1. Spectroscopy ↑
In 1802, British natural philosopher William Hyde Wollaston discovered dark lines in Isaac Newton's colour spectrum. These are now known as spectral lines. Wollaston's discovery may have been attributed to flaws in the prism he used to create a spectrum, but he soon saw that they were always in the same place, whatever prism he used.
Spectral lines in the solar spectrum. Image credit: MaureenV/Phrood/Saperaud/Cepheiden/Public domain.
German optician Joseph von Fraunhofer independently discovered spectral lines in 1814. Fraunhofer mounted a prism in front a small telescope to create a spectroscope. With this new technology, he was able to map over 570 spectral lines, and created the field of study known as spectroscopy.
2. Doppler-shifted lines ↑
French natural philosopher Hippolyte Fizeau showed that light is affected by the Doppler effect in 1848. The Doppler effect was discovered by Austrian natural philosopher Christian Doppler in 1842. Doppler showed that the frequency, and hence wavelength, of sound will change, depending on whether it's moving towards you or away from you.
When it's stationary, the wavelength and frequency of an ambulance siren, for example, will be the same in all directions.
The wavelength appears shorter, and the sound higher pitched, as it moves towards you. This is because the ambulance catches up with the waves it emits, and so the space between emissions is shorter.
The Doppler effect. Image credit: Cool Cosmos/Public domain.
The wavelength appears longer, and the sound lower pitched, if the ambulance is moving away. This is because in the time between emissions, it moves further from the direction of the waves that you hear, and so the space between emissions is longer.
Fizeau found that the Doppler effect causes light to appear more energetic, and therefore bluer, when it is moving towards us, and less energetic, and therefore redder, when it is moving away.
This effect could be seen in the position of spectral lines, and Fizeau showed that the relative velocity of stars could be determined by comparing the position of the lines in their spectra to the position of lines measured in the laboratory.
Image credit: Cool Cosmos/Public domain.
3. Absorption and emission lines ↑
The dark lines found in the spectra of stars are absorption lines. These are caused by clouds of gas that absorb some of the star's light before it reaches Earth. These clouds can then emit this light at the same specific energies, creating emission lines.
Image credit: modified by Helen Klus, original image by Magnus Manske/Jhausauer/Public domain.
Image credit: Magnus Manske/Jhausauer/Public domain.
Kirchhoff and Bunsen determined the energies of lines produced by different elements in the laboratory, and, in 1864, British astronomer William Huggins and Irish-British astronomer Margaret Huggins showed that stars are made of some of these elements, and that they are mostly made of hydrogen.
In 1885, Swiss mathematician Johann Balmer discovered an equation linking the energies of all the hydrogen lines in the visible spectrum. Swedish physicist Johannes Rydberg improved upon this equation in 1888.
In the 20th century, Danish physicist Niels Bohr's theory of the atom was used to explain why particular elements are associated with particular energies.
4. Blackbody radiation ↑
Kirchhoff coined the term 'blackbody' to describe a hypothetical object that emits a continuous spectrum, with no absorption or emission lines. A blackbody absorbs all of the light that hits its surface. This means that it doesn't reflect light and it doesn't let light pass through it.
When a blackbody is cold it is completely black, and as it heats up it remains in thermal equilibrium, emitting light at all wavelengths. There is no such thing as a perfect blackbody, but there are lots of objects that are close, including the filaments of light bulbs, the hob of electric ovens, larva, metals like iron, and stars.
The relationship between a star's energy and temperature was not known until 1879, when Austrian physicist Josef Stefan showed that the total energy emitted by a black body is proportional to its temperature to the power of four. Five years later, Stefan's first PhD student, Austrian physicist Ludwig Boltzmann, explained this using thermodynamics and British natural philosopher James Clerk Maxwell's theory of light.
Stefan and Boltzmann were able to work out the temperature of the Sun's surface by comparing it to other blackbodies found on Earth. They estimated that the surface of the Sun is about 5700 Kelvin (about 5430 °C), which is only about 80 °C less than the currently accepted value. They could then calculate how hot other stars are compared to the Sun.
4.1 Wien's law ↑
In 1893, German physicist Wilhelm Wien showed that the peak wavelength of the light emitted by a blackbody only depends on its temperature. As it heats up, a blackbody will move through the spectrum becoming red, orange, yellow, green, and then blue, no matter what it is made of.
Wien developed his theory by treating light as if it were made of particles. By the end of the century, however, German physicists Heinrich Rubens and Ferdinand Kurlbaum would show that Wien's theory does not apply to infrared light. This problem would be solved by German physicist Max Planck and German-Swiss-American physicist Albert Einstein, using early theories of quantum mechanics.
In 1896, Dutch physicist Pieter Zeeman discovered that spectral lines can be split if the light travels through a magnetic field, and in 1913, German physicist Johannes Stark and Italian physicist Antonino Lo Surdo showed that spectral lines can also be split by electric fields. These affects would later be explained using a quantum theory of the atom devised by Bohr in 1913, and developed by German physicist Arnold Sommerfeld and British physicist Paul Dirac in the 1920s.