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ID#: __ __ __ __ __ __ Note: you only need to hand in the first 3 pages of this handout… TYPES OF STARS Modified from: http://cas.sdss.org/dr5/en/proj/teachers/basic/spectraltypes/lesson.asp When astronomers look through their telescopes, they see billions of stars. How do they make sense of all these stars? The goal of this problem set is for you to understand that astronomers classify stars on the basis of two different criteria: (1) the intensity of one of the H absorption lines (called H), and (2) on the basis of temperature. At the end of this handout, there are typical spectra of 7 stars. In each case there is one spectrum showing the data acquired over the visible wavelength range (the x axis is given in terms of wavelength in Ångstroms, which are just an order of magnitude different than the nm we’ve been using in class) and the y axis is flux (don’t worry about those units!). Below each big plot is a zoomed-in view of the area of the spectrum around 6560 Ångstroms (i.e., 656 nm). Question 1. Star #1 has a conspicuous Hα transition at 6563 Ångstroms. So, the zoomedin view shows the Hα line for the spectrum. Does the Hα line represent more light or less light (relative to the surrounding wavelengths)? [In other words, does it point up or down?] How do you know? Question 2. Given your answer to Question 1, is the Hα line in this spectrum an emission line or an absorption line? How do you know? We can classify stars based on the "strength" of their H lines. Look at the spectra of the seven stars at the end of this worksheet. Using their spectra, rank the seven stars according to the strength of their Hα lines. If you can't rank them all easily, try coloring in the area between the line connecting the triangle centers and the actual spectrum. The bigger the colored area, the greater the line strength. Originally, astronomers classified those stars with the strongest hydrogen lines as 'A' stars, stars with the next strongest lines as 'B' stars, the next strongest 'C' and so on. Eventually, they realized that some letters were unnecessary, and dropped them from the classification system. The letter assigned to a star is called its spectral class. So the spectral classes are A, B, F, G, K, M, and O. In the third column of the table below, write one of these spectral class letters for each star in the table. There is only one star of each class in this data. Question 3: Classifying by Line Strength : Fill in the table below. Line Strength Greatest line area Least line area Star Number Spectral Class Question: Mnemonic Make up a mnemonic to help you remember the 7 spectra classes. Question 5. Star temperature. Look at the spectra of the seven stars again. This time, you can ignore the zoom-in beneath each panel. For each spectrum, trace the underlying continuum shape – in other words, draw a line that shows what the spectrum would look like if it had no emission or absorption lines. In each spectrum, identify the wavelength at which that that continuum peak is most intense. If the peak is not shown on the graph, then estimate where you think it might peak. Rank the stars according to peak wavelength of each star's thermal continuum, using the table below, and calculate their temperature using Wien’s Law. Remember that Wien’s law is expressed as max = 29,000,000/T(K) if wavelength is in Å. Peak Wavelength Peak at shortest wavelength Peak at longest wavelength Peak Wavelength (Å) Temperature (in K) based on Wien’s Law Star Number Question 6. The Connection Between Temperature and Line Strength Look back to the tables you created above. In the first row of the table below, list the stars (1-7) in order of decreasing Hα line strength (strongest on the left, weakest on the right) based on your responses to Question #3. In the second row, list the stars by number, in order of decreasing temperature (hottest on the left, coolest on the right), based on you answers to questions 5 and 6. In the third row, add the spectra class that you decided upon for each star. Strongest Hα Line Hottest Temperature Spectral Class Weakest Hα Line Coolest Temperature Spectral Class Are these two classification schemes the same? Question 7. What two classes of stars have the strongest H lines? Are these the hottest stars? The coolest? Question 8. Which class of stars is the hottest? Which class of stars is the coolest? How strong are the Hα lines for these two classes of stars? Star #1 Star #2 Star #3 Star #4 Star #5 Star #6 Star #7