NASA’s Hubble Space Telescope has unveiled astonishing new details about FU Orionis (FU Ori), a young star in the constellation Orion known for its dramatic brightness outburst nearly a century ago. Using advanced ultraviolet imaging, astronomers discovered that the star’s accretion disk—a swirling structure feeding gas and material to the star—is far hotter and more dynamic than previously believed. These findings challenge long-held theories about the behavior of young, eruptive stars and offer valuable insights into the early stages of stellar evolution.
The results, described as “sizzling” by scientists, provide an unprecedented view into the inner workings of FU Ori, where temperatures reach 16,000 kelvins, nearly three times hotter than the Sun’s surface. This revelation could reshape how astronomers understand the interaction between stars and their surrounding disks.
A Star Like No Other
Discovered in 1936, FU Orionis astonished astronomers by increasing its brightness one hundredfold within a few months, making it one of the brightest young stars ever observed. Unlike an exploding star, FU Ori’s luminosity has declined slowly over decades, revealing it as a peculiar member of the FU Ori class of young, eruptive stars.
FU Ori belongs to a subset of T Tauri stars, youthful stars still forming by accreting material from surrounding disks. However, what sets FU Ori apart is its accretion process. Unlike classical T Tauri stars, where magnetic fields prevent the disk from directly contacting the star, FU Ori’s disk reaches its stellar surface, resulting in a more volatile and extreme interaction.
“The Hubble data indicates a much hotter impact region than models have previously predicted,” explained Adolfo Carvalho of Caltech, the lead author of the study. “In FU Ori, the temperature is 16,000 kelvins [nearly three times our Sun’s surface temperature]. That sizzling temperature is almost twice the amount prior models have calculated. It challenges and encourages us to think of how such a jump in temperature can be explained.”
Unveiling the Inner Accretion Disk
Using Hubble’s Cosmic Origins Spectrograph (COS) and Space Telescope Imaging Spectrograph (STIS), researchers captured the first ultraviolet spectra of FU Ori. The data revealed an extraordinary environment at the disk’s inner edge, where gas spirals toward the star at speeds far exceeding the star’s own rotation. This difference creates an intense shockwave where the gas decelerates and heats up dramatically, emitting ultraviolet light.
The region’s extreme conditions were unexpected. Astronomers hoped to validate models predicting the disk’s maximum temperature but instead encountered temperatures nearly twice as high as expected. Lynne Hillenbrand, a co-author of the study, noted, “I think there was some hope that we would see something extra, like the interface between the star and its disk, but we were certainly not expecting it. The fact we saw so much extra—it was much brighter in the ultraviolet than we predicted—that was the big surprise.”
Implications for Planet Formation
The findings on FU Ori extend beyond stellar evolution, influencing our understanding of planet formation. Stars like FU Ori can shape the chemical and physical environments of their surrounding disks, which are also the birthplaces of planets. However, the violent accretion processes observed in FU Ori can disrupt these environments.
“If the planet is far out in the disk as it’s forming, outbursts from an FU Ori object should influence what kind of chemicals the planet will ultimately inherit,” explained Carvalho. “But if a forming planet is very close to the star, then it’s a slightly different story. Within a couple outbursts, any planets that are forming very close to the star can rapidly move inward and eventually merge with it. You could lose, or at least completely fry, rocky planets forming close to such a star.”
This dynamic highlights the delicate balance required for planetary systems to emerge and survive around stars undergoing such intense activity.
A New Window Into Young Stars
The groundbreaking observations of FU Orionis represent a milestone in stellar astrophysics. By leveraging Hubble’s ultraviolet capabilities, astronomers have peered deeper into the heart of this young star’s accretion process than ever before. These findings not only challenge existing theories about young, eruptive stars but also open new avenues for understanding the complex mechanisms of stellar and planetary formation.
“A lot of these young stars are spectroscopically very rich at far ultraviolet wavelengths,” reflected Hillenbrand. “A combination of Hubble, its size and wavelength coverage, as well as FU Ori’s fortunate circumstances, let us see further down into the engine of this fascinating star-type than ever before.”
The team continues to analyze Hubble’s spectral data, hoping to uncover additional clues about FU Ori’s inflowing and outflowing gas. These insights promise to deepen our understanding of the dynamic processes shaping young stars and their evolving planetary systems.
