The Bridge V Learning Edition 1 ; our solar system | Page 52

The teacher could discuss the answers with students, encouraging them to share their
calculated values of the different types of galaxies found in both the field and the cluster as
well as their answer to the final question about the morphology-density effect. Student
understanding can be assessed by discussion as detailed throughout the activity and by
collecting scripts to mark. As these activities assess student understanding, additional
evaluation tasks are unnecessary. Suggested grading is detailed below. Suggested Grading
ˆ Table 1: 5 points - students provide clear explanations of the classification scheme they
create
ˆ Table 2: 2 points each - Answers (E/S0/SB0 - 2, 6, 9), (S - 1, 8, 12), (SB - 3, 4, 10), (IR - 5, 7,
11)
ˆ Tables 3, 4 and 5 (counting galaxies): not graded - based on student's subjective
interpretation.
ˆ Table 6 and Calculations: 30 points - Graded for completion, not accuracy. Students will
get different numbers, but math should be correct. Answers for percentages are typically
in the following range: (Cluster: E 50%, L 30%, S 20%) (Field: E 20%, L 10%, S 70%).
Students usually find a higher percentage of spirals in the field. Hypothesis Question: 30
points – Student’s hypothesis should mention the effects of interactions and ram-
pressure stripping in changing past gas-rich spirals into current gas-poor ellipticals and
lenticulars in clusters.
MATERIALS
ˆ Image of 40 galaxies.
ˆ Galaxies cards A to D.
BACKGROUND INFORMATION
Galaxy Classification
Astronomers classify galaxies based on their appearance into three main classes: elliptical,
spiral, and irregular galaxies. Edwin Hubble first came up with this classification scheme. Hubble
originally thought that the ‘tuning fork’ sequence represented the evolutionary progression of
galaxies. This concept turned out to be wrong, but astronomers still use these general
categories and labels to describe galaxies.
The Main Galaxy Types
Elliptical (E), Lenticular (S0), Barred Lenticular (SB0), Spiral (S), Barred Spiral (SB) and Irregular
(IR). In detailed on the activity description section.
An additional type of galaxy category
Interacting: Consists of two or more galaxies that are so close together that they are affecting
each other's shape.
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