum showing bright, discrete lines across the visible range. Each line corresponds to a permitted electronic transition in helium. The element-specific pattern exemplifies how emission spectra act as fingerprints (minor extra detail beyond syllabus). Source.
Emission line spectrum: A set of bright spectral lines produced when electrons in excited atoms or ions return to lower energy states, releasing photons of characteristic wavelengths.
In astrophysics, emission lines appear prominently in environments where gases are energised and excited, such as nebulae, stellar coronas, and the outer regions of active galaxies. Because each line corresponds to a unique electronic transition, emission line spectra provide a powerful means of determining the composition and ionisation state of astronomical gases.
Common features observed include:
Hydrogen Balmer lines, often seen in nebulae illuminated by hot stars.
Forbidden lines (e.g. [O III]), emitted under extremely low-density conditions not reproducible in typical laboratories.
Ionised metal lines, indicating high-energy processes.
Emission line spectra are especially useful for studying dynamic and diffuse structures where the density is insufficient to produce a continuous spectrum.
Absorption Line Spectra
An absorption line spectrum appears as a continuous background with dark lines at specific wavelengths where light has been absorbed by cooler gas along the line of sight.
Fraunhofer lines in the solar spectrum illustrate an absorption line spectrum: narrow dark features imprinted on a continuous background. The lines arise in the Sun’s cooler outer atmosphere. Labels mark prominent features for reference (solar-specific context slightly exceeds syllabus scope). Source.
Absorption line spectrum: A continuous spectrum crossed by dark lines formed when cooler gas absorbs specific wavelengths of light from a hotter background source.
Absorption lines allow astronomers to determine the composition, temperature, and density of stellar atmospheres. Because the underlying radiation is continuous, the absorption features stand out clearly, creating unique patterns that are often used for star classification.
Absorption spectra arise due to the following layered physical arrangement:
A hot, dense interior producing a continuous spectrum.
A cooler outer gas layer containing atoms capable of absorbing specific photon wavelengths.
Re-emission in random directions, causing a net loss of light at the absorbed wavelengths when viewed from Earth.
These spectra also enable more advanced measurements, including radial velocities via Doppler shifts, although the OCR specification for this subsubtopic focuses strictly on distinguishing spectral types rather than interpreting shifts.
Comparing the Three Spectra
To distinguish the three types effectively, it is important to relate each to its physical origin and observable appearance.
Physical Conditions
Continuous spectrum: Hot, dense, opaque matter (e.g. stellar surfaces, incandescent solids).
Emission line spectrum: Hot, low-density gas excited by energy input (e.g. nebulae, gas clouds around energetic stars).
Absorption line spectrum: Continuous source viewed through cooler low-density gas (e.g. starlight passing through stellar atmospheres or interstellar gas).
Observational Appearance
Continuous: Smooth blend of all wavelengths.
Emission lines: Bright, isolated lines on a dark background.
Absorption lines: Dark lines superimposed on a bright continuous background.
Astronomical Relevance
Continuous: Indicates temperature and energy distribution.
Emission: Reveals composition and excitation mechanisms in gas clouds.
Absorption: Provides detailed chemical and atmospheric information for stars and planetary atmospheres.
These distinctions form the basis of stellar spectroscopic classification and are essential for understanding the astrophysical processes that shape the observable universe.
FAQ
In low-density gases, atoms are widely spaced, so electrons interact primarily with their own nuclei rather than neighbouring particles. This prevents the broadening of energy levels caused by collisions.
As a result, electrons transition between sharply defined energy levels and emit or absorb photons of only specific wavelengths.
These discrete transitions create bright emission lines or narrow absorption lines rather than a smooth spectrum.
The intensity depends on how many atoms or ions are in the excited state that produces the line. A higher population of excited particles results in stronger lines.
Other important factors include:
Temperature of the gas
Density of the gas
Probability of the specific electronic transition (transition probability)
Presence of external radiation fields that can promote excitation
Absorption lines form in cooler, low-density gas where atoms have relatively low speeds, resulting in minimal Doppler broadening.
In contrast, nebulae often contain hotter or more turbulent gas.
This increases the range of atomic velocities and spreads out the line profile, creating broader emission features.
At high pressures, frequent collisions disturb atomic energy levels, causing both emission and absorption lines to widen and become less distinct.
This broadening can make a spectrum appear more blended, pushing it closer to a continuous shape if the density becomes sufficiently high.
Stars with deep, dense atmospheres show more pressure broadening than diffuse interstellar gas clouds
Although stars may contain similar elements, the visibility of particular absorption lines depends strongly on temperature.
Key influences include:
Ionisation levels: Hotter stars may ionise atoms so strongly that some lines disappear.
Excitation levels: Some transitions occur only when electrons occupy certain excited states.
Atmospheric structure: Variations in density and temperature gradients affect how strongly lines form.
Thus, two stars with similar compositions can display very different spectral patterns.
Practice Questions
Question 1 (2 marks)
State what is meant by:
(a) a continuous spectrum
(b) an emission line spectrum.
Mark scheme:
(a) Continuous spectrum
1 mark: States that it is an unbroken range of wavelengths with no gaps or discrete lines.
(b) Emission line spectrum
1 mark: States that it consists of bright lines at specific wavelengths produced by excited atoms or ions emitting photons.
Question 2 (5 marks)
Explain how an absorption line spectrum is produced when light from a star reaches an observer on Earth.
In your answer, describe the physical conditions needed for absorption lines to form and why the resulting spectral pattern allows astronomers to identify elements present in the star’s atmosphere.
Mark scheme:
1 mark: Light from the star originates as a continuous spectrum from the hot, dense interior or photosphere.
1 mark: Light passes through a cooler, low-density gas layer in the star’s outer atmosphere.
1 mark: Atoms in the cooler gas absorb photons of particular wavelengths corresponding to electron transitions.
1 mark: These absorbed wavelengths appear as dark lines in the spectrum when viewed from Earth.
1 mark: Each absorption line corresponds to a specific element or ion, allowing astronomers to identify composition.
