On average, today’s Universe is an extremely cold place.
In intergalactic space, only the Big Bang’s leftover glow substantially heats up matter.
At 2.725 K above absolute zero, only places that actively cool themselves are colder.
However, numerous mechanisms heat up the Universe’s matter.
Stars, for example, produce radiation that strikes nearby gas and dust.
Heated to tens of degrees above absolute zero, it radiates throughout the far-infrared.
Closer to a newly forming star, the radiation carves out protoplanetary structures.
Heated to hundreds of degrees, these protoplanetary disks radiate throughout the infrared.
Higher-energy phenomena, however, can lead to spectacular astronomical consequences.
The hottest, most massive young stars glow brilliantly in ultraviolet light.
The radiation heats gas to thousands of degrees, ionizing numerous atoms and molecules.
When electrons cascade down their energy levels, they give off a variety of emission signatures.
At a few thousand degrees, hydrogen ionizes, turning nebulae pink with emission lines.
Above ~50,000 K, around dying stars, doubly ionized oxygen glows an eerie green.
Colliding galaxies heat gas further, leading to X-ray emissions.
But radiating neutron stars and black holes can shape entire galaxies.
Producing gamma-ray photons, the highest-energy there are, even the Large Hadron Collider can’t compete.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.