- 3637 - JAMES WEBB - earliest discoveries? - A new generation of astronomy is officially underway. The best is yet to come, each new observation exposes a new portion of our shared cosmic story for the very first time. The Universe, to everyone studying it, will never be the same again. Here is some of what we have already learned:
--------------------- 3637 - JAMES WEBB - earliest discoveries?
- James Webb's very first deep-field image of the lensed galaxy cluster “SMACS 0723” and all the background objects it stretches and magnifies, we've seen deeper into the distant Universe than ever before.
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- There are new features revealed, new galaxies discovered, and new features about previously discovered objects that JWST's unique capabilities have enabled us to uncover.
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- July 11, 2022, the very first science image of the James Webb Space Telescope (JWST) was unveiled to the world. Upon its release, it immediately broke the cosmic record for the deepest view ever taken of the Universe.
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- Prior to JWST’s first science release, the deepest view of our cosmos came from the Hubble eXtreme Deep Field: a region of space so small it takes up just 1/32,000,000 th of the sky. Within it, 5500 galaxies were found, spanning almost the entire history of the Universe: from just 400 million years after the Big Bang until today, or from when the Universe was merely 3% of its current age all the way to its present day.
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- That image represented the deepest view of the Universe for a full decade. But by simply using its suite of instruments to observe a run-of-the-mill galaxy cluster the JWST has shown us the Universe as we’ve never seen it before.
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- Over the course of 50 days, with a total of over 2 million seconds of total observing time (the equivalent of 23 complete days), the Hubble eXtreme Deep Field (XDF) was constructed from a portion of the prior Hubble Ultra Deep Field image. Combining light from ultraviolet through visible light and out to Hubble’s near-infrared limit, this represents humanity’s deepest view of the universe.
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- There are a few keys to taking a “deep” view of the Universe. The first is to find an area of the sky that’s relatively pristine:
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------------------- With no bright stars from within the Milky Way in its field of view,
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------------------- With no bright, extended, extremely nearby galaxies in the vicinity,
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------------------- Where there’s only a small, negligible amount of neutral, light-blocking matter due to the Milky Way’s foreground,
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------------------- Where even the brightest stars and galaxies remaining are faint enough so that they won’t be able to saturate the detectors and instruments under any settings.
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- Then, in as many relevant filters which probe a very specific wavelength range as possible, you take a series of images where you simply gather light, one photon at a time, from the same region of sky.
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- Images that use the same filter, which view the region of the sky in the exact same wavelength range, then get “stacked” together. The light from each individual frame gets added together, exposing the faintest objects perceptible in that wavelength range.
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- Multiple different filters are combined to produce images. Colors are assigned to each filter or set of filters, and then our eyes interpret the data the same way we’d interpret a full-color photograph.
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- The Hubble Space Telescope was primarily designed as an “optical” observatory. It was optimized to look at the same wavelengths of light that human eyes are sensitive to. Because it’s located in space, far above Earth’s atmosphere, it doesn’t have the same limitations that a ground-based telescope possesses: it can look at wavelengths that are otherwise blocked by Earth’s atmosphere.
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- That means that it can see ultraviolet (shorter-wavelength) light as well, where the atmosphere is only partially transparent to it, and it can also see into the infrared (longer-wavelength) portion of the spectrum, where only a selection of wavelength ranges aren’t completely blocked by our atmosphere.
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- Each different wavelength range will have its own unique resolution when it comes to the light that’s gathered. The resolution is determined by the number of wavelengths of light that fit across the diameter of the telescope’s primary mirror. Although human vision extends from 400 to 700 nanometers, telescopes can assign filters to cover whatever wavelength range the operators desire.
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- For Hubble, with its 2.4-meter diameter mirror, that means its “blue” filter of about 400 nanometers has about twice as good a resolution as its “red” filter of about 800 nanometers, and its infrared filters, which span from 1050 nanometers to 1600 nanometers, are again only half-as-sharp at the longest wavelengths.
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- For Hubble:
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------------------- There are three optical filters, at 435, 606, and 814 nanometers, assigned to red, green and blue, respectively,
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---------------- There are four infrared filters, at 1050, 1250, 1400, and 1600 nanometers, all assigned-and-stacked to the same infrared channel,
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---------------- The overall composite combined all seven filters, assigning “blue” to the 435 nm channel, “green” to the 606 + 814 nanometer channel, and “red” to all four infrared channels combined.
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- By looking at details in these channels separately, you can notice a slew of features. Cooler stars and diffuse starlight are brighter in the infrared. Features of a galaxy’s disk are sharpest in the shortest wavelengths, and become more smeared-out at longer and longer wavelengths.
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- A filter at 1600 nanometers isn’t only one-fourth as good as a filter at 400 nanometers; it’s actually only one-sixteenth as good. A galaxy that takes up 64 pixels (8-by-8) in Hubble’s blue filter would only take up 4 pixels (2-by-2) in Hubble’s farthest infrared filter.
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- There are two factors at play that limit, at a fundamental level, what Hubble can see; these two factors are literally the difference-maker between Hubble and JWST.
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- Hubble is warm; JWST is cold. If you want to detect anything and this goes for anything, ever, under any circumstances you need to be able to see a signal from what you’re measuring over and above the noise inherent to your instrument. Infrared radiation is simply a consequence of heat: objects at a certain temperature radiate heat, and that heat shows up as photons of a particular wavelength. The hotter you are, the more photons you’ll emit at higher and higher energies (and shorter and shorter wavelengths).
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- In low-Earth orbit, Hubble, even wrapped in its highly reflective coating, still hovers somewhere around 200 K and up. This is what limits its wavelength capabilities: beyond 2,000 nanometers (2 microns), the observatory produces too much of its own internal radiation to do useful science.
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- JWST with its 5-layer sunshield is passively cooled down to below 40 K for all of its instruments, including NIRCam, the Near-Infrared camera, which can see all the way out to 5,000 nanometers (5 microns) without any active cooling.
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- MIRI, the Mid-Infrared instrument, is actively cooled down to 6 K, enabling it to see light between 5,000 and 28,000 nanometers (5-28 microns) at its limits.
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- You can’t see the most distant objects by looking in the same wavelengths as the ones where that light was initially emitted. The Universe isn’t static, it’s expanding: distant, gravitationally unbound galaxies all recede from one another against the backdrop of expanding space. When a distant object emits light even, very blue light, the expansion of the Universe stretches that light to longer and longer wavelengths.
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- Hubble, even at the longest-wavelengths that it’s sensitive to, can’t really take us back to galaxies that emit light from the first 3% of the Universe’s history. But, NIRCam and MIRI capabilities means JWST can see much longer wavelengths, enabling it to reveal objects that are undetectable by warmer observatories.
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- There are objects out there so distant that their light takes 13,400,000,000 years to reach us, and that takes us beyond the limitations of Hubble. That’s where the cosmic frontier is. If we want to see the earliest galaxies, the first stars, and the youngest objects ever to form in the Universe, we simply have to go beyond the capabilities of current technology. That means longer wavelengths and cooler observatories, and that’s what JWST is delivering.
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- JWST, we can go out to a 14 times the maximum wavelength that the Hubble Space Telescope is sensitive to.
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- Telescope size determines both resolution and light-gathering power. If you want to see farther into the distant Universe, and to see features you couldn’t see before, you have to increase the total amount of light you capture, and also observe the Universe at superior resolutions.
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- With JWST, the NIRCam images, especially at the lowest wavelengths, will produce sharper images than Hubble ever could, owing to its large, 6.5-meter primary mirror that’s 270% the diameter of Hubble’s.
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- At all wavelengths, JWST gathers more than 7 times the total amount of light as Hubble, meaning that it can not only expose fainter, more distant objects than Hubble ever could, but that it can do it with less observing time than Hubble would ever require. What Hubble requires a week for, JWST can do better in a day. What Hubble requires two months for, JWST can outperform in a week.
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- The reason JWST can see what Hubble cannot is because of the difference in size, temperature, wavelength coverage, and the correspondingly-optimized instrument suite.
James Webb will have seven times the light-gathering power of Hubble, but will be able to see much farther into the infrared portion of the spectrum, revealing those galaxies existing even earlier than what Hubble could ever see.
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- The power of gravitational lensing, and strong gravitational lensing explicitly, reveals background sources that could never be practically seen otherwise. But this natural magnifying glass amplifies the light we receive, even at such long wavelengths, so JWST is able to reveal a number of objects never seen before because they were either too faint, too distant. Their wavelength was stretched to too great a value to be seen previously.
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- Contained within the first JWST Deep Field are:
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------------------ A number of faint “blobs” that aren’t seen in the Hubble image, corresponding to galaxies that were too faint and/or too long in wavelength to be seen previously,
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----------------- A number of objects that appeared only as faint smudges, previously, that now have obvious structure to them, such as spiral arms, resulting from JWST’s improved resolution, light-gathering power, and superior wavelength coverage,
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---------------- A brighter, wider glow from the central region: an example of what’s almost certainly intracluster light, inherent to most galaxy clusters that aren’t largely gas-depleted,
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----------------- Numerous instances of “smudges” that were seen previously that are now seen to be multiple galaxies that appear to overlap, a tremendous demonstration of JWST’s superior resolution.
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- Although the official image release doesn’t include scientific data, a number of these objects are claimed to have their light come to us from as far back as 13.5 billion years ago: pushing our earliest cosmic views back by approximately 100 million years.
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- This is just the first science result using this new JWST observatory . Over the coming years an enormous number of cosmic records are going to be broken, most of them over and over again. We will have dozens of galaxies fainter, more distant, and with more pristine stellar populations that go beyond any of the cosmic record-holders lurking in the lensed field of this galaxy cluster.
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- A new generation of astronomy is officially underway. Although the best is yet to come, each new observation, especially at this early stage, exposes a new portion of our shared cosmic story for the very first time. The Universe, to everyone studying it, will never be the same again.
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July 26, 2022 JAMES WEBB - earliest discoveries? 3631
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--------------------- --- Tuesday, July 26, 2022 ---------------------------
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