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Astronomical Spectroscopy: Fingerprinting the Stars
Ancient astronomers were limited to using their naked eyes to observe the heavens, but with the invention of the telescope, astronomers were able to observe celestial objects with previously unimagined precision. This newfound accuracy led to a cascade of discoveries ranging from new planets to laws about planetary motion, gravitation, etc. While progress in scientific disciplines can be slow at times, this is an instance where a single discovery led to exponential growth in the field. This article wishes to introduce the field of Spectroscopy, another such discovery that expanded the margins of our analytical ability and heralded a new age in astronomy.
In the sections that will follow, we will explore the development of this amazing field and its science from a historical
perspective. Along the way, we shall see how spectroscopy helped answer the following big questions in astronomy.
• What are stars made of? How can we possibly know when they are light years away?
• Many enthusiasts of astronomy will know how the doppler shift of the light from celestial objects helped determine their velocities and led to the discovery of the expansion of the universe. But how do we know the wavelength has shifted in the first place? It’s not like we know what it was before it reached earth… Or do we?
• How can we talk about the nature of astronomical bodies and their physical properties when we can barely see them?
However, before spectroscopy could become a reality, an important question needed to be answered. What is light anyway?
A Spectrum of Light
‘We all know what light is; but it is not easy to tell what it is.’ – Samuel Johnson, quoted by Boswell (1776)
From ancient Roman times, colors of light have been an important topic of discussion. Even as early as the 13th century quartz crystals had been used as prisms to make rainbows from white light. Despite this, the nature of the process was misunderstood as many thought the prism was creating the colors. All this was about to change in 1672 when a young professor at the University of Cambridge by the name of Isaac Newton published his first paper. In it, he described how he had used a prism to separate white light into the spectrum and then use another to recombine light into white light. This proved that white light was composed of these colors which the prism was splitting apart, contrary to popular opinion.
As his reputation grew and with the publication of his book on Optics his ideas came to be widely accepted and renewed effort was put into exploring this new spectrum of light. In 1800 William Herchel discovered what we now call Infrared rays beyond the red region of the visible spectrum, using a thermometer. Following this in 1801, William Ritter discovered the existence of rays of light beyond the violet end of the spectrum which we unsurprisingly came to call Ultraviolet rays.
Even with all these developments, the scientific community was still unable to numerically quantify the spectrum and was limited to describing it in vague terms of color. This changed in 1802 due to the now famous double slit experiment by Thomas Young where he showed the wave nature of light and subsequently calculated the wavelength of different colors of light using a diffraction grating. Now scientists had a reliable way to numerically quantify different types of light by the virtue of their wavelength. (Of course, we now know light has a particle nature too, but that won’t be discovered until much later)
Mysterious Dark Lines in the Solar Spectrum.
The next big leap and the true beginning of spectroscopy as an astronomical science wasn’t by an astronomer or a physicist, but by a glassmaker by the name of Joseph Fraunhofer from Bavaria. Being in the glass-making business and specializing in telescopes, Fraunhofer was constantly trying to improve the quality of glass wear. Therefore, he became interested in studying how each color refracted from a given lens. This led him to study the spectrum of different sources of light.
While observing a spectrum from a bright lantern via a telescope on a modified theodolite (thus inventing an early version of a spectroscope) he noticed a bright yellow spot in its spectrum. He wondered if the solar spectrum had the same spot. To check this, he directed sunlight via a thin slit and used a prism to split it into its spectrum. On the contrary, he was shocked to see that there was a dark line in that spot and other countless dark lines in the spectrum.
Fraunhofer wasn’t the first to notice these lines. A few years back, an English scientist of name William Hyde Wollaston discovered these lines but thought them merely to be lines that separated the main colors of the spectrum from each other (This may seem strange in our modern context, but at the time the theory that spectrum could be divided up into the 7 colors of the rainbow was popular). Fraunhofer however, with the use of his telescope realized that the color that was on either side of a given line was quite similar and thus refuted the previously held belief. He then meticulously recorded these observations, counting over 540 lines in a detailed chart. Some of the labels he used for the lines are still used today and are named Fraunhofer lines in his honor.
He began to wonder about the origin of the lines or whether it was some quirk of the experiment. In hope of answers, he started looking at other sources of extraterrestrial light. He observed Venus and found that most of the same lines were present in its light as those of the sun. Then, he had the idea to look at stars with his new equipment. Upon observation, he found that they indeed had dark lines but was shocked to see that at least a portion of the lines did not correspond with the lines in sunlight! Observing further he noticed that some stars had similar spectrums while others differed greatly, which he quotes,
“I have seen with certainty in the spectrum of Sirius three broad bands which appear to have no connection with those of sunlight; one of these bands is in the green, two are in the blue. In the spectra of other fixed stars of the first magnitude one can recognize bands, yet these stars, with respect to these bands, seem to differ among themselves”
He correctly conjectured that at least some of the lines are somehow related to the star’s composition.
It will be a couple more years before it was shown to be true.
Origin of the Dark Lines
Fraunhofer’s discoveries caused renewed interest in the field of spectroscopy. Sir John Herschel managed to find bands in the IR range of the solar spectrum while Sir George Strokes managed to record the spectrum in the UV range using a fluorescent screen. First photographs of the Solar spectrum were also taken during these years thus making headway in the technologies of spectroscopy.
Scientists were also speculating the origin of these lines. One theory was that it was due to the absorption of certain wavelengths of light by the atmosphere. To test this theory, Sir David Brewster designed and experiment. He first measured the intensity of a dark line when the sun is at its peak (when light travels a shorter distance through the atmosphere). Then he measured the intensity of the same line when the sun was near the horizon (when the light travels a longer distance). He found that some of the lines were darker when the sun is near the horizon showing that they were caused by atmospheric effects. Meanwhile, other scientists found that rest of the lines do not show such variation and maintained that they were due to some process in the Sun. The clue to their origin was to be found in the spectra of flames.
While the solar spectrum was being studied, scientists also started studying the spectra of flames infused with chemicals. They found them to be discontinuous and that they were composed of bright bands of color (see the above figure). While several scientists speculated the connection between these spectra and that of the sun, a large part of the honor of establishing the theory goes to Gustav Kirchhoff (of Kirchhoff Circuit law’s Fame).
It had been known for some time that when sodium was introduced into a flame the spectrum that was made consisted of two bright yellow lines that were quite close together. Kirchhoff then sent a continuous spectrum from a lamp through sodium salt in a flame. He expected to see the continuous spectrum with two yellow bright spots. Instead, he saw two dark lines exactly in the spot where the two bright lines would have shown in the normal case. This made him realize that an element could absorb or emit radiation depending on the situation. He summarized his discoveries as follows in his three laws of spectroscopy.
- A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum.
- A low-density gas excited to emit light will do so at specific wavelengths and this produces an emission spectrum
- If light composing a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum with the absorption happening at the same wavelength as the emission spectrum
This meant that each element had an optical fingerprint which could be used to identify the element using its spectrum. He went further and decided that dark bands in sunlight were also due to absorption by elements present in the sun. To demonstrate this, he used an elaborate and sensitive spectroscopy set up to superimpose the spectrum from a sodium lamp and that of sunlight. Upon observation, it was clear that they coincided exactly, thus laying to rest the mystery of the origin of the dark lines in the sun.
As he put it,
“I conclude furthermore that the dark lines of the solar spectrum which are not produced by the Earth’s atmosphere, result from the presence of that substance in the luminous solar atmosphere which produces in the flame spectrum bright lines in the same place…The dark D lines in the solar spectrum allow one therefore to conclude, that sodium is to be found in the solar atmosphere”
The theoretical reason for this wouldn’t come until the next century after the advent of quantum mechanics and the development of the Bohr model of the atom. We now know that electrons of an atom occupy distinct energy levels. During absorption, an incident photon of light is absorbed by an electron as it jumps from a lower energy level to a higher energy level. This can only happen if the energy of the photon is exactly equal to the energy difference between the energy levels. Since the energy of a photon is given by 𝐸 = ℎ𝜈 where 𝜈 is the frequency while ℎ is a constant, only certain frequencies will be absorbed by the atom. Similarly, during the emission of radiation, electrons that are excited to a higher energy level fall back to a lower level emitting a photon of light of the specific frequency equivalent to the energy difference of the energy levels.
The origin of the Dark lines in the spectrum was finally explained, but this discovery that linked the spectrum with the composition of the body was to have an everlasting impact on Astronomy.
Stellar Composition and the Discovery of Helium.
‘We will never know how to study by any means the chemical composition (of stars), or their mineralogical structure” – Auguste Comte (1835)
In the 19th century, as scientists began to learn how far away stars really were using parallax measurements, astronomers feared that the true nature of the stars and their composition would remain a mystery forever (as expressed in the quote above by the 19th century philosopher Auguste Comte). However, as already discussed, the groundwork for figuring out stellar composition had already been made by the time this pronouncement was made! The first major step in this field was due to the joint work of Kirchhoff and Bunsen.
After Kirchhoff formulated his laws, he and the famous chemist Robert Bunsen set out to find the composition of the sun. With the aid of the Bunsen burner (that Bunsen had just invented) which produced a hot flame that was almost colorless (so that it didn’t interfere with the spectrum to be found), they found emission spectra for several elements using pure compounds that Bunsen had synthesized (They even discovered two new elements by identifying spectra that were not seen before!). Next, by comparing their emission spectra with the absorption lines in the sun, they were finally able to shed light on the composition of the sun. They concluded that iron, calcium, magnesium, sodium, nickel, and chromium must be present in the cooler outer layers of the Sun. For iron, the bestobserved element, they found exact coincidences for as many as 60 lines, virtually excluding the possibility that this might have arisen by chance.
Later work by scientists revealed the existence of other terrestrial elements such as hydrogen, oxygen, nitrogen, etc. Scientists such as Sir William Hudgins made great strides in finding the compositions of various stars and nebulae. Finding the compositions of stars such as Sirius, Betelgeuse, and Aldebaran as well as nebulae such as the cat’s eye nebula.
However, an important element was missing from the composition of all these celestial bodies and that was helium. Being the second most abundant element in the universe, it was present in large quantities in all the observed bodies. The reason for the absence was simple, helium was yet to be discovered!
In 1868 there was a solar eclipse, and spectroscopes allowed scientists for the first time to observe the emission spectra of solar prominences observed during a solar eclipse. The presence of Hydrogen emission lines proved they were hot clouds of gas. Researchers argued that the reason for the absence of heavier elements was the distance from the sun meant only lighter gasses could escape to this height. However, they found a line very near the sodium line!
Norman Lockyer later demonstrated that its wavelength was less than either of the sodium lines. Since it had to be from a light gas, he initially postulated it was an undiscovered emission line of hydrogen. But as he was unable to recreate it in the lab, he and his colleague Professor Edward Frankland considered the possibility that it was a new element and named it helium (after Helios, the Greek god of the sun) postulating its existence before it was ever discovered on earth! As Sir William Thomson (who later became Lord Kelvin!) stated in 1871,
“Frankland and Lockyer find the yellow prominences to give a very decided bright yellow line not far from D, but hitherto not identified with any terrestrial flame. It appears to indicate a new substance, which they propose to call Helium”
Later in 1895, Scottish chemist Sir William Ramsay isolated helium on Earth by treating the mineral cleveite with mineral acids, and its emission spectra were shown to have the observed line in the sun. Thus, the second most abundant element in the universe and a vital component in the fusion reaction of stars were discovered.
On the other hand, even though the elements composing the stars were known their relative abundance was widely misunderstood. It was thought the composition of the stars and the sun itself was similar to that of earth. Therefore, the sun was believed to be simply a hot ball that glowed similar to how stones would glow when heated. This erroneous belief was corrected by Cecilia Paynein in 1925, by a careful analysis of spectroscopic data using the ionization theory developed by Meghnad Saha. She found that hydrogen and helium were vastly more abundant in many stars (including the sun) compared to other elements in contrast to popular opinion of the time. This was instrumental in the later development of the theory of fusion developed by Subrahmanyan Chandrasekhar.
The Doppler Effect and Motion of Stars.
The Doppler effect, as many are probably familiar with, is the change in frequency or wavelength of a wave when there is a relative motion between the observer and the source. One might have even experienced it as the change in the pitch of a siren that passes by at speed. As light has wave properties, light also undergoes this phenomenon. Therefore, if the resulting shift can be somehow determined, it can be used to estimate the speed of the light emitting source. Fortunately for us, spectral lines allow us to do this. Since the spectral line positions are already known from the elements that cause them, we essentially know what the spectrum looked like before it was shifted! Therefore, we can use the shift to calculate the velocity of the body.
This effect was first conceived by Christina Doppler as a way to find the radial velocity of stars using spectroscopic data and postulated the effect for sound as well as light. Surprisingly many refused to believe the existence of the effect even for sound and certainly not for light. It was only after experimental confirmation for sound and then later by shifts observed in the spectra of light of stars that the theory was finally accepted.
William Hudgins was one of the first scientists to use the theories developed by Doppler. He used it to calculate the Radial velocity of Sirius while simultaneously confirming the theory as well. The results were somewhat inaccurate as the resolution was not enough to obtain correct shifts of the spectral line and the error was large as ± 22 𝑘𝑚 𝑠−1. Later developments in spectral photography reduced the error to about ±2.6 𝑘𝑚 𝑠−1.Later advent of Photomultipliers and electronics sensors would reduce the error even further.
Using these techniques, the radial motions of several stars were cataloged. Developments of relativity by Einstein and the subsequent development of the relativistic Doppler effect and Gravitational redshifts allowed for more accurate calculation. The doppler effects observed for the spectral lines were even used to empirically confirm these theories by comparing theoretical results with the observed shifts. The data from these proved to be invaluable and some of the discoveries due to these are,
- Confirmation of the expansion theory of the universe.
- Discovery of the first exoplanets by analyzing the periodic shift of the star’s spectral lines due to its periodic wobble due to the planet. (Pegasi 51b was found this way in 1995)
- Discovering the rotation of our galaxy by observing the motion of the stars in it.
And many more.
Pressure, Temperature, Classification, Identification, and More
Aside from the discussed areas the following are some additional areas where spectroscopic analysis is used extensively.
Pressure and Temperature of Stars.
Knowing the Temperature and Pressure of stars is important in the development of the physics of stars and their evolution. Several spectroscopic techniques are used in their determination. One simple way to estimate the temperature of a star is to use the relationships between the Temperature and the wavelength of maximum intensity such as Wien’s displacement law.
𝜆𝑚𝑎𝑥𝑇 = 𝑏 ; 𝑏 = 𝑊𝑖𝑒𝑛′𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
Another way to determine the Temperature is to use doppler broadening. In this effect, the motion of the particles causes a doppler shift in the wavelength of the spectral line. Since the motion is random the shift also happens in different amounts for each particle. Thus, this is perceived as a broadening of the line.
Similar broadening also happens due to the Pressure of the star. Increasing pressure means a higher number of collisions which increases the uncertainty in the energy difference of energy levels and broadens the spectral band.
Finally, different temperatures cause differing amounts of ionization species of the elements present in the Star. This is estimated by Saha’s equation and can be used to estimate the temperature of the star.
Knowing which states are involved gives direct information on the degree of excitation of the system. This can be used to determine the physical conditions, such as the temperature or density of the environment local to the star.
Stellar Classification
Stellar Classification is an important field in Astronomy. It allows scientists to group stars that have similar characteristics together and make studying the countless number of stars easier. The basis for many systems of classification is the spectra of stars. The modern MK system has its roots in the Harvard system which in turn is based on the older Draper and Secchi system established in the late 19th century. These systems are almost entirely based on the presence, absence, and relative strength of certain absorption and emission lines such as the strength of Hydrogen lines and certain Metallic lines.
The relationship between the spectral class and the temperature of the stars is now commonly known, but this was only found in later analysis. The original classification was done without any knowledge of the relationship between the spectra and the properties of the stars, and was purely based on spectral information.
Identification of Astronomical Objects
We now know of the existence of many astronomical objects from galaxies, nebula, and stars. However, for early astronomers distinguishing between these was difficult as most of these tend to look the same under a telescope, especially if the telescope has lower resolution and magnification. Spectroscopical Data allowed Astronomers to differentiate these objects by comparing their spectrographs.
An excellent early example of this is due to the Astronomer William Hudgins. At the time galaxies and nebula were often confused as they looked similar in certain cases. Hudgins observed that the spectrum of galaxies (such as the Andromeda Galaxy) were similar to stars (since they are collections of stars anyway). On the other hand, nebulae such as the Cat’s Eye nebula had emission lines meaning that it was composed of hot and luminous gas and dust. Thus, he was able to differentiate between the two objects, resolving the confusion. Several other celestial bodies that have been confused have been resolved using spectroscopy.
Magnetic Effects
Certain magnetic effects make changes in the distribution of the energy levels of atoms and ions. One such effect is the Zeeman effect. These effects can be seen in spectrographs as the splitting of lines and shifts in their position. By observing these effects, we can find out about the magnetic nature of the source.
Detecting the Presence of Molecules.
In this article lot of attention was paid to the detection of the presence of elements in stars and other astronomical bodies using spectral information. However spectral information can also be used to determine the presence of complex molecules. This is because molecules also can have energy levels that can absorb or emit radiation. Furthermore, the bonds connecting the atoms also have specific frequencies of vibration which can absorb radiations of certain wavelengths. These specific absorption wavelengths depend on the structure as well the atoms that compose the molecule. Therefore, they can be used to determine complex molecules present in celestial bodies.
Conclusion
The first image that comes to mind when you hear astronomy is the telescope and indeed up to the 1800s, it was the instrument that was considered a game changer in astronomy. However, the discovery of spectroscopy opened so many new avenues of analysis that feats that were previously thought unthinkable in astronomy became feasible, making spectroscopy a tool that was almost as revolutionary as the telescope itself. This is best summarized by the following quote by the famous Physicist and Science Communicator Michio Kaku.
“A hundred years ago Auguste Comte… a great philosopher, said that humans will never be able to visit the stars, that we will never know what stars are made out of, that that’s the one thing that science will never ever understand because they’re so far away. And then, just a few years later, scientists took starlight, ran it through a prism, looked at the rainbow coming from the starlight, and said: “Hydrogen!” Just a few years after this very rational, very reasonable, very scientific prediction was made, that we’ll never know what stars are made of.”
― Michio Kaku —
References
- The Analysis of Starlight: Two Centuries of Astronomical Spectroscopy, Second Edition, John B. Hearnshaw University of Canterbury, Christchurch, New Zealand
- ASTRONOMICAL SPECTROSCOPY An Introduction to the Atomic and Molecular Physics of Astronomical Spectra, JONATHAN TENNYSON, University College London, UK
- Spectrum of Belief Joseph von Fraunhofer and the Craft of Precision Optics, Myles W. Jackson
- Stellar atmospheres: A contribution to the observational Study of high temperature in the reversing layers of stars by, Cecilia h. Payne, Published by the observatory Cambridge, Massachusetts, 1925
- Researches on the Solar Spectrum and the Spectra of the Chemical Elements, G. Kirchhoff, 1861 (translated version 1862)
- https://royalsocietypublishing.org/doi/10.1098/rsta.2014.0213
- hysicstoday.scitation.org/doi/10.1063/PT.3.4429
-Praveen Sumanasekara
