LOne of the most famous structures in the earth’s sky has just been revealed in a completely new light.
Mid- and near-infrared observations from the James Webb Space Telescope (JWST/Webb) have revealed previously unseen features in the space cloud known as the Horsehead Nebula. In a stunningly detailed image, the space telescope focuses on the region atop the “horse’s” head, capturing tendrils and filaments in spectacular resolution.
Header image: At the bottom of this Webb/NIRCam image, a small part of the Horsehead Nebula is visible up close, as a curved wall of thick, smoldering gas and dust. Above the nebula, several distant stars and galaxies are visible up to the top of the image. (NASA/ CSA/ ESA/ Webb/ K. Misselt, University of Arizona/ A. Abergel, IAS, Université Paris-Saclay & CNRS)
Using a combination of 23 filters, a team of astronomers achieved such spectacular resolution that they could track the emission of grains smaller than 20 nanometers, including interstellar polycyclic aromatic hydrocarbon particles, as well as light diffused by larger grains and by ionized hydrogen present in the cloud.
The Horsehead Nebula is a distinct cloud some 1,300 light years away, part of the Orion Molecular Cloud. This cloud is covered in dust and gas, so much so that it is as dark as shadows in optical light. In many photos, the cloud appears as a hole in the glowing gas around it.
The region of the nebula imaged by the JWST. (ESA/Webb, NASA, CSA, K. Misselt/University of Arizona, and A. Abergel/IAS/Université Paris-Saclay/CNRS, Mahdi Zamani The Euclid Consortium, Hubble Heritage Project/STScI AURA)
If one zooms in closer or observes the nebula in wavelengths outside the range of normal human vision, the appearance of the nebula changes from dark void to a glowing cloud and swelling. The Horsehead Nebula has no internal light Source, but it is heated by a nearby complex called Sigma Orionis, a system of very young, large and hot stars that shine at temperatures of around 34,600 kelvins.
These features make the Horsehead Nebula an excellent laboratory for understanding stellar nurseries. The “horsehead” itself is a dense clump of material that has collapsed under the influence of gravity and contains small forming stars, hidden by the dust itself.
But the intense radiation from stars outside the nebula has a devastating effect on the matter around them. Far-ultraviolet light causes a process called photodissociation, whereby molecules break apart under the powerful rays, creating an essentially neutral interstellar medium field. Thus, around the Horsehead Nebula there is what is called a photodissociation region (PDR), which the JWST images will make it possible to study.
The new observations may also help understand the process of photo-evaporation, by which gas is ionized by powerful light and evaporates.
The images so far have allowed a team of scientists to highlight the small-scale structures that adorn the lit edge of the Horsehead Nebula, as well as a network of filaments perpendicular to the front of the PDR. This network contains dust and gas which constitute part of the photoevaporative flux.
But this is only the beginning. The next step will be a thorough analysis of the emitted light to determine the chemical composition of the dust and gas, as well as the size and “flow” of the dust grains depending on how the light is scattered. This will establish a detailed model of the evolution of dust in the PDR and help researchers understand how these clouds transform and evaporate, releasing the nascent stars trapped there.
The first study has been accepted for publication in the journal Astronomy & Astrophysics, and is available as a pre-publication in arXiv: JWST observations of the Horsehead photon-dominated region I. First results from multi-band near- and mid-infrared imaging and presented on the ESA website: Horsehead Nebula (NIRCam image) and the Webb Telescope: Horsehead Nebula (Euclid, Hubble and Webb images).
