<p>As mentioned in the comments, a ring will (ideally) only occur if you have perfect alignment of source, lens, and observer. This is true for point sources, for extended sources things get more complicated.</p>

<p>Double and quadruple images occur when the source is small and not in perfect alignment - typically a quasar, as in your example. For the sake of completeness, this would actually be triple or quintuple, but the third / fifth image is very close to the centre, overpowered by the lens, and highly demagnified (faint).</p>

<p>There are examples of rings and almost-rings, but these tend to be extended sources, i.e. galaxies. It is more likely that some part of an extended source happens to be aligned with lens and observer than it is for a point source.</p>

<p><a href="https://i.sstatic.net/YIGJP.jpg" rel="noreferrer"><img src="https://i.sstatic.net/YIGJP.jpg" alt="Cosmic Horseshoe"></a><br />(Image of the <a href="http://apod.nasa.gov/apod/ap111221.html" rel="noreferrer">Cosmic Horseshoe</a>)</p>

<p>In your example, if the host galaxy of that quasar could be seen, it might be an Einstein ring (or an oval, as the lens seems to be somewhat asymmetric).</p>

CC BY-SA 3.0
2016-08-04T21:27:32.673Z
2016-08-05T18:49:41.647Z
2016-08-05T18:49:41.647Z
Alex
<p>I will expect that gravitational lensing should turn an object into a ring image. But I saw in some situations that instead multiple images are formed. For example, the Einstein-cross in the image below, consisting of four images:
<a href="https://i.sstatic.net/CGVQy.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/CGVQy.jpg" alt="enter image description here"></a></p>

<p>Why is it so?</p>

CC BY-SA 3.0
2016-08-03T13:28:44.360Z
Your expectation is incorrect. A ring is only expected in the case of observer, lens and lensed object being in a perfect straight line.
ProfRob
CC BY-SA 3.0
2016-08-03T13:46:28.703Z
So in the above figure, where is the object?
velut luna
CC BY-SA 3.0
2016-08-03T14:07:13.817Z
That would depend on the mass distribution of the foreground object which is a very large galaxy (bigger than the picture, actually). It is shown better here http://astronomynow.com/wp-content/uploads/2015/12/Q2237-0305_930x648.jpg  I should have added to my previous caveat that a ring could only be formed if the lensing object had a nice symmetric mass distribution.
ProfRob
CC BY-SA 3.0
2016-08-05T13:54:46.570Z
@AlphaGo, you need only click to the wiki article, to see many "ring-like" examples ... https://en.wikipedia.org/wiki/Gravitational_lens
Fattie
<gravitational-lensing>
Images of gravitational lensing
velut luna
# Images of gravitational lensing

(Asked by: username_0 on 2016-08-03)

I will expect that gravitational lensing should turn an object into a ring image. But I saw in some situations that instead multiple images are formed. For example, the Einstein-cross in the image below, consisting of four images:
[![enter image description here](https://i.sstatic.net/CGVQy.jpg)](https://i.sstatic.net/CGVQy.jpg)

Why is it so?

**Tags:** gravitational-lensing
> **Comments:**
> **username_1**: Your expectation is incorrect. A ring is only expected in the case of observer, lens and lensed object being in a perfect straight line.  
> **username_0**: So in the above figure, where is the object?  
> **username_1**: That would depend on the mass distribution of the foreground object which is a very large galaxy (bigger than the picture, actually). It is shown better here http://astronomynow.com/wp-content/uploads/2015/12/Q2237-0305_930x648.jpg  I should have added to my previous caveat that a ring could only be formed if the lensing object had a nice symmetric mass distribution.  
> **username_2**: @AlphaGo, you need only click to the wiki article, to see many "ring-like" examples ... https://en.wikipedia.org/wiki/Gravitational_lens  

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## 1 answers

### Answer by username_3 on 2016-08-04. Score: 9

As mentioned in the comments, a ring will (ideally) only occur if you have perfect alignment of source, lens, and observer. This is true for point sources, for extended sources things get more complicated.

Double and quadruple images occur when the source is small and not in perfect alignment - typically a quasar, as in your example. For the sake of completeness, this would actually be triple or quintuple, but the third / fifth image is very close to the centre, overpowered by the lens, and highly demagnified (faint).

There are examples of rings and almost-rings, but these tend to be extended sources, i.e. galaxies. It is more likely that some part of an extended source happens to be aligned with lens and observer than it is for a point source.

[![Cosmic Horseshoe](https://i.sstatic.net/YIGJP.jpg)](https://i.sstatic.net/YIGJP.jpg)  
(Image of the [Cosmic Horseshoe](http://apod.nasa.gov/apod/ap111221.html))

In your example, if the host galaxy of that quasar could be seen, it might be an Einstein ring (or an oval, as the lens seems to be somewhat asymmetric).


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