Nebula
A nebula is a giant cloud of gas (primarily hydrogen and helium) and dust in space—part of the interstellar medium. Nebulae serve as both stellar nurseries and the aftermath of dying stars, making them key players in the life cycle of stars.
There are several main types of nebulae:
Emission nebulae are ionized gas clouds that glow brightly from ultraviolet radiation emitted by nearby hot stars (H II regions), such as the Orion Nebula.
Reflection nebulae are dusty clouds that scatter light from nearby stars, often appearing blue—like NGC 1999.
Dark nebulae are dense dust clouds that block background starlight, seen as dark silhouettes (e.g., the Horsehead Nebula).
Planetary nebulae are shells of gas expelled by dying medium-mass stars, such as the Helix Nebula.
Supernova remnants are expanding nebulae formed from supernova explosions, such as the Cygnus Loop and Veil Nebula, rich in heavy elements.
Nebulae vary widely in size and shape: some span hundreds of light-years and showcase intricate structures like pillars and filaments, which have been imaged by Hubble and JWST.
Despite their vast size, they are very low density—sometimes just 0.1 atoms/cm³—yet they can contain enough mass to form thousands of stars. Gravity and external forces like shockwaves from nearby supernovae trigger regions to collapse and form new stars.
Nebulae enrich galactic chemistry: elements forged in stars are returned to the interstellar medium, fueling future star and planet formation.
They are studied across the electromagnetic spectrum—from radio waves to gamma rays—using observatories like Hubble, JWST, Chandra, and Earth-based telescopes. Their spectacular beauty and scientific significance make them central to both research and public engagement.
Sources
APODs including "Nebula"
R3 PanSTARRS: An Orion Comet
15 May 2026
Comet R3 PanSTARRS might be best remembered as an Orion comet. A key reason is because Comet C/2025 R3 (PanSTARRS) was near its most spectacular -- in terms of tail visibility -- when passing in front of the iconic constellation. Although rare, other bright comets, too, have ventured across Orion, including Lovejoy in 2015, Hale-Bopp in 1997, and the Great Comet of 1264. Best visible in long duration exposures, the featured image was captured last week from the Craigieburn Mountain Range in New Zealand. Visible in the deep background image are the Orion Nebula, Barnard's Loop, and through R3's tail, the bright star Saiph, the sixth brightest star in the constellation of Orion. Comet R3 PanSTARRS continues to fade as it moves further south, passing into the constellation of the Unicorn (Monoceros) in the next few days. Sky Surprise: What picture did APOD feature on your birthday? (after 1995)