Answer

Cas-A is approximately 700 pixels across.

Using the information that 1 physical pixel represents 0.5 arc seconds resolution of the Chandra satellite then Cas-A is 350 arc seconds across.

Using the relationship 206264.8 arc seconds = 1 radian we can determine the radians across Cas-A.

Now using the relationship, s = r , and given that Cas-A is 9.25 X 10¹⁶ kilometers from the Earth, we can determine that the length of Cas-A is approximately 1.57 X 10¹⁴ kilometers.

Remember, radians is not a unit, which is why are answer is given in kilometers.