Compiled and Edited By Barlow Bob
The English philosopher Roger Bacon (1214 – 1294) was the first person to recognize that sunlight passing through a glass of water could be split into colors.
centuries later, Sir
Isaac Newton coined the term “spectrum”
to describe the rainbow of colors produced by sending sunlight through a
prism. In 1666, he directed a beam of sunlight through a glass prism and
noted the light from the
Sun was composed of a continuous
spectrum of colors.
However, two noteworthy spectral discoveries did take place during that period. In 1800, William Herschel tested the heating power of the colors of the solar spectrum. He placed sensitive thermometers at various locations in the spectrum. Herschel found that the maximum heating effect took place outside the red end of the visible spectrum. He discovered infrared (IR) light. The following year, J.W. Ritter was testing the effect of the solar spectrum on various chemicals. His experiment involved a quartz prism placed in an open window. Ritter placed paper that was coated with different chemicals in the spectrum. He found that silver chloride blackened when placed at a point beyond the violet end of the spectrum. Ritter discovered ultraviolet (UV) light.
William Hyde Wollaston (1766 – 1828) was an English chemist and physicist. In 1802, in what was to later lead to some of the more important advances in solar physics, he discovered the spectrum of sunlight is crossed by a number of dark lines. This is considered to be “A Great Moment in the History of Solar Physics” since it was the birth of Solar Spectroscopy.
In the early
The smartest thing the company owners did was to hire a journeyman glassworker by the name of Joseph von Fraunhofer. Orphaned as a young boy, Fraunhofer had been apprenticed to a glass cutter. One day while working, the building he was in collapsed around him trapping him inside and, apparently, turning him into something of a public hero in Bavaria as rescue workers struggled to dig him out. The Elector of Bavaria gave him 18 Ducats, which allowed Fraunhofer to buy out his apprentice’s contract and acquire enough tools to set himself up in business. By 1813 he was in charge of the whole operation at Benediktbeuern. Over the years, he improved the quality of the glass produced by his factory, producing the best optical equipment to be found anywhere in the world. This gave Fraunhofer a new problem. There was not a way that he could measure small differences in the composition of his glass from one melt to another.
To see why this might be important, think about a simple optical instrument, such as a prism. When white light comes into the prism, the different wavelengths are bent through different angles, resulting in a spread of colors. Prisms made of slightly different pieces of glass, however, will bend the same wavelength of light through slightly different angles. What was needed was some sort of calibration standard, some fixed wavelength of light that all laboratories could run through their prisms to compare the way the instruments worked. Today, there are many such standards available. In Fraunhofer’s time, there was nothing.
He turned to a discovery that had been made in 1802. If you pass sunlight through a prism of sufficiently high quality, you notice a series of dark bands superimposed at regular intervals over the colored spectrum of light. No one at the time, including Fraunhofer, had any idea what these lines were. He realized that whether he understood them of not, they could be used to solve his calibration problem.
The same Sun
spectrum and its Fraunhofer Lines.
HERE for more info.
In 1849, Leon Foucault noticed that Fraunhofer’s D line in the Sun corresponded with the bright yellow-orange line in lamps. To show that the two lines matched, he passed sunlight through the arc of a lamp in order to view the superimposed spectra. To his surprise, he found that the absorption lines in the solar spectrum became stronger. Additional experiments using carbon arc lamps showed that a given medium could produce either emission of absorption lines. The results of these and earlier experiments were fundamental to the development of Kirchoff’s Laws.
For more than half a century, the Sun’s Frauenhofer lines remained a useful, but unexplained phenomenon. By the late 1850’s, the University of Heidelberg had become one of the great centers of scientific research in the world. Two of its brightest scientists were a physicist named Gustav Kirchoff and a chemist named Robert Bunsen.
In 1859, they repeated Foucault’s experiment of passing sunlight through a strong sodium flame. To aid their investigation, Bunsen and Kirchoff developed the first spectroscope. Earlier scientists used a prism to diffract the light source to a separate viewing scope, and detailed measurements were difficult to perform. The spectroscope that Bunsen and Kirchoff developed had an integrated slit, prism, and collimator. The results of their experiment were the same as Foucault’s: the sodium flame produced dark lines in the solar spectrum. Kirchoff repeated the experiment with lithium, which had no corresponding solar lines. When he performed the lithium test, he saw absorption lines. How could two superimposed bright light sources produce dark lines? To answer this question, Kirchoff and Bunsen recorded the spectra of lithium, sodium, potassium, calcium, strontium, and barium salts in flames. The showed that the solar absorption spectra were the reverse of the spectra that the solar atmosphere produced. Kirchoff showed strong evidence for the presence of iron, magnesium, sodium, nickel, and chromium in the atmosphere of the Sun.
In conducting these experiments, Kirchoff and Bunsen discovered two new elements: cesium and rubidium. Kirchoff and Bunsen’s use of spectral analysis to determine chemistry is considered to be the beginning of the science of spectroscopy.
invented to burner found in practically every high school chemistry lab.
He was an expert in what is called analytical chemistry. This is the
branch of science devoted to purifying and identifying samples of
chemical compounds. He began doing detailed chemical analyses of the
gases produced in the manufacturer of cast iron. By the 1850’s, he was
Gustav Kirchhoff was beginning a
parallel career through the German university system.
In 1847, he graduated from the
University of Königsberg.
At the age of 21, he formulated the laws that govern the voltages
and currents in electrical circuits, laws which still bear his name.
In 1854, he went to
The two took this idea seriously and put together the world’s first instrument for analyzing spectra from a couple of old telescopes, a prism, and a cigar box. The idea was to look at light from burning materials in a systematic way, cataloging the positions of various spectral lines.
run-up to the main experiments in the autumn of 1859, however, Kirchhoff
stumbled on an observation that was to change the face of astronomy.
He was running sunlight through their apparatus, looking at
Fraunhofer lines. He had already
detected a correspondence between the bright lines in the light from
burning metals and the dark Fraunhofer lines.
What he noticed, however, is that if he passed sunlight through a
sodium flame before bringing it into his apparatus, certain
Fraunhofer lines became darker.
After thinking about this finding overnight, Kirchhoff realized that the dark lines occurred when light was absorbed by atoms. Fraunhofer lines were created when white light from the Sun’s core was absorbed by the sodium atoms in the solar atmosphere. Even more light was absorbed by Kirchhoff’s flame. This is why the lines became darker. This not only explained a fundamental property of radiation, it opened a new window on the universe. For the first time in history, a human being had detected the presence of a specific chemical element (sodium) on a non-terrestrial body.
In 1861 Bunsen and Kirchhoff performed experiments leading to the conclusion that the dark lines in the solar spectrum, observed by Wollaston and Fraunhofer, arise due to the absorption of light by gases in the solar atmosphere that are cooler than those emitting the light.
Today we know that the atom has a heavy, positively-charged nucleus around which electrons move in specified orbits. When an electron jumps from a higher orbit to a lower one, light is emitted. The energy of the light moving away from the atom is precisely equal to the difference in energy of the electron in the two orbits. When an atom absorbs light, on the other hand, the electron jumps from a lower orbit to a higher orbit, and the energy absorbed from the light is precisely equal to the energy needed to lift the electron up.
Kirchhoff didn’t know about electrons, of course. Their discovery was decades in his future. From his experiments, he realized that an atom that emits a particular wavelength of light will also absorb at that wavelength. Why absorption? Where the original photon may have been coming straight along your line of sight, the one that’s re-emitted can head off in any direction. The result is a drop in intensity for the light given off by that element, which shows up as the absorption line.
Armed with their new insight, Kirchhoff and Bunsen displayed the “killer instinct” you would expect from world-class researchers by identifying two new chemical elements: Cesium (from the Latin for “sky blue”) and rubidium (“dark red”) from drops of mineral water. The elements were named for the brightest colors in the spectrum. In the following years many additional elements were identified by their unique spectra.
While chemists were using the newfound ability to analyze spectra to flesh out the Periodic Table, young astronomers were not slow in applying the analysis of spectra to the study of the heavens.
Bright and sunny, with a noontime high of 4000 degrees.
A sunspot next to Earth (for comparison only).
In 1868, he made a rather starling discovery. In the spectrum of the Sun, nestled between two bright yellow sodium lines, he saw a spectral line that just was not supposed to be there. At least, it didn’t belong to any element known to chemists. Lockyer concluded that what he observed was a new element, one that didn’t appear on Earth. He named the element helium, from Helios, the Greek word for Sun.
Today, the use of spectra to analyze the chemical composition of celestial objects is so widespread that a spectrometer is as common as a telescope in an astronomer’s studies. The introduction of spectroscopy by Kirchhoff and Bunsen marked the transition from astronomy to astrophysics, between worrying about where objects are in the sky to worrying about what they are.
William Huggins was one of the wealthy
British “amateurs” who contributed so much to 19th century
science. At age 30 he sold the family business and built a private
observatory outside of
He was the first to distinguish between nebulas and galaxies by showing that some like the Orion Nebula had pure emission spectra characteristic of gases, while others like the Andromeda Galaxy had spectra characteristic of stars. Huggins was assisted in the analysis of spectra by his neighbor, the chemist William Allen Miller. He discovered dark-line spectra in 1863 and in 1864 he examined the spectrum of a planetary nebula, NGC 6543 in Draco, and found that it had a bright emission line spectrum. He therefore concluded that this nebula was not composed of stars, which have a continuous spectrum, but of glowing gas. In 1866, he made the first spectroscopic observation of a nova, Nova Coronae 1866, and found emission lines of Hydrogen. In 1868 he took the spectrum of a comet and identified the spectral lines of ethylene in this spectrum. After 1875 his observations were made jointly with his talented wife, the former Margaret Lindsay Murray, who was a capable astronomer in her own right. She encouraged her husband’s photography and helped to systemize their research.
Since astronomers didn’t employ uniform observing techniques during the late 1800’s, they could not simply pool their accumulating data. However, the Henry Draper Memorial changed all of that. Here, for the first time with the Henry Draper (HD) Catalog, data was consistently gathered on uniform manufactured and developed photographic plates. A single plate could capture 200 spectra for later analysis. This was a year’s work to the previous generation of astronomers. Multiply hundreds of spectra per plate by an endless flow of plates, and you get a feel for the magnitude of the Draper Catalog project as it began in 1886.
Henry Draper, photographic plates, and zoom lens.
The first step in the data organization was to determine the stellar coordinates for assigning HD numbers to the objects. These designations are used to this day. Then came the overwhelming task of identifying spectral features on one-half-inch-long smears of light and dark strips. The placement and intensity of the dark strips needed to be determined precisely to properly classify a particular star.
When all was said and done by the middle of the 20th century, Henry Draper’s namesake catalog would contain positions and spectral information for nearly a quarter of a million stars.
modern observatory, and along with telescopes you will find many
computers. No matter what astronomers study, they need computers to help
them examine the huge amounts of information that telescopes gather.
More than 100 years have passed since “computers” began working
at observatories. However,
at the beginning of this 100 years the computers were not machines at
they were women.
Like the rest of
An early network. The Harvard computers from 1917. Ida E. Woods, Evelyn F. Leland, Florence Cushman, Grace R. Brooks, Mary H. Vann, Henrietta S. Leavitt, Mollie E. O'Reilly, Mabel A. Gill, Alta M. Carpenter, Annie J. Cannon, Dorothy W. Block, Arville D. Walker
end of the 1800’s, American observatories began to resemble factories,
with different workers producing, handling, and studying astronomical
photographs. In the 1880’s,
Harvard College Observatory in
Draper’s ultimate goal was to photograph stellar spectra across the northern and southern hemispheres. He was convinced that in spectra lay the answers to fundamental questions about the universe. Unfortunately, Draper would never achieve his ultimate goal. He died suddenly in 1882. His shocked and grieving widow, Anna Palmer Draper, was determined that her husband’s dream be realized.
Harvard College Observatory (HCO) in
1886, under the financial support of the newly-founded Henry Draper
Memorial Fund managed by Anna Draper and the leadership of
the workrooms at the observatory never held fewer than a dozen
“computers.” For more than
thirty years, 45 women were employed at HCO.
The women computers of HCO were expected to work six days a week,
seven hours a day. Fleming later said in her journal that
The original appeal of stellar spectra lay in their mystery. The first observations of stellar spectra in the early 1800’s hinted to astronomers that there was something significant about the pattern and intensities of the dark lines that chopped the rainbow into slivers, yet they simply did not know the origin of that significance.
Fleming soon found herself in 1881 supervising a platoon of assistants. After four years of tireless, tedious scrutiny, Fleming and her team had managed to classify 10,498 stars with a scheme that the astronomical community coined the “Pickering-Fleming system.” This label indicated the first general recognition of a female astronomer. This system consisted of 15 spectral classes, each of which had specific identifying features such as the strength of the dark lines corresponding to hydrogen. The classes were given simple letter designations, A through Q (I, J, and P were omitted; all oddities were lumped into Q).
work with spectra was so marvelous that
Fleming’s successors was
Annie Jump Cannon.
She was one of the few computers who had training in physics and
astronomy, having graduated from
Wellesley in 1884.
A decade later, after returning home to
The study of
improved spectra was assigned in 1888 to Draper’s niece
Antonia C. Maury.
By noting that some spectra showed double sets of lines, she
discovered a spectroscopic binary. These are stars that appear single
but are actually two stars orbiting each other.
She was the first person to realize that the sharpness of
spectral lines tells important information about a star. Maury
eventually left HCO after disagreements with
Cannon took over Maury’s work. Cannon’s classification scheme retained Fleming’s original letter designation, but followed the overall sequence of Maury’s. By using Maury’s sequence, Cannon saw the need to categorize stars according to some unknown physical property. Cannon created the familiar O, B, A, F, G, K, M, seven major classes, each with ten subclasses: 0 through 9. These subclasses are based on the presence, absence, or intensity of particular spectral features. O stars are hot stars and M stars are relatively cool. The Sun is a G2 star. The spectral class immediately provides information about a star’s physical properties, including temperature. She classified the spectra of more than 350,000 stars during her years at Harvard, while her work in many other areas continued until her death in 1941.
Henrietta Swan Leavitt had a college degree and began working at Harvard in 1895. Leavitt’s forte was photographic photometry. Photometry is the determination of the absolute brightness of the stars from photographic plates. She could get an accurate photometric magnitude scale with the aide of photography and also perform night-to-night comparisons of stars to locate new variable stars. She studied photographs of variable stars and discovered that the periods of variable stars called Cepheids are related to their actual luminosity, or brightness. This period-luminosity relationship helps astronomers measure distances in the universe. A star’s distance can be calculated by comparing its actual brightness to its apparent brightness. She also discovered over 2,400 variable stars this way. At the time, this was about half of the variables known to exist.
Cepheid variable in M100 from the Hubble Space Telescope.
Using photographs of nebulae, Edwin Hubble found Cepheid variable stars. These are yellow supergiant pulsating stars that are very important in establishing distances to groups of stars beyond our own Milky Way galaxy. He analyzed the light of Cepheid stars in three nebulae. Hubble was able to determine the distance to these galaxies in the 1920’s using Henrietta Leavitt’s period-luminosity relationship. His results showed that the farther away a galaxy is from us, the faster it is moving away from us. This shocking evidence that the universe is expanding changed the centuries-old opinion that the universe was static.
until his death at age 73,
Johann Jakob Balmer (1825-1898) was a
high school teacher at a girl’s school in
In the mid-1880’s, Balmer began his examination of the four numbers associated with the hydrogen spectrum. At his disposal were the four numbers measured by Anders Jonas Angström (1814-1874): 6,562.10, 4,860.74, 4,340.1, and 4,101.2. These numbers represented the wavelengths, in units of Angströms, of the four visible spectral lines in the hydrogen atom spectrum.
4,101.2 4,340.1 4,860.74 6,562.10
The Balmer Series for hydrogen.
In 1885 Balmer published a paper in which his successful formulation was communicated to the scientific world. Balmer showed that the four wavelengths could be obtained with the formula that bears his name (wavelength = B x (m^2)/(m^2-n^2), with B = 3645.6 Angströms). He had found a simple mathematical formula that expressed a law by which the hydrogen wavelengths could be represented with striking precision. He further suggested that there might be additional lines in the hydrogen spectrum. Other spectral lines with their own wavelengths were predicted by Balmer and later found by other scientists. Angström measured the wavelengths of the spectral lines of hydrogen, but Balmer showed that the wavelengths of the spectral lines are not arbitrary. The value of the wavelengths are the expression of one particular mathematical formula and this Balmer Series equation altered how scientists thought about spectral lines. Before Balmer published his results, scientists drew an analogy between spectral lines and musical harmonies. They assumed that there were simple harmonic ratios between the frequencies of spectral lines. After Balmer’s work, all scientists recognized that spectral wavelengths could be represented by simple numerical relationships.
Balmer disappeared from the ranks of working scientists and continued his classroom work teaching young ladies mathematics. Neither he nor his students recognized that his paper on the spectrum of hydrogen would bring him scientific immortality. The spectral lines of hydrogen that were the focus of Balmer’s attention are now known as the Balmer Series.
For More Information:
Practical Amateur Spectroscopy
Spectroscopy: The Key to the Stars
Roger Bacon (1214 – 1294) recognizes that sunlight passing through a glass of water could be split into colors.
1666 - Isaac Newton discovers that white light is composed of colored light.
1749 - Thomas Melville studies the spectra of flames and notices the yellow line of sodium in salt.
1800 - William Herschel discovers infrared light.
1801 - Thomas Young uses diffraction to show wave properties of light.
1801 - J.W. Ritter and William Wollaston discover ultraviolet light.
1802 - William Wollaston discovers the solar absorption lines.
1814 - Joseph von Fraunhofer observes the solar spectral lines using an early version of the spectroscope.
1821 - Joseph von Fraunhofer builds the first diffraction grating, composed of 260 close parallel wires.
1823 - Joseph von Fraunhofer observes the spectra of the Moon, Mars, Venus, and several bright stars.
1823 - John Herschel suggests identification of chemical composition from spectra.
1826 - William Henry Fox Talbot introduces the idea of chemical spectral analysis.
1827 - David Brewster produces absorption spectra in a laboratory.
1827 - Joseph Nicephore Niepce produces the first successful photograph.
1832 - John Herschel notes that different flames produce different spectra.
1840 - Auguste Compte suggests that the nature and composition of stars will never be known.
1842 - Christian Doppler develops his theory of the Doppler Effect for light and sound.
1842 - Edmond Becquerel produces the first photographic record showing the Fraunhofer lines in the solar spectra.
1843 - Edmond Becquerel discovers absorption lines in ultraviolet light using photographic methods.
1847 - J.W. Draper suggests the relationship between the temperature and color of light.
1848 - William Simms constructs a spectroscope with a collimator.
1849 - Foucault that a given medium could produce absorption or emission lines. 1849 - The Astronomical Journal was founded.
1853 - Anders Angström measures the hydrogen spectral lines.
1855 - Robert Bunsen and Peter Desaga invent a high-temperature low-luminosity burner to use in spectral analysis.
1857 - Joseph Swan demonstrates that a minute amount of a chemical can produce spectral lines.
1858 - Balfour Stewart shows that at a given temperature, substances that emit a lot of energy also absorb a lot of energy.
1859 - Gustav Kirchhoff shows the presence of sodium in the solar atmosphere.
1860 - Gustav Kirchhoff and Robert Bunsen develop the Bunsen-Kirchhoff spectroscope that uses both a slit and a collimator. This is the first spectroscope.
1860 - James Clerk Maxwell develops his theory of electro-magnetic radiation.
1860 - Gustav Kirchhoff develops his spectral laws and the black body problem.
1860-1 - Robert Bunsen discovers the elements Cesium and Rubidium using the technique of spectral analysis.
1861 - Gustav Kirchhoff and Robert Bunsen systematically show that the solar spectrum is due to absorption in the Sun’s atmosphere.
1862 - Gustav Kirchhoff maps the solar spectrum with elements identified.
1862 - Anders Angström observes hydrogen in the spectra of the Sun.
1864 - Giovanni Battista Donati examines the spectrum of a comet noticing broad line of an unknown glowing gas.
1864 - William Huggins observes the spectrum of a planetary nebula, determining it is made of gas.
1866 - William Huggins makes the first spectroscopic study of a nova.
1868 - Pierre-Jules Janssen observes the lines of helium in the Sun.
1868 - William Huggins examines the spectra of seventy different nebulae.
1868 - Angelo Secchi describes four types of stellar spectra.
1868 - Norman Lockyer and William Crookes recognize and name the element helium.
1868 - Anders Angström publishes a compilation of all the visible lines in the solar spectra.
1868 - William Huggins observes the Doppler shift in stellar spectra.
1870 - C.A. Young discovers a reversing layer near the surface of the Sun showing the bright lines of the Fraunhofer spectra.
1872 - Henry Draper photographs the spectrum of Vega.
1879 - William Huggins photographs the ultraviolet spectra of several stars.
1885 - J.J. Balmer finds a formula for the hydrogen line series. J.R. Rydberg and W. Ritz find the formula of other simple spectra.
1895 - William Ramsey isolates the element helium in a laboratory.
1913 - Niels Bohr develops his theory of the atom which explains the Balmer, Rydberg and Ritz formulas for simple spectra.
1936 - Thanheiser and Heyes use photocells to measure light intensities.
1954 - Alan Walsh invents the atomic absorption spectrometer.
1969 - George Carruthers patents his invention, an Image Converter for Detecting Electromagnetic Radiation at especially in Short Wavelengths.