Auguste Comte was a French philosopher who lived from 1798 to 1857. He was quoted as saying, "There are some things of which the human race must remain forever in ignorance, for example the chemical constitution of the heavenly bodies." He was wrong. He could never have imagined the advances our world would make in the areas of science and technology. Today, scientists use spectroscopy to find the chemical composition of stars. They can even find other information, such as the surface temperature, age, speed of rotation, strength of magnetic field, and the speed of the star towards the Earth or away from the Earth.
Spectroscopy is defined as: "The study of spectra, including the position and intensity of emission and absorption lines."[F1] Scientists can study the light coming from distant stars to find information about those stars. They do this by analyzing the spectrum of light from a star.
When an element is excited, it gives off light in specific wavelengths, or colors. This is because the electrons of this element jump to higher energy levels when they absorb energy. When these electrons fall back down to their ground state energy levels, this energy is given off as light. Since the electrons can only be at specific energy levels, only specific wavelengths of light are emitted from each element. This means that each element has a specific "fingerprint": each line spectrum is different.
When a scientist analyses the spectrum from a star, they will see either dark or bright bands in certain locations. This means that certain wavelengths of light are being absorbed or emitted by the star. In this way, the scientist can discover which elements are present in the star. They compare the line spectra of elements to that of the star, and when there are corresponding bright lines, that element is present in the star. Using spectroscopy, Sir Joseph Norman Lockyer was able to discover the presence of helium in our sun. He announced his findings in 1871 (?). At this point in time, the element helium was not known to exist on Earth. Spectroscopy has also revealed the fact that the sun is composed mainly of hydrogen and helium.
The chemical composition of a star also indicates its age. If a star contains a high percentage of elements other than hydrogen and helium, it is relatively young. Older stars contain few of these other elements. Our own star is middle-aged compared to other stars.
The color of a star also indicates the approximate surface temperature. Blue stars are generally hotter, with temperatures of around 220,000 C. Red stars are cooler, at 17,600 C. Our star, which is yellow, has a surface temperature of 55,000 C. The temperature of a star can also be determined by counting the number of lines in its spectrum. Few lines will indicate a hot star, and many lines will indicate a cool star.
It is also possible to determine the speed of rotation of a star by analyzing its spectrum. The rotation of a star causes the atoms on its surface to advance, retreat, or remain at a constant distance. This causes "smudges" on the spectrum, either towards the blue end or the red end. By measuring the width of these smudges, scientists can determine the speed of rotation of the star.
The velocity at which the star is moving, relative to the Earth, can be determined by taking the Doppler Effect into consideration. The Doppler Effect, or red shift, is a change in wavelength of the light, relative to the observer. If the lines on a star's spectrum are shifted towards the red end of the spectrum, this means that the star is moving away from the Earth. If the lines are shifted towards the blue end of the spectrum, the star is moving towards the Earth. The speed is indicated by the amount that the line shifts: a large shift indicates a greater speed.
The magnitudes of stars and surface temperatures of stars can be plotted on a graph, called a Hertzsprung-Russell diagram. It is named for the Danish astrophysicist Ejnar Hertzsprung and the American astronomer Henry Norris Russell. Most stars can be grouped into one of seven spectral classes, O B A F G K M, remembered by the phrase, "Oh, Be A Fine Guy/Girl: Kiss Me." These spectral classes are shown on the Hertzsprung-Russell diagram. Special stars, such as pulsars and black holes, do not belong to any of these classes, and are not shown on the diagram.
A spectroscope is any instrument that produces a spectrum. In a spectroscope, light is passed through a narrow slit and a lens. It then passes through a prism or a diffraction grating, splitting into its different frequencies, or colors. A viewing telescope is used to observe the lines. A spectrometer is similar to a spectroscope, except that it measures the intensity of the light. A spectrograph is used to photograph the spectra. The spectra are analyzed later. A spectrophotometer is different in that it uses photoelectric cells to show the intensity of light. It is used to measure the absorption of light by different liquids or solutions. A spectroheliograph is used specifically on our own star.
Knowledge of science is expanding all the time, in spectroscopy and in other areas. Scientists learn more and more each day, much more than Auguste Comte could ever have imagined.
[F1] Stars. Virginia: Time-Life Books, n.d.
1666 - Isaac Newton let sunlight pass through a prism, producing a spectrum of color.
1790 - Johann Wolfgang von Goethe said, "The idea of white light being composed of colored lights is quite inconceivable, mere twaddle, admirable for children in a go-cart."
1802 - William Wollaston, an English chemist, discovered that light from the sun did not form a perfect spectrum. The spectrum was slashed by dark lines.
1814 - Joseph Fraunhofer made one of the earliest studies of absorption lines. He discovered that the spectra of various stars had different black lines. He hypothesized that the dark lines were caused by the absence of certain wavelengths of light.
1872 - Henry Draper was the first researcher to successfully photograph the spectrum of a star. It was the star Vega. He went on to record the spectra of over eighty other stars.
1900 - Max Planck, a German physicist, theorized that electromagnetic radiation is emitted from an atom in certain quantities, called photons.
Absorption Line: a dark line or band at a particular wavelength on a spectrum, formed when a substance between a radioactive source and an observer absorbs electromagnetic radiation of that wavelength. Different substances produce characteristic patterns of absorption lines.
Color-Brightness Diagram: a graph showing significant correlations between a star's spectral class and luminosity.
Continuous Spectrum: a spectrum consisting of all wavelengths in a given range, without absorption or emission lines.
Doppler Effect: the change in wavelength observed when a body emitting light is moving toward (blue shift) or away (red shift) from an observer.
Emission Line: a bright band at a particular wavelength on a spectrum, emitted by the source and indicating by its wavelength a chemical constituent of that source.
Energy Level: the quantity of energy associated with an electron. An increase in energy will shift electrons to higher energy levels within an atom.
Ground State: the lowest possible energy level for a given electron.
Intensity: the amount of radiation received from an object; optical astronomers prefer the term "brightness."
Main Sequence: a diagonal region in the Hertzsprung-Russel diagram that indicates 90 percent of all stars.
Main-Sequence Star: one of a class of stars that increase in size, temperature and brightness in a regular progression.
Photometer: a device that measures an object's brightness, or apparent magnitude, by detecting its emitted photons.
Photon: a unit of electromagnetic energy associated with a specific wavelength.
Spectral Class: the classification of stars according to the principle features in their spectra.
Spectrogram: a photograph of an astronomical spectrum.
Spectrograph: an instrument that splits light or other electromagnetic radiation into its individual wavelengths, or spectrum, and records the result photographically.
Spectroscopy: the study of spectra, including the position and intensity of emission and absorption lines.
Spectrum: the array of colors or frequencies obtained by dispersing light, as through a prism; often banded with absorption or emission lines.
Wavelength: the distance from crest to crest or trough to trough of an electromagnetic or other wave. Wavelengths are related to frequency: the longer the wavelength, the lower the frequency.
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