Free Astronomy Magazine January-February 2025

45 ASTRO PUBLISHING temperature, by measuring closer to the inner edge of the accretion disk than ever before,” said Lynne Hillen- brand of Caltech in Pasadena, Cali- fornia, and a co-author of the paper. “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.” Originally deemed to be a unique case among stars, FU Ori exemplifies a class of young, eruptive stars that undergo dramatic changes in bright- ness. These objects are a subset of classical T Tauri stars, which are newly forming stars that are build- ing up by accreting material from their disk and the surrounding neb- ula. In classical T Tauri stars, the disk does not touch the star directly be- cause it is restricted by the outward pressure of the star’s magnetic field. The accretion disks around FU Ori objects, however, are susceptible to instabilities due to their enormous mass relative to the central star, in- teractions with a binary compan- JANUARY-FEBRUARY 2025 T his is an artist’s concept of the early stages of the young star FU Orionis (FU Ori) outburst, surrounded by a disk of material. A team of astronomers has used the Hubble Space Telescope’s ultraviolet capabilities to learn more about the interaction between FU Ori’s stellar surface and the accretion disk that has been dumping gas onto the growing star for nearly 90 years. They found that the inner disk, touching the star, is much hotter than expected—16,000 kelvins— nearly three times our Sun’s surface temperature. That sizzling temperature is nearly twice as hot as previously believed. [NASA-JPL, Caltech]