Spice store clerks have an incredible gift. Just by looking at a powder they can identify it and know if it is baking soda, baking powder, sodium benzoate or any other ingredient. In contrast, some mere mortals cannot distinguish between salt and sugar, which leads to unfortunate culinary consequences.
How can we determine the composition of something if we do not have the aforementioned gift? The answer is simple: we need to experiment. We can evaluate its texture, smell it and, if we dare, even taste it. Furthermore, we could incorporate it into various recipes to verify its impact on the bread, on the cooking point of the meat or on its ability to create gastronomic explosions.
The experimental approach described above is not much different from what has been done for millennia to study chemical compositions. They experimented with all types of materials, many of them dangerous. Among the pioneers, some suffered burns, poisoning, poisoning, asphyxiation and even death from radiation. It was hard-won experiential knowledge.
We depended on direct experimentation to determine the composition of things. The possibility of studying the distant stars seemed unattainable. In 1830, the philosopher Auguste Comte expressed his pessimism by stating that we could never determine the chemical composition of the stars.
Luckily for everyone, Comte was wrong.
Over the rainbow
As often happens in science, help came from a seemingly unrelated area.
Replica of Newton’s telescope. Wikimedia Commons, CC BY
In 1665, Isaac Newton performed his famous optics experiments. He passed a ray of sunlight through a prism, reproducing the visible spectrum of the rainbow.
Later, in 1800, William Herschel discovered a form of light invisible to the human eye, revealed by its heat. He repeated Newton’s experiment by placing thermometers in the different colors. Thus he discovered that heat was registered beyond the red, where light apparently did not reach. He called this invisible light “caloric rays,” a term immortalized by H.G. Wells in his novel War of the Worlds.
Herschel’s discovery of what we call infrared radiation today was only the first step. Then came microwaves, radio waves, ultraviolet radiation, and x-rays and gamma rays. Our seven-color rainbow expanded to form what we now know as the “electromagnetic spectrum.”
Absorption, emission and continuous spectra
In 1814, Joseph Fraunhofer discovered dark lines in the solar spectrum. It seemed that the light of those colors was absorbed.
On the other hand, Thomas Melvill, since 1752, had observed something peculiar when burning salts. Their spectrum was not a continuum of colors, but rather a series of specific bright bands.
Today we call “absorption spectrum” that with dark bands and “emission spectrum” that formed by specific bands. A spectrum without dark bands is called a “continuous spectrum.”
A spectrum says more than a thousand images
We all know that a picture says more than a thousand words, but the magnitude of the information contained in the spectra far exceeds this maxim. Light has various emission mechanisms.
Thermal emission is the result of the dynamic changes in velocity of charged particles in a body. Warmer objects harbor particles with significantly higher average velocities, generating more intense and energetic changes in velocity than colder objects. All of them end up emitting light in a continuous spectrum that covers all colors, although with different relative intensities depending on their temperature.
There is another equally fascinating mechanism that emits light in specific amounts of energy. When electrons make transitions from one orbit to another, they can gain or lose energy in precise magnitudes. This results in absorptions and emissions of light in particular colors.
As each element and each compound has a different number of protons and neutrons in their nuclei, the orbital levels of their electrons have particular energies. It is as if the collections of orbits of each element were stairs with steps of different sizes. So, to go up or down each staircase we would need to take steps of characteristic lengths.
This causes the electronic transitions to form a discrete and distinctive spectrum. Each set of emission lines is like a barcode that reveals the elements present in the emitting substance.
The best doctoral thesis in the history of astrophysics
Cecilia Payne, armed with her knowledge of spectral lines, tackled the challenge of determining stellar composition. In 1925, she presented her results in her doctoral thesis, considered to this day the best thesis in the history of astrophysics.
Payne determined that hydrogen is the dominant element in the atmosphere of stars. He listed the detected elements and established a relationship between the stars’ spectra and their surface temperatures. Thanks to spectroscopy, Payne made possible what Comte considered unattainable a hundred years earlier: knowing the chemical composition of stars.
Cecilia Helena Payne-Gaposchkin en Harvard. Smithsonian Institution/Science Service
From astronomy to the problems of this world
Today, methods developed to study stars help us address the most pressing Earth problems.
Spectroscopy allows us to detect contaminants in air, water and soil.
We can identify toxic substances in the human body through anti-doping analysis, perform food quality and safety testing, and perform tissue studies for non-invasive diagnostic purposes.
In summary, the long-distance study of the stars has provided us with the necessary tools to confront terrestrial problems as serious as environmental pollution, food, and threats to public health.