At NASA/IPAC Extragalactic database, 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 enter image description here

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

  • $\begingroup$ Nice question. What do you mean that they show static structure? Isnt that what they dont show? $\endgroup$ Aug 1, 2021 at 21:57
  • $\begingroup$ 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. $\endgroup$
    – Jim421616
    Aug 1, 2021 at 21:59
  • $\begingroup$ Ah yes, I see what you mean. Interesting. So maybe we could see the depth structure? $\endgroup$ Aug 1, 2021 at 22:02
  • $\begingroup$ Exactly, but I'm not even sure if that's possible. $\endgroup$
    – Jim421616
    Aug 1, 2021 at 22:02
  • $\begingroup$ 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? $\endgroup$ Aug 1, 2021 at 22:07

1 Answer 1


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.

  • $\begingroup$ 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. $\endgroup$
    – Jim421616
    Aug 2, 2021 at 21:43
  • $\begingroup$ 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 $\endgroup$ Oct 7, 2022 at 13:06

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