Starship Asterisk*

Why is it four images rather than a ring?

It was already explained in that picture

I'm sorry, I don't see the explanation. The caption says that the light bent by gravitational lensing produces four images of the distant quasar, but it does not explain why there are not eight, or sixteen, or thirty-seven images, or a uniform ring.

I can't find perfect explanation link, but in a few words it goes like this: you see ring when light source is directly behind the mass; if it is not so "directly" behind, you see arcs; if it is not behind "at all", you see multiple images. Try reading here.

Update: things are getting even more complex, if you 'll consider a number of masses on the light path:

http://www.cita.utoronto.ca/%7Ehoekstra/lensing.html wrote:
The paths of three light rays travelling through a computer simulation. The deflections have been grossly
enhanced (in the real universe the deflections are very small). All the filaments and clusters change the
paths and the shape of the galaxy we observe at Earth has been changed (it original shape can be seen at
the left side of the image). Credit: S. Colombi (IAP), CFHT Team.

Thanks for the link and the very informative diagram from the CFHT group. I took a quick look and decided it's worth mentioning Dr Kaiser's course on galaxies at Southampton:
http://www.astro.soton.ac.uk/~crk/PH227/
The webpage you referenced is one of its nodes.

BANG wrote:It was already explained in that picture

Wow...noob, aka BANG, getting snappy about an explanation...that's ironic...

Orca wrote:...getting snappy about an explanation...that's ironic...

No, that's rather typical. People learn from other people, and that ain't necessary good things every time

naaaaaaa. i am just stupid, thats all!