Comment by sbierwagen | Hacker News Reader

Comment by sbierwagen | Hacker News Reader

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Image stacking to remove noise and optical artifacts, careful use of color filters to enhance contrast and pull out detail. The press release says it used Red: F444W, Orange: F335M, Yellow: F470N, Green: F200W, Cyan: F187N, Blue: F090W. The N filters are narrowband. F470N is only 54 nanometers wide: https://jwst-docs.stsci.edu/jwst-near-infrared-camera/nircam...

Almost all the light in this image is way off the red end of the human visual spectrum, of course. The shortest wavelength filter is F090W which has a center wavelength of 902nm, about the same color as the light coming out of a TV remote infrared LED, which is barely visible in pure darkness.

This is what it looks like through a film SLR, without the detail enhancing filters: http://www.phys.ttu.edu/~ozprof/3372f.htm Here's a 20 minute exposure through a telescope: http://www.phys.ttu.edu/~ozprof/3372fk.jpg Maybe what you would see with your own eyes through binoculars at a dark site well away from city lights. A dim red smudge, hints of finer detail.

While the JWST uses the red end of the human visual spectrum and narrow filters these objects are still broadband and can be imaged as such. Adam did an excellent job on widefield https://www.adamblockphotos.com/ngc-3372-carina-nebula.html and there are lots of deep field images in RGB on Astro imaging sites. The detail is phenomenal with JWST but a lot of people are saying one wouldn't see this with their eyes or with a color cam - and they're "wrong" (its complex). Your eyes just don't "collect" photons like a camera, but a color camera would see this nebula beautifully... We use NB in hobby Astro imaging a lot just to reduce the impact of light pollution.

How does this "blueshift" compare to what we'd get if we just corrected for the relative-speed-induced redshift?

NGC3372 is inside our galaxy, just 8500 light years away. It's not redshifted by metric expansion to any appreciable degree, (A calculator I just checked gave me a z of 0.000000617) and radial velocity is a sedate ~34 km/s. (z = 0.000000115)

The redshift on the other JWST images is because most of them are of objects that are much, much, much farther away. Infrared telescopes are great for observing those, but that's not the only thing they're used for.

Maybe my question would be better asked for other objects images then, but I can just google how far things are redshifted at extreme distances as well.

My understanding is that the IR here is used to see "through" the "smoke", so you can see more details that would normally be obstructed.

A good way to see this is comparing it to Hubble [1], a lot of the extra detail you see is thanks to IR letting you see the stars behind.

[1] https://johnedchristensen.github.io/WebbCompare/

Understood!

What I was asking is: Is the target's normally-visible light redshifted into the same bands that JWST is measuring, higher? or lower frequency?

That doesn't have anything to do with why JWST uses IR.

Redshift refers to how the wavelength of a photon can change if the observer is moving relative to it, (Doppler shift, redshift if you're moving away from the photon, blueshift if you're moving towards it) or cosmological redshift. (The fabric of the universe expanding, reducing photon energy)

NGC3372 is a cloud of (relatively) hot gas and dust. It's emitting broad spectrum blackbody radiation: it's emitting on all wavelengths. You can look at the same cloud at different wavelengths and see different things, telling you what parts of the cloud are at what temperature, or relative chemical composition, or what parts are ionized: http://legacy.spitzer.caltech.edu/uploaded_files/graphics/fu... Nothing here is redshifted, Spitzer is just capturing different light entirely.

In the side by side of JWST and Hubble https://pbs.twimg.com/media/FXecm6vXwAMPhoc?format=jpg&name=... https://pbs.twimg.com/media/FXecnp2XkAE4Rs5?format=jpg&name=... you see broadly the same thing, but Hubble is almost all visible-light while JWST goes deeper into infrared and sees cloud structure that Hubble doesn't.

Ok, disclaimer: I am not an astronomer and this might all be rubbish. I suspect I'm neglecting relativistic effects that might be important, for example.

The NIRCAM instrument on JWST has a wavelength range of about 600 - 5000nm [1]. The human eye is sensitive to around 380nm - 700nm.

To shift blue light (380nm) down to the upper frequency range of NIRCAM (600nm) requires a redshift of:

z = Δλ / λ0 = 0.58

This is related to the velocity of the object by:

z = v / c

and the velocity is related to distance (approximately) by the Hubble constant (H0 ~ 71 km/s / Mpc):

d = v/ H0

So we can rearrange and solve for distance to get:

d = z c / H0 = 8 billion light years.

The southern ring nebula is more like 2000 light years from us, so not even vaguely far enough that NIRCAM would see "originally-visible" light. The deep field image might actually be far enough... the faintest galaxies there might be something like 12 billion light years away [2].

[1] https://www.stsci.edu/jwst/instrumentation [2] https://www.nasa.gov/content/discoveries-hubbles-deep-fields

> to see "through" the "smoke"

What causes this "smoke"?

Different wavelengths of light scatter more or less as they pass through gas or dust. This is why the Earth's sky is blue on a clear day, and sunsets are red --- the red light scatters less, blue scatters more.

In smokey or smoggy air, the red light is also scattered.

The "smoke" in a nebula is mostly gas and dust. It's either left-over primordeal matter (hydrogen gas, some helium), or ejecta from novas and supernovas --- star-smoke if you will, though it's created by nuclear fusion rather than chemical combustion.

JWST's IR sensors can cut through that dust more readily than Hubble's optical-range sensors could, and pull out more detail on the dust to boot (based on my own viewing of comparative images).

I'm not sure if the dust is reflecting light or glowing from heat, though my hunch is it's mostly reflecting. Stellar gas that gets hot enough will also glow in infrared (or higher) wavelengths, and that might also be picked up by JWST. I suspect there will be targets demonstrating this in future.

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