Pre-exploration Study and Information
I. Introduction
At least 95% of the celestial information that we receive on Earth is in the form of light (electromagnetic radiation). Most of what astronomers know about stars, galaxies, nebulae, and planetary atmospheres come from spectroscopy, the study of the wavelengths of light emitted by such objects.
Anatomconsists of anucleusandelectronsthat “orbit” around the nucleus. An atom emits energy when an electron jumps from a high-energy orbit around the nucleus to a smaller low-energy orbit. The energy appears as aphotonof light having energy exactly equal to the difference in the energies of the two electron levels. Aphotonis a wave of electromagnetic radiation the wavelength (distance from one wave crest to the next) of which is inversely proportional to its energy: high-energy photons have short wavelengths, low-energy photons have long wavelengths (Fig. 1). Because each element has a different electron structure — and, therefore, different electron orbits — each element emits a unique set of spectral lines.
Figure 1. — Emission of photons of light from an atom. See text for explanation.
Vocabulary/Definition
Word
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Definition
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Spectrum (plural: spectra)
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The pattern light produces when passed through a prism or diffraction grating, as seen through a spectrograph
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Spectroscopy
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A field of study that analyzes radiation energy as a function of its wavelength or frequency. Allows us to identify chemical compositions, temperatures, velocities, gas densities, pressures, and magnetic fields of many kinds of objects.
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Spectrograph
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A device that allows one to see a spectrum, which usually has a prism or diffraction grating inside. (Fig. 2)
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Diffraction Grating
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When light bends around an obstacle or through a small opening. In Fig. 2, it’s a sheet of material with thousands of evenly spaced parallel openings. At a unique angle for each wavelength, the waves constructively interfere (crests add to crests, troughs to troughs). At that angle, we see a color image formed by that wavelength of light.
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Continuous Spectrum
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The rainbow that white light is composed in which each color is equally bright.
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Emission Spectrum
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The bright lines that appear through the spectrograph against a dark background. This occurs when when electrons jump from a high-energy orbit to a smaller low-energy orbit.
Therefore an emission spectrum is characterized by radiation only at specific wavelengths and appears as several bright lines with dark gaps in between.
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Absorption Spectrum
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Dark lines that appear against the continuous spectrum seen through a spectrograph, indicating that certain wavelengths of the photons are being absorbed by rarefied gas on its way to the observer.
Here, the passing photon of light is absorbed by an atom and provides the energy for an electron to jump from a low-energy orbit to a high-energy orbit. The energy must be exactly equal to the difference in the energies of the two levels, otherwise the photon will pass by the atom unaffected.
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Light Source
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Any object that produces light.
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Table 1. Partially taken from:http://lasp.colorado.edu/home/education/k-12/project-spectra/
Figure 2. — Schematic diagram of a hand-held spectroscope and its operation. Light enters the spectroscope through a slit and strikes a grating (or prism) that disperses the light into its component colors (wavelengths, energies). Each color forms its own separate image of the opening; a slit is used to produce narrow images, so that adjacent colors do not overlap each other.
The human eye perceives different energies (wavelengths) of visible light as different colors. The highest-energy (shortest-wavelength) photon detectable by the human eye has a wavelength of about 400 nanometers (one nanometer equals 10-9 meters) and is perceived as “violet.” The lowest energy (longest wavelength) photon the eye can detect has a wavelength of about 750 nanometers (nm), and appears as “red.”
In astronomy we can make observations of many types of radiation, (light bulb, your TV, a planet, star, galaxy). When we pass this light through a spectrograph, a unique rainbow pattern will be produced. The intensity of each color of light can be plotted on a line graph. Intensity will be found on the y-axis and color (wavelength) on the x-axis.
Below are examples of a continuous spectrum, emission and absorption.
Figure 3.
Graphing the Rainbow
II. Stellar spectra
Stellar spectra can tell us about the temperatures and luminosities of stars. The sequence shown in the table below has the hottest stars, O stars, at the top, and at the bottom are the coolest stars, M stars. Various spectra will be seen to have prominent line features shown as in the Strong Line column.
Table 2.Principal classes and characteristics of stars showing color, temperature and strong lines.
Figure 4. Absorption spectra is shown for various stars ranging from hot to cool temperatures.
Figure 5.Absorption spectrais shown in a plot of relative brightness vs. wavelength. Chemical lines are shown such as Ca (calcium), Na (sodium), He (helium).
Let's look at the distinguishing characteristics of each spectral type, as illustrated by the example spectra above. The Fig. 5 shows spectra for seven stars of decreasing temperature: O5, B3, A6, F6, G7, K5 and M4. Note the following characteristics:
1. As temperature decreases, the spectrum shifts from peaking towards the short wavelength (blue) end, to the long wavelength (red) end. This reflects the overall, rough blackbody shape of stellar spectra, and Wien’s Law for blackbodies (see your textbook).
2. Hotter stars have more energetic photons that cooler stars. One consequence of this is for helium. It takes rather energetic photons to raise helium atoms to excited levels and thus produce an absorption line. Thus, helium absorption lines are only present in the hottest stars, even though helium itself is present in all stars. By the same reasoning, the coolest stars do not show absorption lines of hydrogen, even though they are mostly hydrogen! They do not have many photons energetic enough to raise hydrogen atoms to excited levels.
3. But hotter stars don’t show hydrogen lines either. Why? The reason is ionization. Energetic photons in hotter stars also cause more ionization. A consequence is that hydrogen is ionized in the hotter stars. Since it has lost its only electron, there are no electrons left to make transitions between energy levels and produce spectral lines. Calcium is easy to ionize, and even moderate temperature stars have energetic enough photons to do it. The pair of calcium lines is from ionized calcium atoms that have lost one electron, and are seen in moderate temperature stars. They get weaker in the hottest stars because such stars have energetic enough photons to cause calcium to lose more electrons. The sodium line is from neutral sodium and is seen in cooler stars, because even they can cause neutral sodium atoms to be in excited states. In moderate and hot stars, sodium is ionized, so we don’t see the neutral sodium line. The same is true for the neutral magnesium line marked. (You can review theindividual lineshere.)
4. In the coolest stars, atoms move relatively slowly and as a consequence can bond together when they collide, producing molecules. Molecules have rich absorption patterns of their own, often producing very broad features (wider). Prominent, broad trough-like features due to the TiO molecule are seen in the coolest stars. Note that we have covered the major spectral classes, but we can go even further and divide each letter into subdivisions, 0 through 9 following the letter and going from hotter to colder. Thus, A0 is colder than B9 and hotter than A1. Here is an example of a B-type star with subdivisions. Note as the flux gets lower, that relates to a cooler temperature, but you will still see the same general lines.
Note that we have covered the major spectral classes, but we can go even further and divide each letter into subdivisions, 0 through 9 following the letter and going from hotter to colder. Thus, A0 is colder than B9 and hotter than A1. Here is an example of a B-type star with subdivisions. Note as the flux gets lower, that relates to a cooler temperature, but you will still see the same general lines.
Figure 6.Spectra of B type starwith its subdivisions.
Activity
Analyzing Spectra
You have been observing stars with a spectrograph and have collected a number of spectra. For each of the stellar spectra in your Stellar Observation Sheet, you will categorize the star according to the example spectra in Fig. 5, determining which type it most closely resembles.
Required:Print out the Observations Sheet to make notes about your chemical lines. Please write your name and date next to each star, photo it and insert it into your lab report.NOTE:you still need to type out your answers to the below.
Usethis Flow Chartand the Luminosity Comparison Spectra below to Decide on the Approximate Spectral Class.
You will create a lab report that has a section for EACH star. Each section should have:
The star number.
Identify the stellar class (O, B, A, F, G, K, M). Consider the overall shape and the spectral lines present. If it appears to be between two types, note this too and note which one it is most like. IMPORTANT: write a couple of sentences about how you decided on the spectra type.
Note at least 3 of chemical lines that you see, and at what wavelength you see them at. (Try to use a straight edge, such as a ruler, to determine the wavelength of the spectral lines you are focusing on.)
From Table 2 of spectral types and temperatures, roughly estimate the temperature of each star.
Summarizing What We Learned
What did you learn from this lab regarding analyzing stellar spectra? From the prerequisite information for this lab, the video of a Ted Talk from Garik Israelian which talks about some of the applications of spectroscopy, using the internet find an article that deals with one of the possible applications of spectroscopy and summarize any important findings.(Summary needs to be typed and a minimum of 150 words long.) Be sure to include references.
Submit
Submit your data sheets to the proper assignment folder (using a digital camera or scanner).Ensure that your name and date are next to each star that you photo.
AST1120 Spectroscopy Lab Observation Sheet The below are stellar spectra that you have observed. For each star on your lab report, provide the following information: 1. The star number. 2. Identify the stellar class (O, B, A, F, G, K, M). Consider the overall shape and the spectral lines present. If it appears to be between two types, note this too and note which one it is most like. IMPORTANT: write a couple of sentences about how you decided on the spectra type. 3. Note at least 3 of chemical lines that you see, and at what wavelength you see them at. (Try to use a straight edge, such as a ruler, to determine the wavelength of the spectral lines you are focusing on.) 4. From Table 2 of spectral types and temperatures, roughly estimate the temperature of each star. Star 1. Star 2. Star 3. Star 4. Star 5.