Credit: NASA, ESA, CSA, Olivia C. Jones (UK ATC), Guido De Marchi (ESTEC), Margaret Meixner (USRA)
Stars usually form in groups and with numbers that depend on the mass and density of the cloud from which they form. Regardless of the number, the combination of the intense brightness of the new stars and the bright gas around them contrasts with the dense molecular gas and dark dust of these forming regions, giving rise to spectacular shapes with complex patterns that tell us something about the parent cloud.
The most spectacular star-forming regions can be found in our Galaxy; however, with powerful telescopes like Hubble and James Webb, we can also reach into the birthplaces of the stars in our satellite galaxies known as the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). One of these regions is the star-forming region in the open cluster NGC 346, about 200 light-years across, and found among many other clusters and nebulae of the SMC.
Credit: David Malin, Anglo-Australian Obs./Royal Obs. Edinburgh]
Because of their proximity, both galaxies are considered satellites of our own Galaxy. The SMC is about 200,000 lightyears away, while the LMC is only about 150,000 light years away. Both galaxies are on the other side of the disk of the Milky Way, making them visible in the southern sky at the top of the Hydrus constellation or “the water snake.” While LMC is considered a dwarf spiral galaxy with a bar, the SMC is a dwarf irregular galaxy. The irregularity is most likely due to their own interaction and their interaction with the Milky Way.
credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg
From the observations taken by the Hubble Space Telescope (HST) in 2005, we learned that the open cluster NGC 346 is not one but probably more than three sub-clusters, each with many smaller and compact clusters covering the whole region. These sub-clusters contain dozens of hot, blue, high-mass stars, which collectively contain more than half of the known high-mass stars in the SMC galaxy [1]. We can see the bright stars and a pink nebulosity in the HST observations. This is the atomic gas that the intense radiation from the new stars produced by dividing or dissociating the molecular gas into atoms. These atoms are then ionized and re-emitted in a combination of heated gas and ionized radiation that HST captures as a pink nebula. The light dissociating the parent molecules continues beyond what seems to be a large arch of dense gas and dust[2].
credit: NASA, ESA, and A. Nota (STScI/ESA)
credit: NASA, ESA, and A. Nota (STScI/ESA)
Recent observations of this region taken with the James Webb Space Telescope (JWST) reveal the detailed structure of the dust and the cold and hot molecular gas that remains at the edges of all these clusters. The pink-hued gas in the JWST image provides new information about this region that was not revealed with the Hubble observations. JWST highlights the warm molecular gas, with temperatures around 10,000 C. Even at these temperatures, this gas cannot be seen with HST or appear as brown or dark regions between or around the stars. To make the contrast more apparent between the light coming from these two observatories, we also show the observations of HST with a blue hue, probably more adequate to the bluish light this powerful observatory detects. In the JWST image, the dust appears as bright clouds, almost white, contrasting with the dark or obscured regions in the HST image. These dusty regions also have cold molecular gas, which has temperatures of about -200 C or less. Looking carefully at this image, we can see that the dust and cold gas seem to be packed in semicircular regions around many of the smaller clusters scattered throughout the nebula. Invisible to HST, the JWST observation reveals a striking red emission at the top left of the image.
Credit: NASA, ESA, CSA, Olivia C. Jones (UK ATC), Guido De Marchi (ESTEC), Margaret Meixner (USRA
When observing this cluster in the mid-infrared with the instrument MIRI of JWST, we can see the dusty silicates and molecules known as polycyclic aromatic hydrocarbons, or PAHs. These are highlighted with the blue tendrils, while the warm dust heated by the brightest and most massive stars in the heart of the region is shown with the diffuse red emission[5].
Credit: NASA, ESA, CSA, STScI, N. Habel (JPL). Image Processing: P. Kavanagh (Maynooth University).]
If I were to find a shape in NGC 346, I would say it looks like a pasodoble dancer, throwing into flight a ruffled skirt. When combining the images of these two observatories we see how HST shows the full skirt and upper body of the dancer. JWST reveals the inner ruffles and folds of the skirt and the torso of the ballerina. The red emission at the top, made of PAHs, evokes a red “peineta” and a “mantilla” that is hidden from the HST light.
Photoshop combined R. Diaz
Astronomers targeted this cluster with JWST to understand more about the star formation process in the SMC. Because SMC has fewer metals than we have in the Milky Way, the star formation would mimic that of our Universe when it was somewhat younger, at a time more or less in between the formation of the first stars and the present time. Previous studies focused on the heavier stars, about 5 to 8 times the mass of the Sun, but with JWST, the team can look into the less massive stars; as small as a tenth of our Sun, and see if the lower metal content in the the SMC affected the formation process we see in regions with more metals, like those in our Galaxy. Remarkably, the scientists were able to identify about 1,001 pinpoint sources of light, most of them young stars still embedded in their dusty cocoons. These observations indicate that the planets might have formed earlier in the Universe than they previously thought and that the presence of metals is not a prerequisite for the formation of planets[4].
References:
[1] https://hubblesite.org/contents/media/images/2005/35/1818-Image.html
[2]https://www.esa.int/Science_Exploration/Space_Science/Star-forming_region_in_nebula_NGC_346
[3] https://en.wikipedia.org/wiki/Small_Magellanic_Cloud#/media/File:Tucana_IAU.svg
[4] https://webbtelescope.org/contents/media/images/
2023/101/01GNYHXG26ZPW9DW7KTXQH116G
[5] https://webbtelescope.org/contents/news-releases/2023/news-2023-101
SMC image from COSMOS: https://astronomy.swin.edu.au/cosmos/s/Small+Magellanic+Cloud