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By NASA
This graphic features data from NASA’s Chandra X-ray Observatory of the Cassiopeia A (Cas A) supernova remnant that reveals that the star’s interior violently rearranged itself mere hours before it exploded. The main panel of this graphic is Chandra data that shows the location of different elements in the remains of the explosion: silicon (represented in red), sulfur (yellow), calcium (green) and iron (purple). The blue color reveals the highest-energy X-ray emission detected by Chandra in Cas A and an expanding blast wave. The inset reveals regions with wide ranges of relative abundances of silicon and neon. This data, plus computer modeling, reveal new insight into how massive stars like Cas A end their lives.X-ray: NASA/CXC/Meiji Univ./T. Sato et al.; Image Processing: NASA/CXC/SAO/N. Wolk The inside of a star turned on itself before it spectacularly exploded, according to a new study from NASA’s Chandra X-ray Observatory. Today, this shattered star, known as the Cassiopeia A supernova remnant, is one of the best-known, well-studied objects in the sky.
Over three hundred years ago, however, it was a giant star on the brink of self-destruction. The new Chandra study reveals that just hours before it exploded, the star’s interior violently rearranged itself. This last-minute shuffling of its stellar belly has profound implications for understanding how massive stars explode and how their remains behave afterwards.
Cassiopeia A (Cas A for short) was one of the first objects the telescope looked at after its launch in 1999, and astronomers have repeatedly returned to observe it.
“It seems like each time we closely look at Chandra data of Cas A, we learn something new and exciting,” said Toshiki Sato of Meiji University in Japan who led the study. “Now we’ve taken that invaluable X-ray data, combined it with powerful computer models, and found something extraordinary.”
As massive stars age, increasingly heavy elements form in their interiors by nuclear reactions, creating onion-like layers of different elements. Their outer layer is mostly made of hydrogen, followed by layers of helium, carbon and progressively heavier elements – extending all the way down to the center of the star.
Once iron starts forming in the core of the star, the game changes. As soon as the iron core grows beyond a certain mass (about 1.4 times the mass of the Sun), it can no longer support its own weight and collapses. The outer part of the star falls onto the collapsing core, and rebounds as a core-collapse supernova.
The new research with Chandra data reveals a change that happened deep within the star at the very last moments of its life. After more than a million years, Cas A underwent major changes in its final hours before exploding.
“Our research shows that just before the star in Cas A collapsed, part of an inner layer with large amounts of silicon traveled outwards and broke into a neighboring layer with lots of neon,” said co-author Kai Matsunaga of Kyoto University in Japan. “This is a violent event where the barrier between these two layers disappears.”
This upheaval not only caused material rich in silicon to travel outwards; it also forced material rich in neon to travel inwards. The team found clear traces of these outward silicon flows and inward neon flows in the remains of Cas A’s supernova remnant. Small regions rich in silicon but poor in neon are located near regions rich in neon and poor in silicon.
The survival of these regions not only provides critical evidence for the star’s upheaval, but also shows that complete mixing of the silicon and neon with other elements did not occur immediately before or after the explosion. This lack of mixing is predicted by detailed computer models of massive stars near the ends of their lives.
There are several significant implications for this inner turmoil inside of the doomed star. First, it may directly explain the lopsided rather than symmetrical shape of the Cas A remnant in three dimensions. Second, a lopsided explosion and debris field may have given a powerful kick to the remaining core of the star, now a neutron star, explaining the high observed speed of this object.
Finally, the strong turbulent flows created by the star’s internal changes may have promoted the development of the supernova blast wave, facilitating the star’s explosion.
“Perhaps the most important effect of this change in the star’s structure is that it may have helped trigger the explosion itself,” said co-author Hiroyuki Uchida, also of Kyoto University. “Such final internal activity of a star may change its fate—whether it will shine as a supernova or not.”
These results have been published in the latest issue of The Astrophysical Journal and are available online.
To learn more about Chandra, visit:
https://science.nasa.gov/chandra
Read more from NASA’s Chandra X-ray Observatory Learn more about the Chandra X-ray Observatory and its mission here:
https://www.nasa.gov/chandra
https://chandra.si.edu
Visual Description
This release features a composite image of Cassiopeia A, a donut-shaped supernova remnant located about 11,000 light-years from Earth. Included in the image is an inset closeup, which highlights a region with relative abundances of silicon and neon.
Over three hundred years ago, Cassiopeia A, or Cas A, was a star on the brink of self-destruction. In composition it resembled an onion with layers rich in different elements such as hydrogen, helium, carbon, silicon, sulfur, calcium, and neon, wrapped around an iron core. When that iron core grew beyond a certain mass, the star could no longer support its own weight. The outer layers fell into the collapsing core, then rebounded as a supernova. This explosion created the donut-like shape shown in the composite image. The shape is somewhat irregular, with the thinner quadrant of the donut to the upper left of the off-center hole.
In the body of the donut, the remains of the star’s elements create a mottled cloud of colors, marbled with red and blue veins. Here, sulfur is represented by yellow, calcium by green, and iron by purple. The red veins are silicon, and the blue veins, which also line the outer edge of the donut-shape, are the highest energy X-rays detected by Chandra and show the explosion’s blast wave.
The inset uses a different color code and highlights a colorful, mottled region at the thinner, upper left quadrant of Cas A. Here, rich pockets of silicon and neon are identified in the red and blue veins, respectively. New evidence from Chandra indicates that in the hours before the star’s collapse, part of a silicon-rich layer traveled outwards, and broke into a neighboring neon-rich layer. This violent breakdown of layers created strong turbulent flows and may have promoted the development of the supernova’s blast wave, facilitating the star’s explosion. Additionally, upheaval in the interior of the star may have produced a lopsided explosion, resulting in the irregular shape, with an off-center hole (and a thinner bite of donut!) at our upper left.
News Media Contact
Megan Watzke
Chandra X-ray Center
Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu
Corinne Beckinger
Marshall Space Flight Center, Huntsville, Alabama
256-544-0034
corinne.m.beckinger@nasa.gov
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Last Updated Aug 28, 2025 EditorLee MohonContactCorinne M. Beckingercorinne.m.beckinger@nasa.govLocationMarshall Space Flight Center Related Terms
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By NASA
5 min read
NASA’s Apollo Samples, LRO Help Scientists Predict Moonquakes
This mosaic of the Taurus-Littrow valley was made using images from the Narrow Angle Cameras onboard NASA’s Lunar Reconnaissance Orbiter. The orbiter has been circling and studying the Moon since 2009. The ancient-lava-filled valley is cut by the Lee-Lincoln thrust fault, visible as a sinuous, white line extending from South Massif (mountain in the bottom left corner) to North Massif (mountain in the top center) where the fault abruptly changes direction and cuts along the slope of North Massif. The Lee-Lincoln fault has been the source of multiple strong moonquakes causing landslides and boulder falls on both North and South massifs. The approximate location of the Apollo 17 landing site is indicated to the right of the fault with a white “x”. NASA/ASU/Smithsonian As NASA prepares to send astronauts to the surface of the Moon’s south polar region for the first time ever during the Artemis III mission, scientists are working on methods to determine the frequency of moonquakes along active faults there.
Faults are cracks in the Moon’s crust that indicate that the Moon is slowly shrinking as its interior cools over time. The contraction from shrinking causes the faults to move suddenly, which generates quakes. Between 1969 and 1977, a network of seismometers deployed by Apollo astronauts on the Moon’s surface recorded thousands of vibrations from moonquakes.
Moonquakes are rare, with the most powerful ones, about magnitude 5.0, occurring near the surface. These types of quakes are much weaker than powerful quakes on Earth (magnitude 7.0 or higher), posing little risk to astronauts during a mission lasting just a few days. But their effects on longer-term lunar surface assets could be significant. Unlike an earthquake that lasts for tens of seconds to minutes, a moonquake can last for hours, enough time to damage or tip over structures, destabilize launch vehicles on the surface, or interrupt surface operations.
“The hazard probability goes way up depending on how close your infrastructure is to an active fault,” said Thomas Watters, senior scientist emeritus at the Smithsonian’s National Air & Space Museum in Washington.
Watters is a long-time researcher of lunar geology and a co-investigator on NASA’s LRO (Lunar Reconnaissance Orbiter) camera. Recently, he and Nicholas Schmerr, a planetary seismologist at the University of Maryland in College Park, developed a new method for estimating the magnitude of seismic shaking by analyzing evidence of dislodged boulders and landslides in an area, as the scientists reported on July 30 in the journal Science Advances. Studies like these can help NASA plan lunar surface assets in safer locations.
Unlike an earthquake that lasts for tens of seconds to minutes, a moonquake can last for hours, enough time to damage or tip over structures, destabilize launch vehicles on the surface, or interrupt surface operations.
There are thousands of faults across the Moon that may still be active and producing quakes. Watters and his team have identified these faults by analyzing data from LRO, which has been circling the Moon since 2009, mapping the surface and taking pictures, providing unprecedented detail of features like faults, boulders, and landslides.
For this study, Watters and Schmerr chose to analyze surface changes from quakes generated by the Lee-Lincoln fault in the Taurus-Littrow valley. NASA’s Apollo 17 astronauts, who landed about 4 miles west of the fault on Dec. 11, 1972, explored the area around the fault during their mission.
By studying boulder falls and a landslide likely dislodged by ground shaking near Lee Lincoln, Watters and Schmerr estimated that a magnitude 3.0 moonquake — similar to a relatively minor earthquake — occurs along the Lee Lincoln fault about every 5.6 million years.
“One of the things we’re learning from the Lee-Lincoln fault is that many similar faults have likely had multiple quakes spread out over millions of years,” Schmerr said. “This means that they are potentially still active today and may keep generating more moonquakes in the future.”
The authors chose to study the Lee-Lincoln fault because it offered a unique advantage: Apollo 17 astronauts brought back samples of boulders from the area. By studying these samples in labs, scientists were able to measure changes in the boulders’ chemistry caused by exposure to cosmic radiation over time (the boulder surface is freshly exposed after breaking off a larger rock that would have otherwise shielded it).
This cosmic radiation exposure information helped the researchers determine how long the boulders had been sitting in their current locations, which in turn helped inform the estimate of possible timing and frequency of quakes along the Lee-Lincoln fault.
This 1972 image shows Apollo 17 astronaut Harrison H. Schmitt sampling a boulder at the base of North Massif in the Taurus-Littrow valley on the Moon. This large boulder is believed to have been dislodged by a strong moonquake that occurred about 28.5 million years ago. The source of the quake was likely a seismic event along the Lee-Lincoln fault. The picture was taken by astronaut Eugene A. Cernan, Apollo 17 commander. NASA/JSC/ASU Apollo 17 astronauts investigated the boulders at the bases of two mountains in the valley. The tracks left behind indicated that the boulders may have rolled downhill after being shaken loose during a moonquake on the fault. Using the size of each boulder, Watters and Schmerr estimated how hard the ground shaking would have been and the magnitude of the quake that would have caused the boulders to break free.
The team also estimated the seismic shaking and quake magnitude that would be needed to trigger the large landslide that sent material rushing across the valley floor, suggesting that this incident caused the rupture event that formed the Lee-Lincoln fault.
A computer simulation depicting the seismic waves emanating from a shallow moonquake on the Lee-Lincoln fault in the Taurus-Littrow valley on the Moon. The label “A17” marks the Apollo 17 landing site. The audio represents a moonquake that was recorded by a seismometer placed on the surface by astronauts. The seismic signal is converted into sound. Both audio and video are sped up to play 10 times faster than normal. The background image is a globe mosaic image from NASA’s Lunar Reconnaissance Orbiter’s Wide-Angle Camera. Red and blue are positive (upward ground motion) and negative (downward ground motion) polarities of the wave. Nicholas Schmerr Taking all these factors into account, Watters and Schmerr estimated that the chances that a quake would have shaken the Taurus-Littrow valley on any given day while the Apollo 17 astronauts were there are 1 in 20 million, the authors noted.
Their findings from the Lee-Lincoln fault are just the beginning. Watters and Schmerr now plan to use their new technique to analyze quake frequency at faults in the Moon’s south polar region, where NASA plans to explore.
NASA also is planning to send more seismometers to the Moon. First, the Farside Seismic Suite will deliver two sensitive seismometers to Schrödinger basin on the far side of the Moon onboard a lunar lander as part of NASA’s CLPS (Commercial Lunar Payload Services) initiative. Additionally, NASA is developing a payload, called the Lunar Environment Monitoring Station, for potential flight on NASA’s Artemis III mission to the South Pole region. Co-led by Schmerr, the payload will assess seismic risks for future human and robotic missions to the region.
Read More: What Are Moonquakes?
Read More: Moonquakes and Faults Near Lunar South Pole
For more information on NASA’s LRO, visit:
Media Contacts:
Karen Fox / Molly Wasser
Headquarters, Washington
202-358-1600
karen.c.fox@nasa.gov / molly.l.wasser@nasa.gov
Lonnie Shekhtman
NASA’s Goddard Space Flight Center, Greenbelt, Md.
lonnie.shekhtman@nasa.gov
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Lonnie Shekhtman
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Last Updated Aug 14, 2025 Related Terms
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