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Comment by skrause | Hacker News Reader

All the very red galaxies in the JWST image are mostly or completely invisible in the Hubble image. That’s because they’re so redshifted that they’re out of the spectrum Hubble can see. Those are the galaxies that are really far away.

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Here's a variant of that GIF that separates out the blue, blue+green, and blue+green+red channels, to (hopefully) highlight which differences are due to the longer wavelengths (and which look more like exposure time difference). Webb's color mapping roughly aligns with the RGB channels, so I think this is meaningful [0].

https://i.ibb.co/D8dW6v5/jwst.webp

[0] https://webbtelescope.org/contents/media/images/2022/038/01G...

("...the assigned colors are: Red: F444W Orange: F356W Green: F200W + F277W Blue: F090W + F150W")

Steps to reproduce:

    convert 9uyhwijeo0b91.gif[1] -resize 4537x4630 aligned-hst.png
    cp STScI-01G7JJADTH90FR98AKKJFKSS0B.png jwst-rgb.png
    convert jwst-rgb.png -channel R -fx "u*0" jwst-gb.png
    convert jwst-gb.png -channel G -fx "u*0" jwst-b.png
    convert -delay 150 -loop 0 jwst-{b,gb,rgb}.png aligned-hst.png my.gif
    ffmpeg -i my.gif -loop 0 my.webp

Comparing both images, there is a perfectly round red dot that's just a few pixels wide, a little up from the most prominent star, which doesn't correspond to another object in Hubble's image. Is that an image artifact or some laser guide?

Could be an internal reflection. The optical path for many JWST instruments is, uh, compact: https://www.esa.int/ESA_Multimedia/Videos/2021/09/Webb_MIRI_... (Every kilo on telescope structure you save by folding the optical path is a kilo you can add in propellant, extending the working life of the spacecraft)

Instrument internals are painted black and heavily baffled, but nothing in optics is perfect. Dithering the direction the telescope is pointed in and image stacking should cancel out most optical artifacts, but internal reflections will be worse for bright objects like stars, which JWST probably isn't usually going to be observing with the imaging instruments.

It is a bit suspicious looking, but there's a lot of very red objects that the Hubble image doesn't catch. Presuming that the red colors in the images are the deeper infrared wavelengths (and thus the most heavily-redshifted objects) I would guess that Hubble just didn't have the detectors to see those.

Aren't the red ones moving really fast away from us and the white ones more "stationary" from our point in the universe?

Yes, that's generally true. Galaxies that are redshifted are moving very fast relative to us. But the only reason for them to be moving so fast is due to the expansion of the universe, and these galaxies being sufficiently far away.

The large white elliptical galaxies in the center of the image are in the "foreground", while the orange-ish galaxies are much farther away in the background. This is why the light from the more distant galaxies is curved and distorted by the foreground objects, creating the lensing effect that we see.

Hubbles law says that astronomical objects that are further away also move away from us more quickly with the constant of proportionality being the Hubble constant

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