How Big is CAS-A???
  
Now lets use Chandra to find out how big an object is.  We can do this with some simple calculations and the Chandra software.

1)  Open the Chandra software and load the CAS-A image.  Draw a circle around the image and got "get info" from the "REGION" option on the tool bar.

2)  Find the radius of the region.  The example above is about 338 pixels.  Therefore the diameter of Cas-A is approximately 676 pixels.

3)  What can we do with pixels?  Well 1 pixel is approximately 0.5 arc-seconds.  Therefore, Cas-A spans about 338 arc-seconds.

4)  How many radians does 338 arc-seconds equal?  You know the magic number...you figure it out.
click here for answer

5)  We found out in a previous lesson that Cas-A is 1 x 10^17 km from earth.  Now we can use small angle approximation to find out how big Cas-A is.

  
Now you can use Chandra to find out how big an object is (assuming you know how far away it is using the Doppler Effect).