Emission & Absorption Spectra in Stars
- There are three types of light spectra:
- Continuous emission spectra
- Emission line spectra
- Absorption line spectra
- Continuous spectrum: created when photons of all wavelengths are emitted
- Appearance: a broad range of colours (depending on a star's temperature)
- Produced by: hot, dense sources, such as the cores of stars
- Emission spectrum: created when photons are emitted by excited electrons in a hot gas
- Appearance: discrete wavelengths represented by coloured lines on a black background
- Produced by: hot, low-pressure gases, such as a nebula surrounding a star
- Absorption spectrum: created when photons are absorbed by electrons in a cool gas
- Appearance: discrete wavelengths represented by dark lines on a continuous spectrum
- Produced by: light passing through cool, low-pressure gases, such as the photosphere of a star
- Note: the lines in an absorption spectrum correspond to the same lines in the emission spectrum of the same element
The difference between continuous, emission & absorption spectra
An absorption spectrum is the combination of an emission spectrum on top of a continuous spectrum
Chemical Composition of a Star
- Stellar spectral lines are caused by the interactions between photons and the atoms present in gaseous layers of stars
- Photons produced by fusion reactions in a star’s core move towards the layers of gas in the outer atmosphere of the star
- The photons produced in the core form a continuous spectrum
- Photons are absorbed by the gas atoms, which excite and re-emit other photons of various frequencies in random directions
An absorption line will appear in a spectrum if an absorbing material is placed between a source and the observer
- Each gas produces a unique pattern of spectral lines due to the specific transition between the element’s energy levels
- The presence of absorption lines in a star’s spectrum act as fingerprints
- They can be used to determine the presence of a certain element within the star
- The chemical composition of a star can be investigated even when extremely distant
- If the element is present in the star, its characteristic pattern of spectral lines will appear as dark lines in the absorption line spectrum of the star
- The Sun is predominantly made up of hydrogen and helium gas
- This can be verified by comparing the emission line spectra of hydrogen and helium with the absorption line spectrum of the Sun
The Hydrogen Spectrum
- Each element produces its own unique set of lines corresponding to specific energy level transitions
- The spectrum of hydrogen was the first to be studied in great detail
The full hydrogen spectrum
- In spectra of hydrogen:
- The Lyman series converges on the ground state n = 1
- The Balmer series converges on the second energy level n = 2
- The Ritz-Paschen converges on the third energy level n = 3, and so on
- The Lyman series photons will have the most energy since they have the shortest wavelength
- The Pfund series photons will have the least energy since they have the longest wavelength
- Note: in this course, you only need to remember the Balmer series, the others are only mentioned here for context
Electron transitions in the hydrogen spectrum
- The discovery of these electron transitions has enabled astronomers to study the nature and chemical composition of objects in the Universe
Worked example
Which of the following electron transitions in a hydrogen atom would result in the emission of visible light?
A. n = 1 to n = 2
B. n = 2 to n = 3
C. n = 2 to n = 1
D. n = 3 to n = 2
Answer: D
- A photon is emitted when an electron moves from a higher energy level to a lower energy level
- This eliminates options A & B
- Emission in the visible region occurs for an electron transitioning from any higher energy level to n = 2
- Therefore, the transition n = 3 to n = 2 would result in the emission of visible light
Worked example
Explain why:
(a)
Hot, dense sources produce continuous spectra
(b)
Hot, low pressure gases produce emission spectra
(c)
Hot, dense sources observed through cool, low pressure gases produce absorption spectra
Answer:
(a) Hot, dense sources, such as the cores of stars, produce continuous spectra because:
- In a hot, dense material, the atoms or molecules are so close together that they interact with one another
- This leads to a spread of energy states that are not clearly defined
- Therefore, photons of all frequencies are emitted leading to an uninterrupted band of colour
(b) Hot, low pressure gases produce emission line spectra, because:
- Hot gases produce emission line spectra when photons are emitted due to the transition of electrons between discrete energy levels in atoms of the gas
- The line spectrum has certain, fixed frequencies related to the differences in energy between the various energy levels of the atoms of the gas
- In a low pressure gas, the atoms or molecules are not close together
- This means the energy levels of the gas atoms or molecules are clearly quantised and well-defined
- Therefore, only photons which correspond to the differences in energy between the energy levels of a bound electron are seen
(c) Hot, dense sources observed through cold gases produce absorption spectra because:
- Atoms of different elements in the cold gas absorb energy emitted from the hot source but only at particular energy values
- These particular energy values correspond to the differences in energy between the energy levels of a bound electron
- This means that particular frequencies of light are absorbed, creating black lines in the continuous emission spectrum
Examiner Tip
Given an absorption line spectrum for a specific star, you can be asked to identify a star of similar chemical composition. It is important to pay attention to the spacing between the lines to be able to correctly identify the most similar star to the given one.
