NASA’s Fermi Spots Young Star Cluster Blowing Gamma-Ray Bubbles
This image of super star cluster Westerlund 1 was captured with the Near-InfraRed Camera on NASA’s James Webb’s Space Telescope. The cluster is largely hidden at visible wavelengths by dust clouds, which infrared light penetrates. Westerlund 1’s large, dense, and diverse stellar population of massive stars has no other known counterpart in the Milky Way.
Credit: ESA/Webb, NASA & CSA, M. Zamani (ESA/Webb), M. G. Guarcello (INAF-OAPA) and the EWOCS team
Alt text: A cluster of multicolored stars
Image Description: Many stars in this image have six large and two small spikes created by the instrument optics on NASA’s James Webb Space Telescope. The stars have a variety of sizes depending on their brightness and distance within the cluster and different colors reflecting different stellar types. Patches of billowing red gas can be seen in and around the cluster, lit up by the stars. Small stars in the cluster blend into a background of distant stars and galaxies on black.
For the first time, astronomers using NASA’s Fermi Gamma-ray Space Telescope have traced a budding outflow of gas from a cluster of young stars in our galaxy.
The cluster, called Westerlund 1, is located about 12,000 light-years away in the southern constellation Ara. It’s the closest, most massive, and most luminous super star cluster in the Milky Way. The only reason Westerlund 1 isn’t visible to the unaided eye is because it’s surrounded by thick clouds of dust. Its outflow extends below the plane of the galaxy and is filled with high-speed, hard-to-study particles called cosmic rays.
Super star clusters like Westerlund 1 contain more than 10,000 times our Sun’s mass. They are also more luminous and contain higher numbers of rare, massive stars than other clusters.
Scientists think that supernova explosions and stellar winds within star clusters push ambient gas outward, propelling cosmic rays to near light speed. About 90% of these particles are hydrogen nuclei, or protons, and the remainder are electrons and the nuclei of heavier elements.
Because cosmic ray particles are electrically charged, they change course when they encounter magnetic fields. This means scientists can’t trace them back to their sources. Gamma rays, however, travel in a straight line. Gamma rays are the highest-energy form of light, and cosmic rays produce gamma rays when they interact with matter in their environment.
Most gamma-ray observations of stellar clusters have limited resolution, so astronomers effectively see them as indistinct areas of emission. Because Westerlund 1 is so close and bright, however, it’s easier to study.
Fermi’s sensitivity and resolution allowed researchers to filter out other gamma-ray sources in the cluster like rapidly spinning stellar remnants called pulsars, background radiation, and Westerlund 1 itself.
What was left was a bubble of gamma rays extending over 650 light-years from the cluster below the plane of the Milky Way. That means the outflow is about 200 times larger than Westerlund 1 itself.
The researchers call this a nascent, or early stage, outflow because it was likely recently produced by massive young stars within the cluster and hasn’t yet had time to break out of the galactic disk. Eventually it will stream into the galactic halo, the hot gas surrounding the Milky Way.
Westerlund 1 is located slightly below the galactic plane, so the researchers think the gas expanded asymmetrically, following the path of least resistance into a zone of lower density below the disk.
This artist’s concept shows the location of Westerlund 1 relative to our Sun as seen from the underside of our Milky Way galaxy. The magenta bubble illustrates what the nascent outflow might look like in gamma rays. Westerlund 1 is located slightly below the middle of the galactic disk, so stellar activity pushes gas preferentially along a path of lower density beneath the disk.
Credit: NASA’s Goddard Space Flight Center/Conceptual Image Lab
Alt text: A view of the underside of the Milky Way galaxy
Image description: This artist’s concept shows the underside of the Milky Way galaxy. In the distance near bottom center, the galactic bulge extends slightly outward from a brownish disk flecked with blue-white stars, which makes up most of the image. The Sun’s location is labeled at the center of the image. Westerlund 1 is labeled below and to the Sun’s right, closer to the galaxy’s center. A magenta bubble extends from Westerlund 1’s location, representing the location of its nascent outflow of high-speed particles called cosmic rays, which we see indirectly through the gamma rays they produce.
Unlabeled version of the above.
Credit: NASA’s Goddard Space Flight Center/Conceptual Image Lab
Alt text: A view of the underside of the Milky Way galaxy
Image description: This artist’s concept shows the underside of the Milky Way galaxy. In the distance near bottom center, the galactic bulge extends slightly outward from a brownish disk flecked with blue-white stars, which makes up most of the image. A magenta bubble extends from Westerlund 1’s location, representing the location of its nascent outflow of high-speed particles called cosmic rays, which we see indirectly through the gamma rays they produce.
Westerlund 1 is located closer to the center of the Milky Way than our Sun, as shown in this artist’s concept. Westerlund 1 is one of only a few known super star clusters in our galaxy and is the closest, brightest, and most massive one discovered so far.
Credit: NASA’s Goddard Space Flight Center/Conceptual Image Lab
Alt text: A top-down view of the Milky Way galaxy
Image description: This artist’s concept shows our Milky Way galaxy. The center is a bright yellowish oval. Spirals of gas and stars swirl outward from the center. A label for the location of Westerlund 1 is placed slightly below and to the right of the center. A label for the location of the Sun is located below center, closer to the galaxy’s edge.
Unlabeled version of the above.
Alt text: A top-down view of the Milky Way galaxy
Image description: This artist’s concept shows our Milky Way galaxy. The center is a bright yellowish oval. Spirals of gas and stars swirl outward from the center.
Data from NASA’s Fermi Gamma-ray Space Telescope reveal the budding gas bubble of star cluster Westerlund 1. Brighter colors indicate a stronger likelihood that gamma rays arise from specific types of point sources, notably two pulsars located at center and in the brightest portion of the image. Pink contours denote steep changes in likelihood. An underlying orange-magenta feature extends down the image, starting from the cluster’s location, and represents the nascent outflow. The grey lines indicate distance below the galactic plane. The bubble is over 650 light-years long and angles slightly away from us. Westerlund 1’s stellar activity more easily pushes gas outward into lower-density regions of the galaxy’s disk.
Credit: NASA's Goddard Space Flight Center/Lemoine-Goumard et al. 2025; ESA/Webb, NASA & CSA, M. Zamani (ESA/Webb), M. G. Guarcello (INAF-OAPA) and the EWOCS team
Alt text: Fermi and Webb data of Westerlund 1 and its early outflow
Image description: A multicolored amoeba-like blob at the center of the image represents gamma-ray data of Westerlund 1. The outer edges are purple, shading to pink, orange, and finally yellow closer to the center. The blob has a bold pink outline and thinner pink lines trace contours within it. Six gray, irregularly spaced lines of varying thicknesses divide the image horizontally. The first is labeled “Galactic plane.” The second is labeled “130 light-years.” The sixth is labeled "650 light-years.” A small smattering of stars near the top of the blob is labeled “Westerlund 1.” The image is titled “Westerlund 1’s Early Outflow.”
An unlabled version of the Fermi gamma-ray data.
Credit: NASA's Goddard Space Flight Center/Lemoine-Goumard et al. 2025
Alt text: Fermi data of Westerlund 1
Image description: A multicolored amoeba-like blob at the center of the image represents gamma-ray data of Westerlund 1. The outer edges are purple, shading to pink, orange, and finally yellow closer to the center.
Westerlund 1 Multiwavelength Reel
Westerlund 1 is a massive cluster of young stars located in our Milky Way galaxy. This super star cluster has more than 10,000 times our Sun’s mass, and it contains a larger number of rare stars than many other clusters.
Though it’s the closest, largest, and most luminous cluster in the galaxy, Westerlund 1 actually isn’t visible to the unaided eye. It’s surrounded by thick clouds of gas and dust. Many space- and ground-based telescopes have peered at cosmic activities occurring in Westerlund 1 in various wavelengths of light. For example, recently our Fermi Gamma-ray Space Telescope helped astronomers trace a budding outflow of gas pouring out from Westerlund 1.
Image credits:
- ESA/Webb, NASA & CSA, M. Zamani (ESA/Webb), M. G. Guarcello (INAF-OAPA) and the EWOCS team
- ESA/Hubble & NASA
- ESO
- NASA/ESA/STScI; Image Processing: NASA/CXC/SAO/L. Frattare
- NASA/CXC/INAF/M. Guarcello et al.; Image Processing: NASA/CXC/SAO/L. Frattare
- NASA's Goddard Space Flight Center/Lemoine-Goumard et al. 2025
Music: “Underwater,” Laswell, Ambient Dreams
Complete transcript available.
Story and individual images can be found [here](https://svs.gsfc.nasa.gov/14930/){target=_blank}. Visual description: 0:00 The sparkling stars of Westerlund 1 fill the screen with shimmering, colorful dots in an infrared view from @NASAWebb. The brightest have thin spikes of light extending from them, many of which overlap. 0:02 The text “Westerlund 1: Multiwavelength” appears. 0:09 The cluster of stars shines brightly in light blue in the shape of a footprint, with a smaller number of stars marked with spikes of light in an optical and infrared view from @NASAHubble. 0:14 The optical and infrared view from the Wide Field Imager on the European Southern Observatory’s MPG/ESO ground-based telescope shows a slightly fuzzier look at the star cluster, now appearing in shades of orange and red. 0:19 An optical view from Hubble shows a rainbow of stars with a handful that appear brighter and orb-like. 0:24 Stars dot a bright pink cloud surrounded by an orange glow and a small ball of teal and blue near the bottom in the X-ray view from @NASAChandraXray. 0:29 The image of Westerlund 1 then spins and zooms out to the gamma-ray view from Fermi. At the center is a bright orange blob shaped roughly like a bowling pin. It is surrounded by layers of red-orange and purple against a dark background. 0:35 The video fades to black with a NASA insignia.NASA’s Hubble Space Telescope captured this view of Westerlund 1 in visible and infrared light. Scientists think most of Westerlund 1’s formed in the same burst of activity, meaning that they have similar ages and compositions. The cluster is relatively young in astronomical terms — around 3 million years old compared to our own Sun, which is some 4.6 billion years old.
Credit: ESA/Hubble & NASA
Alt text: Hubble image of Westerlund 1
Image description: White, yellow and blue stars cluster in the center of this image. Many have four spikes created by the instrument optics on NASA’s Hubble Space Telescope. The stars vary in size and brightness. Scattered across the black background are many more stars of different colors.
A visible-light-only view of Westerlund 1 from Hubble.
Credit: NASA/ESA/STScI; Image Processing: NASA/CXC/SAO/L. Frattare
Alt text: Hubble image of Westerlund 1
Image description: Multicolored dots of various sizes are strewn across a black background in his Hubble visible-light image. A cluster of yellow stars in the center is the Westerlund 1 super cluster.
This image shows data from NASA’s Chandra X-ray Observatory. The X-rays show young stars (mostly represented as white and pink) as well as diffuse heated gas throughout the cluster (colored pink, green, and blue, in order of increasing temperatures for the gas).
Credit: X-ray: NASA/CXC/INAF/M. Guarcello et al.; Image Processing: NASA/CXC/SAO/L. Frattare
Alt text: Chandra image of Westerlund 1
Image description: In this image of Westerlund 1, the black canvas of space is peppered with colored dots of light of various sizes, mostly in shades of red, green, blue, and white. At the center of the image is a semi-transparent, red and yellow cloud of gas encircling a grouping of tightly packed white stars. The shape and distribution of stars in the cluster call to mind effervescent soda bubbles dancing above the ice cubes of a recently poured beverage.
Credits
Please give credit for this item to:
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Producer
- Scott Wiessinger (eMITS)
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Science writer
- Jeanette Kazmierczak (University of Maryland College Park)
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This page was originally published on Thursday, December 18, 2025.
This page was last updated on Wednesday, December 17, 2025 at 4:05 PM EST.