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| This collage compares images of the Flame Nebula: the left shows a near-infrared view from NASA's Hubble Space Telescope, while the right features near-infrared images from NASA's James Webb Space Telescope, revealing a more translucent cloud and highlighting young stars and brown dwarfs. Webb’s images offer a deeper look into this star-forming region. |
The Flame Nebula, located approximately 1,400 light-years from Earth, is a vibrant and dynamic region where stars are still in the process of being born. Within this cosmic nursery, astronomers have discovered objects that are too small to ignite hydrogen fusion—the defining characteristic of stars—called brown dwarfs. These so-called “failed stars” are cooler and dimmer than full-fledged stars, which makes them notoriously hard to detect, especially from vast distances.
However, when these brown dwarfs are young, they are still relatively bright and warm, making them easier to observe, even through the dense dust and gas that fills the Flame Nebula. Thanks to the cutting-edge capabilities of NASA’s James Webb Space Telescope, astronomers have been able to peer through this cosmic haze and detect faint infrared light from these elusive objects.
A recent study, led by Matthew De Furio from the University of Texas at Austin, used Webb to explore the lowest mass limits of brown dwarfs within the nebula. The team found free-floating objects with masses about two to three times that of Jupiter. Webb's sensitivity even reached objects as small as half the mass of Jupiter.
De Furio explained, "The goal of this project was to explore the fundamental low-mass limit of the star and brown dwarf formation process. With Webb, we're able to probe the faintest and lowest mass objects."
Fragmentation and the Formation of Brown Dwarfs.
The study focused on the process of fragmentation, a key mechanism in star and brown dwarf formation. When large molecular clouds, which are the birthplace of both stars and brown dwarfs, break apart, they create smaller fragments. The balance of temperature, pressure, and gravity plays a crucial role in this process.
As these fragments contract, their cores heat up. If a fragment is massive enough, nuclear fusion of hydrogen begins, and the object stabilizes into a star. However, if a fragment's mass is insufficient to ignite fusion, it remains a brown dwarf, continuing to contract while radiating its internal heat.
Scientists previously theorized that fragmentation stops when a fragment becomes opaque enough to absorb its own radiation. This prevents further cooling and collapse, with theories placing the lower mass limit between one and ten times the mass of Jupiter. Webb’s findings have narrowed this range significantly, with fewer brown dwarfs detected below three Jupiter masses.
As De Furio noted, "We don’t really find any objects below two or three Jupiter masses, and we expect to see them if they are there, so we are hypothesizing that this could be the limit itself."
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| This near-infrared image from NASA’s James Webb Space Telescope showcases three low-mass objects in the Flame Nebula, detected using Webb's sensitive instruments as part of a study on the lowest mass limit of brown dwarfs. |
A Legacy of Discovery: Building on Hubble's Data.
Brown dwarfs have always been difficult to detect, making them a challenging target for astronomers. NASA’s Hubble Space Telescope, though limited in its ability to observe brown dwarfs at the low masses Webb can detect, played a crucial role in identifying promising candidates for further study. Over the past three decades, Hubble’s data helped guide Webb’s follow-up observations in the Flame Nebula.
De Furio reflected on this collaboration, saying, "Having existing Hubble data over the last 30 years allowed us to know that this is a really useful star-forming region to target. We needed to have Webb to be able to study this particular science topic."
Massimo Robberto, an astronomer at the Space Telescope Science Institute, further explained, "It's a quantum leap in our capabilities between understanding what was going on from Hubble. Webb is really opening an entirely new realm of possibilities."
What’s Next?
The team continues to study the Flame Nebula using Webb’s spectroscopic tools to further characterize the different objects within this dusty region. As Michael Meyer of the University of Michigan pointed out, distinguishing between very low-mass brown dwarfs and planets is a key challenge in the years to come. "There’s a big overlap between the things that could be planets and the things that are very, very low-mass brown dwarfs," he said. "And that's our job in the next five years: to figure out which is which and why."
These groundbreaking results have been accepted for publication in The Astrophysical Journal Letters, shedding new light on the formation and limits of brown dwarfs, and expanding our understanding of the building blocks of stars and planets.
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