Spectroscopy-The Astronomers Amazing Tool! August 2, 2012Posted by dwelchscience in Astronomy, physics.
Tags: physics, spectra
Can we really solve all the mysteries of the Universe? Well, maybe not but SPECTROSCOPY certainly holds the key to unfolding many planetary secrets! What benefit do we get from the study of spectroscopy? Basically….
Almost all knowledge of the composition of the solar system comes from spectroscopy.
Scientists are also looking at:
Nasa’s Hubble telescope, which is delving into the far reaches of the universe looking for large scale structures with a new tool, the “cosmic origins spectrograph.”
Nasa is planning to launch a telescope that will help us see beyond the Hubble’s capabilities, in order to examine Infrared light from the oldest parts of the universe.
What is SPECTROSCOPY?
Spectroscopy relates to the dispersion of an object’s light into its individual colors or energies. By analyzing the object’s light, astronomers can determine the various physical properties of that object, such as temperature, mass, luminosity and composition.
Before going further it is important to review some basics of light: What is light? How does it behave? It’s been a very long haul through time to solve these two very fundamental questions. During this century alone, with the development of quantum mechanics have we understood quantitatively how light and atoms work.
Dual nature of Light:
Light behaves like a wave. Light has a particle nature also, but for now let’s focus on the wave nature of light. Waves are disturbances that possess a height (amplitude), in which the peak is the crest and the base is the trough. When a certain number of waves pass per unit of time, such as the second, this is called frequency . The unit for frequency is nu, ν, and means cycles per second, or hertz (Hz). The characteristic speed the wave moves through a medium is the wave speed. The distance between successive waves is the wavelength, given the symbol lambda, ( λ). Often given in meters or nanometers. The diagram below demonstrations two types of waves, low and high energy:
We can think of the above wave diagram like the water in the ocean. Or, let’s direct our thoughts to waves of LIGHT. The wave speed is the speed of light ,(c) a nd different wavelengths of light manifest themselves as different colors! The energy of light wave is inversely proportional to its wavelength, or low-energy waves have long wavelengths and low frequency. High energy waves have short-wavelengths and high frequency.
The Electromagnetic Spectrum:
Physicists classify light waves by their energies or wavelengths. Below is the Electromagnetic Spectrum which gives us a chart of this. The shorter the wavelength beyond visible light the more dangerous the form of energy to organisms. The human eye can only respond to the visible light spectrum, between 700-400 nm (ROYGBBIV).
When we look at the universe in non-visible wavelengths, we can probe different kinds of physical conditions-enabling us to see new objects in space! For example, today we have high energy gamma-ray and X-ray telescopes in which we can visualize galaxies, remnants of dying stars, matter around black holes, etc. However, visible light telescopes are best to probe light produced by stars. Longer-wavelength telescopes are used to study dark, cool, obscure structures: star-forming regions, dark cold molecular clouds, and primordial radiation emitted by the formation of the Universe.
“By studying astronomical objects at different wavelengths astronomers can piece together a comprehensive picture of how the Universe works.”
Types of Spectra:
There are 2 general types of spectra: continuous and discrete. The continuous spectrum is composed of light that is a continuous range of colors (energies). With discrete spectra, the light consists of dark or bright bands of very distinct colors (energies). The discrete spectra with bright bands is referred to as emission spectra, and those with dark lines are absorption spectra.
The continuous spectra generates from dense gases or solid objects that radiate heat away due to the production of light. These objects produce light over a broad spectrum of wavelengths, therefore the spectrum appears “continuous”. Stars emit light generally in a continuous spectrum but not always. Other examples of objects emitting light: light bulbs, electric stove filaments, cooling fire embers, flames and YOU! Yes, believe it or not you are emitting a continuous spectrum but at infrared wavelengths that are not visible to your eye. Infrared (IR), would be lower energy and longer wavelengths than red light.
Light & Matter: Arapaho.nsuok.edu
Discrete spectra are the result of the atom. There are two forms of discrete spectra: emission (bright line) and absorption (dark line) spectra.
Emission Line Spectra:
Unlike the continuous spectrum which can have any energy by changing the temperature, the electron cloud surrounding the nuclei can only have very “specific, discrete packets of energy” or quanta. Each element on the periodic table has its own set of energies and most are very distinct and identifiable.
The diagram below shows a hydrogen atom dropping from the 2nd energy level to the first. As it drops it emits a wave of light, photon, with an energy equal to the difference between the second and first energy levels. This energy corresponds to a specific color or wavelength of light, therefore we see a bright line at that exact wavelength. This is an emission spectrum, see example below:
Very small changes in energy generate photons with small energies and long wavelengths, such as radio waves. Large changes in energy in an atom will result in high-energy, short-wavelength photons: UV, X-ray, and gamma rays. The diagram below shows an excited Hydrogen atom emitting a photon resulting in a bright line emission line.
Absorption Line Spectra:
What about reversing this process? What happens if we fire a photon back into a ground state atom? The atom would become excited and jump to a higher energy level. If a star with a “continuous spectrum” is shining upon an atom, the wavelengths of each energy transition would be absorbed and we would not see them. This would be a dark line absorption spectrum. The diagram below shows this process of a hydrogen atom absorbing the exact photon to produce a dark absorption line.
In the diagram below, the absorption spectrum example is from medium resolution of the Sun’s spectrum. The dark lines are called Fraunhofer lines, or solar absorption lines. These lines are a combination of lines produced as sunlight passes through the outer layers of the Sun’s atmosphere AND lines from the sunlight that goes through Earth’s atmosphere. A comparison can be made to identify & remove the Earth’s lines, therefore allowing scientists to determine the atoms and molecules coming from the Sun’s spectrum. Also observe the second diagram below for analyzing stars.
In the above diagram we see the continuous spectrum of the sun produced as its light moves outward towards us. When the light reaches the cooler chromosphere (solar atmosphere) some colors or light are absorbed. So astronomers can see an absorption spectrum due to the absorption of the chromosphere.
Spectrometer: A Powerful Applications in a Simple Device!
A spectrometer is an instrument used to measure properties of light over a certain portion of the Electromagnetic Spectrum, with a purpose of spectroscopic analysis in order to identify different materials. Usually, the variable being measured is light intensity, however, other factors could be looked at such as polarization state of light. The independent variable is the wavelength of light, or a unit inversely proportional to the photon energy or electron volts and wavenumber. A spectrometer is used to produce spectral lines and measuring their intensities and wavelengths. Spectrometer is designed to operate over a wide range of wavelengths, from gamma, x-rays, or to the far-infrared. There are several kinds of spectrometers, such as spectrographs that measure wave frequency by using photographic paper as a detector. The star spectral classification and the discovery of the main sequence, Hubble’s Law, and the Hubble sequence were all made using photographic paper. Today, detectors are electronic, such as CCD, which can be used for visible and UV light. The choice of detector depends upon the wavelength of light being analyzed.
A spectrometer consists of 3 basic parts:
1) A small telescope collects light which is focused into a thin beam by using a narrow slit and specific lenses.
2) A diffraction grating acts like a prism and a spectrum is produced from the incident light.
3) A detector, or photocell, measures the intensity of the light in various regions of the spectrum. A voltmeter can be used as a detector.
4) The detector “scans” the spectrum and intensity of each point of light is graphed. The result is shown in the graph below:
Most people have a general idea of how a telescope works, but not a spectrometer. The following diagrams below explain the basic structure of a spectrometer. There are many types, this example is one that was used at an Astronomy Space Camp for kids.
- Light beam enters the spectrometer.
- The focal point is directed to the slit, which is imaged on the detector.
- The slit is set at an angle and the area around it is silvered so that the beam not passing through the slit can be re-routed to the eyepiece for easy guiding of the telescope.
Diagram #2- this picture shows the light passing through the slit and being bounced off of a collimating mirror in order to parallelize the light before it goes to the diffraction grating.
Diagram #3- the diffraction grating disperses the parallel beams of light into its individual colors, wavelengths, and energies. Each “individual” beam coming out of the grating is bent at a different angle or wavelength. The image we see looks like a rainbow of colors.
The colored-dispersed beam of light is focused and imaged on the detector using a 35 mm camera lens. The space camp spectrometer used an eyepiece or CCD.
(All hand-drawn diagrams taken from Spectroscopy Space Camp- loke.as.arizona.edu)
So, putting all the parts together, the diagram above shows the complete Spectrometer. An important point to make concerning Spectroscopy: astronomers are not looking at ALL the light, but a certain “region” of wavelength of color. Because surface brightness is lower when taking images a bigger telescope is needed to get a good spectrum of an object in space.
The narrower the slit and farther the light is dispersed, the better the resolution. This enables astronomers to view the more subtle characteristics of the spectrum. The down side is, the spectrum becomes dimmer and more diffuse. Retaining clarity is difficult, magnifying an image usually results in a more blurred object. Therefore, high resolution spectroscopy requires a BIGGER telescope and brighter objects to view! For the dim objects in the night sky, resolution will be sacrificed.
Examples of Spectroscopy in Astronomy:
A lot of valuable information can be obtained from the use of spectroscopy in Astronomy. We can gather information about the temperature, density, composition and physical processes of objects in space.
When peering into the far reaches of space some questions that we may ask: What is it? How did it get here? What is it made of? How did the universe begin? What will happen during the object’s life cycle? Astronomers can gather valuable data using spectroscopy and other means to attempt to answer such questions.
Comets: comets are formed from materials dating back to the earliest times when our solar system was forming. The composition of these “dirty snowballs” can give us clues into our universe’s early history.
Two images taken 9 minutes apart showing the comet and mag 9 star SAO 80381 embedded in the comma. A rather noisy image with only 5 frames taken due to cloud cover, however, the spectrum is visible. Taken by 80 mm and f5 refractor.
Hale Bopp- March-April 1997: the strong background lines are from the Mercury streetlights. Authors heard that a newly discovered tail was composed of Sodium, so they set out to prove if this was correct. The slit was centered on the tail and a sodium emission line at 589 nm was the result, it was Sodium!
Star Formation in Colliding Galaxies:
When the universe was forming billions of years ago, there were intense, rapid star formation going on. This has since decreased a great deal. Astronomers know that galaxies demonstrate violent and extreme star formation. “Starburst” galaxies show up best in the Infrared and Radio wavelengths. This is because they harbor so much dust and gas that this prevents the penetration of visible light, especially in the center where star formation takes place. The above spectrum shows Infrared between 20,000-25,000 Angstroms of two starburst galaxies, most of which is H2 gas—what stars are made of! From the hydrogen emissions the molecular gas is very warm. In the top galaxy the gas is excited by shock heated gas. The bottom spectrum shows molecular H2 excited by UV light from a recently formed, young and hot star.
Quasar were discovered in the 1960’s and determined to be red-shifted due to the speeding away or expansion of quasars from us. This process is explained in the Big Bang Theory of cosmology, which states that the faster its speeding away from you the more distant it is. Quasars are the most distant astronomical bodies known to man. The diagram below is the spectrum for a Quasar, in which the most obvious emission line is Hydrogen at 1216 Angstroms. The hydrogen atom is making the transition from the first excited state to the ground state. The emission line at 1216 Angstroms is in the UV where Earth’s atmosphere is opaque, quasars are expanding very rapidly away from us so that this emission line is red-shifted into the visible light portion of the spectrum at 4,000-7,000 Angstroms.
Planetary Nebulas & Why Pollution Filters Work:
Image of M57 aka the “Ring Nebula” with spectrometer slit shown in red. This is 180 second exposure on a 10” Meade Schmidt-Cassagrain telescope. The resulting image on the spectrum shown below has an emission line at 4861 Angstroms. This comes from hot, excited atomic hydrogen. Highly – excited atoms in M57’s gaseous shell begins at energy level 4 and will probably drop to level 2, giving up energy by the difference between the two levels. The two brightest lines at 4959 and 5007 indicate that conditions in this nebula are indeed very harsh! Temperatures consist of several thousand Kelvins and very thin density (1-100 atoms/cm3). A LINE spectrum is seen, proving that planetary nebulae are hot rarified gases! This spectrum also reinforces the fact that using light-pollution filters help viewers get great contrast from reflection/emission nebulae by blocking out all wavelengths due to skyglow and allow for only the wavelengths in the range needed for viewing nebulas.
Sirius-The Dog Star:
Shown below is a ½ second exposure of Sirius centered near 4,000 Angstroms in the blue, near-UV and shows a series of deep absorption lines. Sirius’s outer atmosphere is cooler and mildly excited H atoms in the 2nd energy level are “zapped” by photons. This sends them to higher excited states. The dark absorption line in the diagram is the result of each transition upward in the H atom. The higher energy transitions on the left are the result higher energy absorption in the UV. (Balmer series).
Stars are classed by their temperatures, determined by their spectral features. Hottest stars are called O-stars, then B..A, F, G, K and lastly, M-stars. Hotter stars give off more light. Sirius is a hot A-type star with a temperature of 10,000 Kelvin. This type of star has the strongest hydrogen features due to their temperature. These hot stars ionize the hydrogen atoms that form the spectral lines!
Below is a another diagram of the classification scheme of stars, (OBAFGKM). Each category further divided into 10 subclasses. A mnemonic for remembering the sequence is: Oh Be A Fine Girl/Guy Kiss Me. Spectral type tells you about the surface temperature of a star. Notice there are few spectral lines for the hot O & B region. This indicates the simple atomic structure related to high temperature.
The appearance of stars is related more to the continuous spectrum of the inner parts of the star than the absorption at its surface. The continuous spectra for the interior of stars is described by Planck Curves shown in the two figures below. As the temperature increases, total amount of light energy (area under curve) increases also and the peak wavelength moves to a smaller more energetic wavelength.
Spectral Classification- Identifying a Star’s Lifespan
Using information obtained from Planck’s Curve and wavelength we can now determine star temperature and color in order to get an idea of a star’s approximate life expectancy. The following table lists corresponding values of color, temperature, mass and life expectancy of the stars in the OBAFGKM system.
Below are two charts, a plot of star luminosities versus stellar temperatures, is called an H-R diagram and a more detailed chart of radius, mass, luminosity, etc. Students can make comparisons of basic properties of each class type using this chart.
About 90% of all stars lie in the “main sequence” which stretches from hot, bright blue supergiants and giants through intermediate stars such as our sun to cool red dwarfs. By using spectroscopy astronomers can determine the luminosity class of a star. A valuable tool for astronomers, the spectrometer and spectrograph, that gives them insight into our distance stellar neighbors.
Students can conduct a further study by going to astro.unl.edu/naap under Spectral classification, “Practice exercises” and answer the following:
1) What are the surface temperatures and colors of O2, M3, and G2 stars? 2) What is the spectral type of a star with surface temperature of a) 10,000K and b) 5,000K? 3) What is the color of a star with spectral type A0 and surface temperature 4,000K? Answers to these can be determined by using the “sliding” star graph on the website.
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