The Universe seen by the James Webb space telescope is not in color as we see it. The sumptuous colors of the images released by NASA on July 12, in fact, are a choice made by scientists: in this way they become usable even by an audience of non-experts, and also much more spectacular. But as he explains to Italian tech Massimo Stiavelli, who heads the Space telescope science institute (STScl) in Baltimore, “the early release science it is a tradition for all observers, in general it is above all the effect of showing beautiful images to the general public, we have tried to make images that are also scientifically significant ”. And he adds: “So far we have only joked.”
Space
The new photos of the Webb telescope released by NASA
by Matteo Marini
Stiavelli oversees all operations of the James Webb mission, from maneuvers to aiming for observations. Until the daily data download. Which are downloaded twice a day: “Every day there are about 50 gigabytes – he points out – the download takes place at 28 megabits per second. It was a good internet speed ten years ago. Except that we are one and a half million kilometers away ”. Stiavelli tells of the great success they have had, with “400 terabytes downloaded from our cloud in the first day alone”.
The Universe in infrared
The James Webb observes the Universe in infrared, light that the human eye does not see. Simplifying a lot: if it had an eyepiece like amateur telescopes, and we approached our eye, we would not see anything outside of a few reddish spots (only a very small part of the frequencies fall on this side of the limit of the visible spectrum). It was designed like this because it is necessary to look much further than Hubble, for example, at the beginning of the Universe https://www.repubblica.it/cronaca/2022/07/12/news/la_prima_foto_a_colori_del_james_webb_svelata_da_joe_biden_galassie_del_lontano_universo_35748/35748 through dense clouds of dust, or analyzing the chemical elements, of the atmosphere of planets passing in front of their stars.
So you have to arbitrarily assign colors to “develop” the photos. Upstream of the whole process are the filters that are applied directly by the instruments on board the telescope: “Any camera for astronomical use does not have color – explains Stiavelli – therefore, when observations are made in different bands, a filter. This is how color images are obtained. You make an observation with the blue filter, one with the red filter, one with the yellow filter and then they can be combined. In the case of our observations it becomes a little less intuitive because, being in the infrared, the filters we use have nothing to do with the visibility to the human eye. It’s a convention ”.
James Webb collects light from near infrared (shorter wavelengths) to mid infrared (longer). “For example, if I have an image that has a near infrared and a medium infrared and an even more distant medium infrared, I assign the blue to the near infrared, maybe the yellow to the nearest medium infrared and the red to the farthest infrared” , explains the astrophysicist.
Develop photos
Once they reach the ground, the “development” takes place. “The telescope sends us the images for each color separately – adds Stiavelli – when they reach the ground, these data are combined to produce the images”. The difference between the wavelengths serves above all for scientific purposes, different for each instrument on board: “If we look at the image of the planetary ring nebula made by the NirCam and Miri instruments, we see for example the blue in different areas precisely because the two instruments work in different regions of the spectrum ”.
Stiavelli gives another example, that of the image of the Carina nebula, which left the whole world speechless: “For a scientific observation, filters are chosen on the basis of the laws and physical quantities to be measured. In this case it also corresponds to an aesthetically pleasing choice. Blue indicates a wavelength emitted by ionized hydrogen, it shows us the distribution of gas that is ionized by the ultraviolet light of those young and very bright stars that illuminate the entire region. The reddish, darker part was made by choosing a filter sensitive to a frequency emitted by interstellar dust, and then shows us these layers of dust that are on the surface of the gas cloud. Then there is a third filter which is more sensitive to starlight ”. Result: many more stars are seen in the James Webb photo.
In “Stephan’s Quintet”, the dance of galaxies observed in infrared by the Webb, reveals more than Hubble could have told us: “The NirCam image of Quintet, unlike the Carina nebula, it is almost the same as the Hubble one, at a slightly higher resolution – Stiavelli points out – you can see these gas filaments in a little more detail. Instead that of Miri is completely different precisely, because of the powders ”. Miri, Mid-Infrared Instrument, has the ability to see through matter like an infrared camera. This allows you to observe the stars immersed in dense nebulae. Or capture the activity of black holes that devour matter and partially convert it into energy.
Born in Montecatini Terme, Massimo Stiavelli arrived in the USA in 1995 as an ESA astronomer in the Hubble program. Five years later he is, as he puts it, “passed to the Americans”. He led the Hubble ultra deep field team: the observation made by the space telescope in 2004 that led to capturing the light of the most distant galaxies, in a corner of the sky where it seemed there was only darkness. On the wall of his studio next to a drawing made by his son, there is a framed painting of the Ultra deep field, which he covered with 18 golden hexagons “bought at Ikea”, to symbolize the 18 mirrors of the James Webb. Stiavelli also expects a lot from the new observations: “The image of distant galaxies taken by James Webb was designed to show how it was possible to reach Hubble’s performance in much less time. But the tool proved more sensitive than expected. so so far we have only joked, we will go much deeper ”. And much further away.