Emission & Absorption Spectra

Three types of spectra

• Continuous spectrum: produced when photons of all wavelengths are emitted. Appears as a broad, uninterrupted band of colours. Created by hot, dense sources such as the cores of stars.
• Emission line spectrum: produced when photons are emitted by excited electrons in a hot gas. Appears as discrete coloured lines on a black background. Created by hot, low-pressure gases such as a nebula surrounding a star.
• Absorption line spectrum: produced when photons are absorbed by electrons in a cool gas. Appears as discrete dark lines on a continuous spectrum. Created by light from a hot, dense source passing through cool, low-pressure gas such as the photosphere of a star.

How stellar absorption lines form

• Photons produced by fusion in a star's core form a continuous spectrum. These photons travel outward through the cooler gaseous layers of the star's atmosphere.
• Atoms in these outer layers absorb photons at specific wavelengths, exciting their electrons to higher energy levels. The absorbed photons are then re-emitted in random directions.
• The key part is that the dark lines in an absorption spectrum correspond to exactly the same wavelengths as the bright lines in the emission spectrum of the same element. Each element has a unique pattern of spectral lines that acts as a fingerprint.

Chemical composition of stars

• By comparing a star's absorption line spectrum with the known emission spectra of different elements, astronomers can determine which elements are present in the star's atmosphere, even at enormous distances.
• 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

• The spectrum of hydrogen was the first to be studied in great detail. Each spectral seriesA group of spectral lines that all converge on the same energy level. Named series include the Lyman series (n = 1), Balmer series (n = 2), and Ritz-Paschen series (n = 3). corresponds to electron transitions converging on a particular energy level.
• The Lyman series converges on the ground state $n = 1$ (ultraviolet region).
• The Balmer series converges on $n = 2$ (visible region).
• The Ritz-Paschen series converges on $n = 3$ (infrared region).
• Crucially, for this course you only need to remember the Balmer series. The others are mentioned for context.