Free Astronomy Magazine July-August 2024

JULY-AUGUST 2024 T his image showcases three views of one of the most distinctive objects in our skies, the Horsehead Nebula. The first image (left), released in November 2023, features the Horsehead Nebula as seen in visible light by ESA’s Euclid telescope, which has contributions from NASA. The second image (middle) shows a view of the Horsehead Nebula in near-infrared light from NASA’s Hubble Space Telescope. This image reveals a beautiful, delicate structure that is normally obscured by dust. The third image (right) features a new view of the Horsehead Nebula from NASA’s James Webb Space Telescope’s NIRCam (Near-Infrared Camera) instrument. [NASA, ESA, CSA, Karl Misselt (University of Arizona), Alain Abergel (IAS, CNRS), Mahdi Zamani The Euclid Consortium, Hubble Heritage Project (STScI, AURA)] T his image of the Horsehead Neb- ula from NASA’s James Webb Space Telescope focuses on a portion of the horse’s “mane.” It was taken with Webb’s MIRI (Mid-Infrared In- strument). Mid-infrared light cap- tures the glow of substances like dusty silicates and soot-like mole- cules called polycyclic aromatic hy- drocarbons. In this image, blue represents light at wavelengths of 5.6, 7.7, and 10 microns; green is 11, 12, and 15 microns; and red is 18, 21, and 25 microns. [NASA, ESA, CSA, Karl Misselt (University of Arizona), Alain Abergel (IAS, CNRS)] surrounding the Horsehead have already dissipated, but the jutting pillar is made of thick clumps of ma- terial and therefore is harder to erode. Astronomers estimate that the Horsehead has about five mil- lion years left before it too disinte- grates. Webb’s new view focuses on the illuminated edge of the top of the nebula’s distinctive dust and gas structure. The Horsehead Nebula is a well- known photodissociation region, or PDR. In such a region, ultraviolet (UV) light from young, massive stars creates a mostly neutral, warm area of gas and dust between the fully ionized gas surrounding the mas- sive stars and the clouds in which they are born. This UV radiation strongly influences the chemistry of these regions and acts as a signifi- cant source of heat. These regions occur where interstel- lar gas is dense enough to remain mostly neutral, but not dense enough to prevent the penetration of UV light from massive stars. The light emitted from such PDRs pro- vides a unique tool to study the physical and chemical processes that drive the evolution of interstellar matter in our galaxy, and through- out the universe from the early era of vigorous star formation to the present day. Due to its proximity and its nearly edge-on geometry, the Horsehead Nebula is an ideal target for as- tronomers to study the physical structures of PDRs and the molecu- lar evolution of the gas and dust within their respective environ- ments, and the transition regions between them. It is considered one of the best regions in the sky to study how radiation interacts with interstellar matter. Thanks to Webb’s MIRI and NIRCam instruments, an international team of astronomers has revealed for the first time the small-scale structures of the illuminated edge of the Horsehead. As UV light evaporates the dust cloud, dust particles are swept out away from the cloud, car- ried with the heated gas. Webb has detected a network of thin features tracing this move- ment. The observations have also al- lowed astronomers to investigate how the dust blocks and emits light, and to better understand the multi- dimensional shape of the nebula. Next, astronomers intend to study the spectroscopic data that have been obtained to gain insights into the evolution of the physical and chemical properties of the material observed across the nebula. !