Separating Elliptical and Spiral Galaxies

In the paper, SDSS astronomers found a line that separates galaxies into early (elliptical) and late (spiral) types. That line corresponds to a u-r value of 2.2. You are now going to add that line to your graph. Unfortunately, this is tedious to do in Excel, so this page will show you how to add the line by hand.

Exercise 6: Print a copy of the more detailed color-color diagram you made in Exercise 5. Use a ruler and a pencil to sketch a line with the equation y = -x + 2.2 onto your graph. This line will separate elliptical and spiral galaxies.
Question 6: In the observable universe, 77% of galaxies are spiral (late) and 20% of galaxies are elliptical (early). How does your percentage of early and late galaxies compare to the universe? Would you say the galaxies in the vicinity of Abell 2255 are representative of the universe as a whole?