<p>The quick answer is yes, we indeed see real changes in the structure of the jet.</p>
<p>Longer answer;  There are several factors that define how the jet of 3C 273 looks like: frequency of observation, telescope/array used, and real changes in structure.
If we observe with interferometry technique from Earth, the higher the frequency - the better the resolution and the worse is sensitivity. In simple words, at higher frequency one can see finer but only the brightest details.</p>
<p>The images you’d shown are indeed taken with some VLBI array: a number of telescopes observing simultaneously at the same frequency. The more telescopes one has at once - the bettter image is. With less telescopes or problems even at a single telescope of the array, the image is more noisy</p>
<p>Finally, the jet itself changes. In case of 3C 273, the changes are visible with monthly observations.
There are two excellent programs to monitor jets and see real structural changes:
<a href="https://www.physics.purdue.edu/MOJAVE/sourcepages/1226+023.shtml" rel="nofollow noreferrer">https://www.physics.purdue.edu/MOJAVE/sourcepages/1226+023.shtml</a>
And
<a href="https://www.bu.edu/blazars/VLBA_GLAST/3c273.html" rel="nofollow noreferrer">https://www.bu.edu/blazars/VLBA_GLAST/3c273.html</a></p>
<p>UPD:
The matter in these jets indeed moves with immense speed, exceeding 0.99 of the speed of light. But because the jet in 3C 273 is pointing almost towards us, there is another funny effect that increases <strong>apparent</strong> speed: <a href="https://en.wikipedia.org/wiki/Superluminal_motion" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Superluminal_motion</a></p>
<p>It can be imagined that emitting plasma and its emission are moving towards us almost at the same speed and on almost parallel trajectories. In projection on the sky plane, the apparent speed of any feature in the jet would be larger than the speed of light. <strong>It does not imply that the matter moves faster than the speed of light.</strong> It is just an apparent effect.</p>
<p>The measurements of the apparent speed are made on the basis of positions of bright spots/knots moving along the jet. The most probable nature of these bright features is shock waves traveling downstream the jet.</p>

CC BY-SA 4.0
2021-08-02T21:43:34.110Z
I'm not sure how your answer describes why the jet is changing. It doesn't look like the telescope or frequency is changing (from your first paragraph), and from your second, surely to show a change of ~10 mas over a few months would require an immense speed.
Jim421616
CC BY-SA 4.0
2022-10-07T13:06:41.330Z
The reasons for the structural changes in the jets themselves are: 
1) travelling shock waves that compress plasma and make it look brighter. 
2) ejection of new plasma blobs into the jet from the accretion process. Both 1 and 2 can move along the jet and make it image change. 
3) the jet precession of large-scale instabilities can make the whole jet to turn
Mikhail Lisakov
CC BY-SA 4.0
2021-08-02T06:04:46.083Z
2021-08-09T16:38:32.100Z
2021-08-09T16:38:32.100Z
Mikhail Lisakov
<p>At <a href="https://ned.ipac.caltech.edu/byname?objname=3C273&amp;hconst=67.8&amp;omegam=0.308&amp;omegav=0.692&amp;wmap=4&amp;corr_z=1" rel="nofollow noreferrer">NASA/IPAC Extragalactic database</a>, we can see images of the jet of matter being ejected from the quasar 3C 273. Here are a couple from the radio spectrum as examples:</p>
<p><a href="https://i.sstatic.net/TeMVU.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/TeMVU.png" alt="enter image description here" /></a>
<a href="https://i.sstatic.net/Xp3fo.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Xp3fo.png" alt="enter image description here" /></a></p>
<p>My question is: why are the images so different? Is it because the jet itself is changing between the images (which are taken on different dates)? Or are the images somehow showing static structure within the jet? Or is it an artefact of the image processing?</p>
<p>EDIT: I've made a quick animation of the images spanning July 1998 to March 2001, in case it sheds any light, so to speak:
<a href="https://i.sstatic.net/K5ydi.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/K5ydi.gif" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2021-08-01T21:57:56.180Z
Nice question. What do you mean that they show static structure? Isnt that what they *dont* show?
Deschele Schilder
CC BY-SA 4.0
2021-08-01T21:59:58.770Z
By that, I meant it reminds me of looking through a microscope at a slide; as you focus the microscope, you get slightly different views as the focal plane moves through the 3-dimensional structure of the sample. I was wondering if we were somehow seeing at different 'depths' through the 3-d structure of the jet.
Jim421616
CC BY-SA 4.0
2021-08-01T22:02:13.970Z
Ah yes, I see what you mean. Interesting. So maybe we could see the depth structure?
Deschele Schilder
CC BY-SA 4.0
2021-08-01T22:02:48.570Z
Exactly, but I'm not even sure if that's possible.
Jim421616
CC BY-SA 4.0
2021-08-01T22:07:18.223Z
It would mean that in every image we see a selected plane. Im not sure too if this is possible. Its more likely the emission varies. There are 6 years there. Can things change that fast?
Deschele Schilder
<quasars><image-processing>
Why are the radio images of the jet of 3C 273 different?
Jim421616
# Why are the radio images of the jet of 3C 273 different?

(Asked by: username_0 on 2021-08-01)

At [NASA/IPAC Extragalactic database](https://ned.ipac.caltech.edu/byname?objname=3C273&hconst=67.8&omegam=0.308&omegav=0.692&wmap=4&corr_z=1), we can see images of the jet of matter being ejected from the quasar 3C 273. Here are a couple from the radio spectrum as examples:

[![enter image description here](https://i.sstatic.net/TeMVU.png)](https://i.sstatic.net/TeMVU.png)
[![enter image description here](https://i.sstatic.net/Xp3fo.png)](https://i.sstatic.net/Xp3fo.png)

My question is: why are the images so different? Is it because the jet itself is changing between the images (which are taken on different dates)? Or are the images somehow showing static structure within the jet? Or is it an artefact of the image processing?

EDIT: I've made a quick animation of the images spanning July 1998 to March 2001, in case it sheds any light, so to speak:
[![enter image description here](https://i.sstatic.net/K5ydi.gif)](https://i.sstatic.net/K5ydi.gif)

**Tags:** quasars, image-processing
> **Comments:**
> **username_1**: Nice question. What do you mean that they show static structure? Isnt that what they *dont* show?  
> **username_0**: By that, I meant it reminds me of looking through a microscope at a slide; as you focus the microscope, you get slightly different views as the focal plane moves through the 3-dimensional structure of the sample. I was wondering if we were somehow seeing at different 'depths' through the 3-d structure of the jet.  
> **username_1**: Ah yes, I see what you mean. Interesting. So maybe we could see the depth structure?  
> **username_0**: Exactly, but I'm not even sure if that's possible.  
> **username_1**: It would mean that in every image we see a selected plane. Im not sure too if this is possible. Its more likely the emission varies. There are 6 years there. Can things change that fast?  

---

## 1 answers

### Answer by username_2 on 2021-08-02. Score: 3 | Accepted answer

The quick answer is yes, we indeed see real changes in the structure of the jet.

Longer answer; There are several factors that define how the jet of 3C 273 looks like: frequency of observation, telescope/array used, and real changes in structure.
If we observe with interferometry technique from Earth, the higher the frequency - the better the resolution and the worse is sensitivity. In simple words, at higher frequency one can see finer but only the brightest details.

The images you’d shown are indeed taken with some VLBI array: a number of telescopes observing simultaneously at the same frequency. The more telescopes one has at once - the bettter image is. With less telescopes or problems even at a single telescope of the array, the image is more noisy

Finally, the jet itself changes. In case of 3C 273, the changes are visible with monthly observations.
There are two excellent programs to monitor jets and see real structural changes:
<https://www.physics.purdue.edu/MOJAVE/sourcepages/1226+023.shtml>
And
<https://www.bu.edu/blazars/VLBA_GLAST/3c273.html>

UPD:
The matter in these jets indeed moves with immense speed, exceeding 0.99 of the speed of light. But because the jet in 3C 273 is pointing almost towards us, there is another funny effect that increases **apparent** speed: <https://en.wikipedia.org/wiki/Superluminal_motion>

It can be imagined that emitting plasma and its emission are moving towards us almost at the same speed and on almost parallel trajectories. In projection on the sky plane, the apparent speed of any feature in the jet would be larger than the speed of light. **It does not imply that the matter moves faster than the speed of light.** It is just an apparent effect.

The measurements of the apparent speed are made on the basis of positions of bright spots/knots moving along the jet. The most probable nature of these bright features is shock waves traveling downstream the jet.

> **Comments:**
> **username_0**: I'm not sure how your answer describes why the jet is changing. It doesn't look like the telescope or frequency is changing (from your first paragraph), and from your second, surely to show a change of ~10 mas over a few months would require an immense speed.  
> **username_2**: The reasons for the structural changes in the jets themselves are:  1) travelling shock waves that compress plasma and make it look brighter.  2) ejection of new plasma blobs into the jet from the accretion process. Both 1 and 2 can move along the jet and make it image change.  3) the jet precession of large-scale instabilities can make the whole jet to turn  

---

