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Webb explores Sagittarius B2, the largest star-forming cloud in the Milky Way

The NASA/ESA/CSA James Webb Space Telescope has revealed a colourful array of massive stars and glowing cosmic dust in the Sagittarius B2 (Sgr B2) molecular cloud, the most massive and active star-forming region in our Milky Way galaxy.

A wide view of a region of space filled with stars and clumps of orange clouds. — Stars, gas and cosmic dust in the Sagittarius B2 molecular cloud glow in near-infrared light, captured by Webb’s NIRCam (Near-Infrared Camera). In this light, astronomers see more of the region’s diverse, colourful stars, but less of its gas and dust structure. Webb’s instruments each provide astronomers with important information that help build a more complete picture of what is happening in this intriguing portion of the centre of our galaxy.
Credit: NASA, ESA, CSA, STScI, A. Ginsburg (University of Florida), N. Budaiev (University of Florida), T. Yoo (University of Florida). Image processing: A. Pagan (STScI)

Sagittarius B2 is the Milky Way galaxy’s most massive and active star forming cloud, producing half of the stars created in the galactic centre region despite having only 10 percent of the area’s star-making material. Now, Webb has revealed stunning new views of the region, using both its near-infrared and mid-infrared instruments, to capture both its colourful stars and gaseous stellar nurseries in unprecedented detail.

Sagittarius B2 is located only a few hundred light-years from the supermassive black hole at the heart of the galaxy called Sagittarius A*, a region densely packed with stars, star-forming clouds, and complex magnetic fields. The infrared light that Webb detects is able to pass through some of the area’s thick clouds to reveal young stars and the warm dust surrounding them. Astronomers think that analysis of Webb’s data will help unravel enduring mysteries of the star formation process, and why Sagittarius B2 is forming so many more stars than the rest of the galactic centre.

However, one of the most notable aspects of Webb’s images of Sagittarius B2 are the portions that remain dark. These ironically empty-looking areas of space are actually so dense with gas and dust that even Webb cannot see through them. These thick clouds are the raw material of future stars and a cocoon for those still too young to shine.

Cosmic clouds of pink and purple, some with bright centres, are surrounded by dark areas that appear like black space dotted with bright blue stars. A group of small clouds to the right is more red than any other area of the image. — Webb’s MIRI (Mid-Infrared Instrument) shows the Sagittarius B2 (Sgr B2) region in mid-infrared light, with warm dust glowing brightly. To the right is one clump of clouds that captured astronomers’ attention. It is redder than the rest of the clouds in the image and corresponds to an area that other telescopes have shown to be one of the most molecularly rich regions known. Additional analysis of this intriguing region could yield important insights into why Sgr B2 is so much more productive in making stars than the rest of the galactic centre. Only the brightest stars in this region emit mid-infrared light that can be picked up by Webb’s MIRI instrument, which is why this image has so many fewer stars than that captured by Webb’s NIRCam (Near-Infrared Camera). The darkest areas of the image are not empty space but areas where cosmic dust and gas are so dense that light cannot penetrate them to reach the telescope.
Credit: NASA, ESA, CSA, STScI, A. Ginsburg (University of Florida), N. Budaiev (University of Florida), T. Yoo (University of Florida). Image processing: A. Pagan (STScI)

The high resolution and mid-infrared sensitivity of Webb’s MIRI (Mid-Infrared Instrument) revealed this region in unprecedented detail, including glowing cosmic dust heated by very young massive stars. The reddest area, known as Sagittarius B2 North, (note: north is to the right in these Webb images) is one of the most molecularly rich regions known, but astronomers have never seen it with such clarity.

Sagittarius B2 (Sgr B2) molecular cloud Cosmic clouds of pink and purple, some with bright centres, are surrounded by dark areas that appear like black space dotted with bright blue stars. A small cloud to the lower right is more red than any other area of the image. At the top right are compass arrows indicating the orientation of the image on the sky. The north arrow points to just past 3 o’clock, and the east arrow points to just past 12 o’clock. At the lower right is a scale bar labeled 5 light-years. The length of the scale bar is about 7 times the total width of the image. Below the image is a colour key showing which filters were used to create the image and which visible-light colour is assigned to each filter. From left to right, the filters are: F770W in blue, F1280W in green, and F2550W is red. — This image of the Sagittarius B2 (Sgr B2) molecular cloud, captured by Webb’s MIRI (Mid-Infrared Instrument) includes compass arrows, scale bar, and colour key for reference. To create this image, mid-infrared wavelengths of light have been translated into visible-light colours. The colour key at the bottom shows which MIRI filters were used, and which visible-light colour was assigned to that filter. The north and east compass arrows show the orientation of the image on the sky. Note that the relationship between north and east on the sky (as seen from below) is flipped relative to direction arrows on a map of the ground (as seen from above).
Credit: NASA, ESA, CSA, STScI, A. Ginsburg (University of Florida), N. Budaiev (University of Florida), T. Yoo (University of Florida). Image processing: A. Pagan (STScI)

The difference longer wavelengths of light make, even within the infrared spectrum, are stark when comparing the images from Webb’s MIRI and NIRCam (Near-Infrared Camera) instruments. Glowing gas and dust appear dramatically in mid-infrared light, while all but the brightest stars disappear from view.

In contrast to MIRI, colourful stars steal the show in Webb’s NIRCam image, punctuated occasionally by bright clouds of gas and dust. Further research into these stars will reveal details of their masses and ages, which will help astronomers better understand the process of star formation in this dense, active galactic centre region. Has it been going on for millions of years? Or has some unknown process triggered it only recently?

Sagittarius B2 (Sgr B2) molecular cloud A region of space filled with stars and clumps of orange clouds. At the top right are compass arrows indicating the orientation of the image on the sky. The north arrow points to 3 o’clock, and the east arrow points to 12 o’clock. At the lower right is a scale bar labeled 10 light-years. The length of the scale bar is about 5 times the total width of the image. Below the image is a colour key showing which filters were used to create the image and which visible-light colour is assigned to each filter. From left to right, the filters are: F150W, F182M, and F187N are blue; F212N and F210M are cyan; F300M and F360M are green; F405N is yellow; F410M is light orange; F466N and F480M are orange. — This image of the Sagittarius B2 (Sgr B2) molecular cloud, captured by Webb’s NIRCam (Near-Infrared Camera) instrument includes compass arrows, scale bar, and colour key for reference. To create this image, near-infrared wavelengths of light have been translated into visible-light colours. The colour key at the bottom shows which NIRCam filters were used, and which visible-light colour was assigned to that filter. The north and east compass arrows show the orientation of the image on the sky. Note that the relationship between north and east on the sky (as seen from below) is flipped relative to direction arrows on a map of the ground (as seen from above).
Credit: NASA, ESA, CSA, STScI, A. Ginsburg (University of Florida), N. Budaiev (University of Florida), T. Yoo (University of Florida). Image processing: A. Pagan (STScI)

Astronomers hope Webb will shed light on why star formation in the galactic centre is so disproportionate. Though the region is stocked with plenty of gaseous raw material, on the whole it is not nearly as productive as Sagittarius B2. While Sagittarius B2 has only 10 percent of the galactic centre’s gas, it produces 50 percent of its stars.

Bibliographic information:

Nazar Budaiev, Adam Ginsburg, Ashley T. Barnes, Desmond Jeff, Taehwa Yoo, Cara Battersby, Alyssa Bulatek, Xing Lu, Elisabeth A.C. Mills, Daniel L. Walker, JWST’s first view of the most vigorously star-forming cloud in the Galactic center — Sagittarius B2, DOI: https://doi.org/10.48550/arXiv.2509.11771

Press release from ESA Webb.

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