Spectroscopy and Types of Spectra – Spectroscopy Tells Astronomers the Composition of Celestial Objects. Spectroscopy allows astronomers to deduce the chemical compositions of distant astronomical objects. What is astronomical spectroscopy and how does it work?
Spectroscopy, the branch of optics that measures the wavelength and intensity of a light spectrum, helps astronomers determine the composition and motion of stars and other heavenly bodies that give off light.
Invention of Spectroscopy
The 17th century physicist Isaac Newton – the same scientist who contributed to our understanding of gravity – was also the first to understand that varying wavelengths account for the different rainbow colors in the light spectrum we see. Each color is a product of electromagnetic radiation from an object reaching our field of vision at different frequencies, or wavelengths. It is possible to measure these wavelengths by using an instrument called a spectroscope.
Isaac Newton discovered that passing light through a prism could break it up into its component colors, like a rainbow. A rainbow is simply a low resolution spectrum of the Sun produced by water droplets in the atmosphere.
In 1814 Josef von Fraunhofer (1787 – 1826) built the first spectroscope and used it to observe the spectrum of the Sun. He discovered that the solar spectrum has dark lines, called absorption lines, which are specific wavelengths where light is missing.
Spectroscopy began in earnest in 1859 when Gustav Robert Kirchoff (1824 – 1887) and Robert Wilhelm Bunsen (1811-1899) built an improved spectroscope. Bunsen had recently invented the Bunsen burner and the two scientists used their spectroscope to study the spectra of materials burning in the Bunsen burner. (One wonders if they were pyromaniacs as well as scientists.)
How Spectroscopy is Used in Astronomy?
The spectroscope, aided by a powerful telescope, provides a picture called a spectrogram or spectrograph, which spreads the light patterns out into a rainbow. Looking at the spectrograph gives scientists information on the position of emission and absorption bands – the “fingerprints” of atoms and molecules – causing the unique light spectrum coming from that particular object.
An emission band results from an electron losing energy after dropping to a lower orbit around the nucleus of an atom. An absorption band is the result of an electron gaining energy as it climbs to a higher orbit. There is also a continuous band, which is the fingerprint of an electron that emits all the wavelengths of the colors that compose white light. The location and spacing of each band is unique for each atom, so spectroscopy helps astronomers learn many details about stars, planets, comets and galaxies: what the object is made of, what direction it’s moving and how fast, its temperature and its density.
For example, a spectrograph of the sun (the largest star in our solar system) will show dark lines caused by its atmosphere absorbing light at different wavelengths. The absorption process causes the intensity of the light at this wavelength to decrease, and that shows up as a dark line. If absorption bands show up as dark lines, emission bands show up as bright lines, also in a unique pattern for each element.
Since each of the atoms that make up the sun’s atmospheric gases can only absorb and emit specific wavelengths of light, in a particular pattern of lines, spectroscopy gives astronomers the clues they need to determine exactly which gases compose the sun’s atmosphere. The absorption lines of hydrogen can be found in the spectrograph of virtually every star, but in addition to hydrogen, there are other kinds of gases that may be present.
Using principles discovered by Newton and an 18th-century German scientist named Joseph von Fraunhofer, two scientists developed spectroscopy in 1859. One of these scientists was Robert Bunsen, who invented the Bunsen burner still used in many chemical laboratories today. The other was Gustav Kirchhoff. These two men made a prism that separated visible light emitted from substances vaporized in the flames of the Bunsen burner. Analyzing what he saw, Kirchhoff discovered that each gas produced a different spectrum. He observed that when emitted light passed through a cooler gas of the same substance, dark spectral lines replaced the bright ones on the same orbits. Using this knowledge, Kirchhoff was the first to explain that the sun was a hot, gaseous star.
However, it was not until several years later that British astronomer William Huggins and his wife, Margaret, began using spectroscopy to study the sun and other stars and to determine that they were all made mostly of hydrogen gas. Margaret gave particular attention to the spectrographic study of comets and nebulae. She identified nebulae as glowing, interstellar clouds of dust and gases, some of which are clusters of stars and some of which are the remains of dead or dying stars. This groundbreaking work paved the way for more sophisticated uses of spectroscopy that astronomers rely on to this day.
Types of Spectra
Bunsen and Kirchoff found that there are three types of spectra: continuous spectra, emission (or bright) line spectra, and absorption (or dark) line spectra.
A continuous spectrum has no sharp changes in brightness at different wavelengths, so no specific wavelengths are significantly brighter or darker than the adjacent wavelengths. A hot solid, liquid, or compressed opaque gas will produce a continuous spectrum. For example, an incandescent light bulb glows when an electric current heats a tungsten wire, so as a hot solid it produces a continuous spectrum.
An emission line spectrum is dark at most wavelengths, but certain specific wavelengths are bright. These bright wavelengths are called emission lines or bright lines. Emission line spectra are produced by a hot transparent gas and the wavelengths of the emission lines depend on the chemical composition of the gas. A neon sign glows when an electric current heats neon gas in a glass tube. Looking at a neon sign through a spectroscope reveals the emission line spectrum of neon. Neon signs glow bright red because the spectrum of neon has a large number of lines at red wavelengths. A neon sign that is not red is not really a neon sign. It works on the same principle but contains a gas other than neon.
In astronomy, emission nebulae are interstellar clouds of gas at a temperature of about 10,000 Kelvins. Their spectra are emission line spectra because they are a hot transparent gas.
An absorption line spectrum looks like a continuous spectrum with certain specific wavelengths missing. These dark lines are the absorption lines. An absorption spectrum occurs when light having a continuous spectrum passes through a transparent gas that is cooler than whatever produced the continuous spectrum. The wavelengths of the absorption lines depend on the chemical composition of the cool transparent gas. Most stars have absorption line spectra because their cores are much hotter and more compressed than their surface layers. The hot opaque core produces a continuous spectrum. When this light passes through the outer layers of the star, that are cooler than the core, absorption lines are produced.
Note that each type of atom or molecule has its own unique set of spectral lines that can be either emission or absorption lines depending on the physical condition producing the spectrum. This unique set of spectral lines produces a unique spectral signature for each element or compound and allows astronomers to determine the chemical compositions of astronomical objects. Chemists also use this technique to determine the chemical compositions of unknown samples on Earth, but chemists also have other techniques that are not available to astronomers.
Kirchoff and Bunsen did not know what causes the spectral lines. That required understanding atomic structure.