The diagram illustrates physical size and angular size as seen by the observer. We can estimate the size in light years (or parsecs or km) of M31 from
A) its angular size in degrees.
B) its distance in light years (or parsecs or km).
C) both answers A and B
D) It is impossible to estimate the size of M31.

Question

The diagram illustrates physical size and angular size as seen by the observer. We can estimate the size in light years (or parsecs or km) of M31 from 
A) its angular size in degrees. 
B) its distance in light years (or parsecs or km). 
C) both answers A and B 
D) It is impossible to estimate the size of M31.

Quizplus · Accepted Answer

To estimate the size of M31, we need both the angular size in degrees and the distance in light years (or parsecs or km). These can be obtained from the diagram. The angular size of M31 is approximately 3 degrees. From the diagram, we can see that the distance between M31 and Earth is approximately 2.4 million light years. Hence, the size of M31 can be estimated to be around 220,000 light years (2.4 million light years x tan(1.5 degrees)), assuming it is roughly circular in shape.

The Diagram Illustrates Physical Size and Angular Size as Seen