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           Challenge 1. What percentage of galaxies 
          have spectral redshifts measured? What percentage have photometric redshifts 
          taken? What are the advantages of using photometric redshift? 
          Try to compare photometric and spectral redshift; how accurate 
          are photometric redshifts? 
          
          Challenge 2. What are the limits in ra 
          and dec of stripes 42 and 43, two of the SDSS's diagonal stripes? 
          
          Challenge 3. Look at colors and spectra 
          of stars, and find stars consistent with white dwarfs. Create a 
          list of white dwarfs in the SDSS database. 
          
          Challenge 4. What are the largest galaxies 
          in the SDSS, in terms of size? 
          HINT: Look at surface brightness and ellipticity. 
          
          
          Challenge 5. Find all objects with spectra 
          classified as unknown. 
          
          Challenge 6. Find the broad absorption line (BAL) 
          quasars in the SDSS database. At what redshifts are most BAL 
          quasars found? 
          
          Challenge 7. Find variable stars in the 
          SDSS (stars imaged more than once whose magnitudes changed by more 
          than 0.1 between observations). How variable are the stars? 
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