Elliptical Galaxies - The categorization of elliptical galaxies involves measuring the two axis - the longest and the shortest, and using those values to determine just how elliptical one of these things is.
Formula: e = 10 (a-b)/a where:

• e = elliptical value, rounded to the nearest integer
• a = longest axis distance, measured in any sort of units
• b = shortest axis distance, measured in any sort of units

Typical Problems

1. Galaxy NGC 2033 has a long axis that is 74,000 pc long, and a short axis that is 32,000 pc long. What type of elliptical galaxy is it?

Solution: Pop the values into the formula
e = 10 (a-b)/a
e = 10 (74,000-32,000)/74,000
e = 10 (42,000)/74,000
e = 10 (0.57)
e = 5.7, rounded up to 6. The galaxy is a E6 type.

2. Galaxy NGC 4535 has a long axis that is measured as 15.7" long, and a short axis that is 12.3" long. What type of elliptical galaxy is it?

Solution: Different units, but since they are the same (arc-seconds), you can just pop them into the formula
e = 10 (a-b)/a
e = 10 (15.7-12.3)/15.7
e = 10 (3.4)/15.7
e = 10 (0.22)
e = 2.2, rounded down to 2. The galaxy is a E2 type.