Continuous, Emission, and Absorption Line Spectra

Transcription

1 name Continuous, Emission, and Absorption Line Spectra In this lab we will be looking at emission and absorption lines and understand what type of information we can gain from them. There are four parts to this lab. a) Looking at a Light Bulb and analyzing Black Body Radiation b) Looking at Gases c) Looking the Solar Spectrum d) Classifying Stellar Spectra Below is a graphical illustration of the differences between Continuous Spectra, Emission Line Spectra and Absorption Line Spectra (this is also known as Kirchoff s Laws ). Make sure you understand the concept of this. Spectral Line Formation Lab 8 1

2 Part I Lamps and Black Bodies 1) Observe the Intensity and the Color of the Light from the light bulb. Using the Variable Switch turn on the Light Bulb and slowly increase the Voltage. Do not use the spectroscope. What happens to the intensity as you increase the electricity? What happens to the temperature as you increase the electricity? What happens to the color as you increase the electricity? 2) Use the Spectroscope to analyze the light from that bulb. Pick an intermediate Intensity and describe what you see. List the colors from left to right. Using color pens, draw what you see. Identify and label wavelengths, i.e., write the appropriate wavelengths below the drawing. Draw an arrow in the direction of increasing wavelengths. 2 Lab 8 Spectral Line Formation

3 3) Drawing the Spectrum in the form of a graph. Are all colors equally bright? Which color is brightest? Now turn the variable switch to a setting where yellow is the brightest color. Let s draw a graph of what you see. In the previous drawing you looked at the colors, and labeled the wavelengths this corresponds to the x-axis of your graph. The y-axis is the intensity of each color. This is shown below. Intensity very bright bright somewhat fainter much fainter Wavelength Compare the relative intensities of those colors. (How much fainter is the red light compared to the yellow? What about the blue light? Use the words somewhat fainter and much fainter.) Draw this into the above graph. (For example, for yellow draw a point the Intensity is bright ; repeat for the other colors). Connect all pints with a smooth curve that goes through all those points. And voila this is your spectrum it the Black Body Spectrum (or the Planck Spectrum same thing, other name). Now repeat the above analysis for two more settings the case where the blue light is the most intense, and where orange is most intense. Draw those points into the above graph and connect the points of each of the two settings. In WORDS describe what happens to the shape of the black body spectrum as the electricity is turned down? Spectral Line Formation Lab 8 3

4 4) Understanding the Correlations In WORDS describe the correlation between the peak wavelength of the black body spectrum and the overall color of the light. In WORDS describe the correlation between the temperature and intensity of the light bulb. Write a formula for that correlation into the box below. This is Stephan Bolzman s Law. If we were to make record the exact measurements of temperature and intensity, we would end up with a better mathematical description of that formula. Look up that version of Stephan Bolzman s Law and write it into the box below. In WORDS describe the correlation between temperature and the peak wavelength of the black body spectrum (denote the peak wavelength by λ max ). Write a formula for that correlation into the box below. This is Wien s Law. If we were to make record the exact measurements of temperature and wavelengths, we would end up with a better mathematical description of that formula. Look up that version of Wien s Law and write it into the box below. 4 Lab 8 Spectral Line Formation

5 Part II Gasses Go back to your lab and look at the spectra of the gas tubes. Draw which lines you see, and then identify the gases by comparing the spectra you see to the chart in the hallway. Gas: Hydrogen Gas 1: Gas 2: Gas 3: Gas 4: Gas 5: Spectral Line Formation Lab 8 5

6 Part III Understanding Spectral Lines 1) Consider the Hydrogen gas. Draw the Bohr Model and indicate which line corresponds to which color. Write down the names of each line. (You should include the Lyman, Balmer and Paschen series and also indicate which lines are the H α, H β, H γ, and H δ lines.) Which of these lines are seen in the visual part of the spectrum? What are their colors? Which of these lines is the strongest one? What is the overall color (without a prism or diffraction grating) of Hydrogen gas? 2) Why are the positions of the spectral lines different for different gases? (This is the reason why each gas has its own fingerprint. ) 6 Lab 8 Spectral Line Formation

7 3) How can we use spectroscopy to determine the chemical make-up of Emission Line Nebulae? 4) Explain how you would determine the chemical composition of stars. 5) The atmospheres of different types of stars all have roughly the same chemical composition, but their spectra vary considerably. Explain WHY their spectra are different. 6) Explain how we use spectroscopy (not photometry) to determine the surface temperature of stars. Spectral Line Formation Lab 8 7

8 Part IV- Classifying Stellar Spectra There are two methods of how to classify stellar spectra. As in the main Stellar Spectra Lab you can look at the intensity trace (where dark lines correspond to dips in the intensity trace) of a spectrum, identify many details and get relatively accurate classifications. While this method is good, obtaining individual spectra may take a long time. Thus astronomers sometimes take several spectra during one observation. Oftentimes these spectra are of poorer quality and rough eye-ball classifications are sufficient. You will do this with the picture on the next page. At first sight the spectra will look similar, but if you look carefully, you ll notice details. Pay particular attention to the presence or absence of major lines and compare relative intensities. You should be able to classify the stars accurate to within three subdivisions as explained below. The picture on the next page was obtained with the 24-inch Burrell Schmidt telescope at Warner and Swasey Observatory in Ohio. It is a picture of one of the richest star clouds in the constellation Cygnus. The stars magnitudes range roughly from 5 th to 9 th magnitude. The comparison stars (Figure 7) are placed top to bottom in order of decreasing surface temperature in the following order: O B A F G K M (to be remembered using the lyric O h B e A F ine G uy /G irl K iss M e ). The stars are subdivided further into ten classes: A0, A1, A2,, A9. Class B9 precedes A0, and A9 is followed by F0. In spectral types B, A, and F, the pattern of hydrogen lines plays an important role. The secret for distinguishing B from A and F is the relative strength of the H and K lines of ionized calcium. For G and K spectra check the G band at 4307Å and the neutral calcium line at 4227Å. In addition there are also Wolf-Rayet stars, which are very hot and contain bright emission lines (all other stars have absorption, not emission lines). Note that violet is to the left, longer wavelengths to the right. Let s investigate the spectra of the first six stars. Notice that star 1 and 5 have similar, almost featureless, continua; 2 and 6 are related, both exhibiting many hydrogen lines; 3 is an emission-line star; and 4 is a cool star (the intense part of 4 is very short and is concentrated on the red end). Star 5 is also relative featureless, but is strong in ultraviolet light (so it is a hot star). Now compare 2 and 6 more carefully. The hydrogen lines are comparatively stronger in 2, but the most striking difference is the appearance of the H and K pair in 6, which spoils the regular pattern of the Balmer series. This means that 2 is an A star and 6 a F star. In star #4 the H and K lines are clearly visible, but the hydrogen series has, vanished, thus ruling out type 0 and earlier. The dark line some distance to the right of the N and K pair is the G band. Since the 4227 calcium line is not visible, this must be an early K, actually K2. It is instructive to compare this spectrum with 26, 27, and 28, as well as with the key. Examples 1 and 5 are clearly much earlier than A0, belonging in the O-B classes, but an exact classification is difficult on the reproduction because the critical absorption lines are very fine. The long smooth spectrum 5, with no trace of hydrogen lines, reveals an 0 star. In star #1 the Balmer series is faintly visible, indicating a BO star. Using the photograph of the comparison stars (Figure 2) classify the remaining stars in Figure 1. Astronomers who are trained to look at these types of Spectra can classify the stars accurate to two subclasses (e.g., when you identify a star as an A5 star, it could actually be any star of the types A3, A4, A5, A6, or A7). You should be able to classify the stars accurate the five sub-classes. The photocopies do not show enough detail, please download Figure 1 (or this entire exercise) from the Web and view it with a 200% magnification. 8 Lab 8 Spectral Line Formation

9 Fig1 Spectral Line Formation Lab 8 9

10 Star Spectral Type Optional Comments Lab 8 Spectral Line Formation