A small part of the SMACS 0723 deep field is the first image published by James Webb showing hundreds of galaxies

In this image you can see a cluster of 3 stars in the bottom left, one big star in the middle up and many more identical stars scattered throughout the image.

Are they all the same star? They all look identical (the only difference being the size), so it can't just be a coincidence that there are just a bunch of identical stars so near to one another.

The same image, but with highlights on all the identical stars I've found

If they are all the same, then why does it appear multiple times?

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    $\begingroup$ They're different stars. and they're not identical. It's just that they're much brighter to the telescope than the galaxies that the image was taken to highlight, so they're whited out, and the diffraction spikes from the telescope structure are much more prominent on them. $\endgroup$
    – notovny
    Dec 13, 2022 at 13:13
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    $\begingroup$ To expand on that - all nearby stars photographed by JWST have that pattern of eight large spikes and six small spikes. This is caused by the diffraction from the struts which support the secondary mirror and the hexagonal shape of the main mirror segments. $\endgroup$
    – Skyler
    Dec 13, 2022 at 21:07
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    $\begingroup$ Have a look at If I can't unscramble an egg, how do Astronomers unscramble views gravitationally lensed by complex mass distributions? in Astronomy SE $\endgroup$
    – uhoh
    Dec 14, 2022 at 0:44

1 Answer 1


Those are different overexposed foreground stars. You can tell by the diffraction spikes that they are overexposed point source objects, i.e., they are stars in our galaxy. The diffraction spikes are caused by the JWST rather than by the stars themselves. The image shown in the question was the first JWST deep field view image. The goal of this image was to capture images of gravitationally lensed remote galaxies. This inevitably meant that any foreground stars in the image would be overexposed.

The stars in the referenced image all look the same (pure white and similar diffraction spikes) because they are overexposed point sources of light. The overexposure results in every channel being saturated, and hence white. The diffraction spikes are a consequence of overexposure, the near point source nature of stars, and the way the JWST was constructed. In particular, the diffraction spikes in a JWST image are a consequence of the segmented mirrors used by the JWST and the vanes that support the secondary mirror. To look at deep field views such as this you need to ignore those overexposed foreground objects; these overexposed objects are an undesirable but unavoidable consequence of the long exposure times needed to see the very remote background galaxies. The objects of interest in images such as this are the smeared out, gravitationally-lensed, low-intensity background objects.

There are at least 22 objects in that image that are duplicates as a result of gravitational lensing; see the image below and the references below. The image portrays several of the remote galaxies that are duplicated due to gravitational lensing. However, the ones circled in the question are not duplicates.

Portion of the SMACS J0723.3-7327 image taken by the JWST. Several remote galaxies that are duplicated due to gravitational lensing are highlighted. The foreground stars highlighted in the question are not highlighted as duplicates.


phys.org, "Improved model for the mass distribution of galaxy cluster SMACS J0723.3−7327 based on Webb telescope image"

Caminha, G. B., et al. "First JWST observations of a gravitational lens: Mass model from new multiple images with near-infrared observations of SMACS J0723. 3-7327." Astronomy and Astrophysics 666 (2022): L9.

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    $\begingroup$ "You can tell by the diffraction spikes that they are stars." - shouldn't that read "You can tell by the diffraction spikes that they are very much brighter than the other objects - only nearby stars can appear so bright, because galaxies are too far away." $\endgroup$
    – asdfex
    Dec 14, 2022 at 8:46
  • $\begingroup$ @asdfex Your longer sentence is correct, but so is my much shorter "You can tell by the diffraction spikes that they are stars." $\endgroup$ Dec 14, 2022 at 9:23
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    $\begingroup$ Yes, sure. Technically correct, maybe a bit short for non-experts. Might lead to the question "Why do stars have spikes, but galaxies don't?" $\endgroup$
    – asdfex
    Dec 14, 2022 at 9:28
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    $\begingroup$ I feel that this answer would be improved by explaining why the suspected duplicates are all “identical” even though they are not actually duplicates. (Point source, that diffraction spikes are caused by the JWST not the star, etc.) $\endgroup$
    – KRyan
    Dec 14, 2022 at 14:21
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    $\begingroup$ @KRyan I've tried once again to add details. Let me know if this last set of edits does a better job. $\endgroup$ Dec 15, 2022 at 14:42

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