Outer space

What happens when the Universe turns up the heat?

On average, today’s Universe is an extremely cold place.

universe temperature

At any epoch in our cosmic history, any observer will experience a uniform “bath” of omnidirectional radiation that originated back at the Big Bang. Today, from our perspective, it’s just 2.725 K above absolute zero, and hence is observed as the cosmic microwave background, peaking in microwave frequencies. At great cosmic distances, as we look back in time, that temperature was hotter dependent on the redshift of the observed, distant object.

(Credit: Earth: NASA / BlueEarth; Milky Way: ESO / S. Brunier; CMB: NASA / WMAP)

In intergalactic space, only the Big Bang’s leftover glow substantially heats up matter.

Visible (left) and infrared (right) views of the dust-rich Bok globule, Barnard 68. It has a temperature of less than 20 K, which is why it remains invisible in both visible and near-infrared light, but it is still quite warm when compared with the temperatures of the cosmic microwave background.

(Credit: ESO)

At 2.725 K above absolute zero, only places that actively cool themselves are colder.

coldest place in the universe

A color-coded image of the Boomerang Nebula, as taken by the Hubble Space Telescope. The gas expelled from this star has expanded incredibly rapidly, causing it to cool adiabatically. There are places within it that are colder than even the leftover glow from the Big Bang itself.

(Credit: NASA, ESA and The Hubble Heritage Team (STScI / AURA))

However, numerous mechanisms heat up the Universe’s matter.

The image shows the central region of the Tarantula Nebula in the Large Magellanic Cloud. The young and dense star cluster R136 can be seen at the lower right of the image. The tidal forces exerted on the Large Magellanic Cloud by the Milky Way are triggering a wave of star-formation in there, resulting in hundreds of thousands of new stars. The injection of energy into the Universe provided by star-formation is one of the primary sources that heat up the matter within galactic environments.

(Credit: NASA, ESA, and P. Crowther (University of Sheffield))

Stars, for example, produce radiation that strikes nearby gas and dust.

This far-infrared image of Messier 16, the Eagle Nebula, showcases a variety of neutral atoms heated to between 10 K (red) to 40 K (blue) by the stars that have already formed inside. Below the center of the image, the famed Pillars of Creation can be seen in far-infrared light, a unique view of this object thanks to the still unmatched capabilities of ESA’s Herschel observatory.

(Credit: ESA / Herschel / PACS / SPIRE / Hill, Motte, HOBYS Key Program Consortium)

Heated to tens of degrees above absolute zero, it radiates throughout the far-infrared.

The famed Pillars of Creation inside the Eagle Nebula are a location where new stars are forming in a race against the evaporating gas. In the visible light view, at left, the new stars are largely obscured, while infrared light allows us to peer through the dust to the newly formed stars and proto-stars inside. Still cooler gas will radiate at longer wavelengths.

(Credit: NASA, ESA, and the Hubble Heritage Team (STScI / AURA))

Closer to a newly formed star, the radiation carves out protoplanetary structures.

A sample of 20 protoplanetary disks around young, infant stars, as measured by the Disk Substructures at High Angular Resolution Project: DSHARP. Observations such as these taught us that protoplanetary disks form primarily in a single plane, and tend to support the core accretion scenario of planet formation. The disk structures are seen in both infrared and millimeter / sub-millimeter wavelengths.

(Credit: SM Andrews et al., ApJL, 2018)

Heated to hundreds of degrees, these protoplanetary disks radiate throughout the infrared.

The star forming region Sh 2-106 showcases an interesting set of phenomena, including illuminated gas, a bright central star that provides that illumination, and blue reflections off of gas that has yet to be blown away. The various stars in this region likely come from a combination of stars of many different pasts and generational histories, but none of them are pristine: they all contain significant quantities of heavy elements in them.

(Credit: ESA / Hubble and NASA)

Higher-energy phenomena, however, can lead to spectacular astronomical consequences.

This image showcases the open star cluster NGC 290, as imaged by Hubble. These stars, imaged here, show a variety of colors because they are at different temperatures, and so the hotter stars emit more blue-than-red light while the cooler ones emit more red-than-blue. Different colors can only be revealed by imaging stars in multiple different wavelengths, but it’s the bluest, hottest, most luminous stars that primarily drive heating and ionization of the surrounding matter.

(Credit: ESA & NASA; Acknowledgment: David de Martin (ESA / Hubble) and Edward W. Olszewski (University of Arizona))

The hottest, most massive young stars glow brilliantly in ultraviolet light.

Most galaxies contain only a few regions of star-formation: where gas is collapsing, new stars are forming, and ionized hydrogen is found in a bubble surrounding that region. In a starburst galaxy, pretty much the entire galaxy itself is a star-forming region, with M82, the Cigar Galaxy, being the closest one with those properties. The radiation from hot, young stars ionizes a variety of atomic and molecular gases, creating emission signatures that can be revealed visually with the right astronomical filters.

(Credits: NASA, ESA and the Hubble Heritage Team (STScI / AURA); Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (National Science Foundation))

The radiation heats gas to thousands of degrees, ionizing numerous atoms and molecules.

planetary nebula

When the central star in a dying stellar system heats up to about temperatures of ~ 30,000 K, it becomes hot enough to ionize the previously ejected material, creating a true planetary nebula in the case of a Sun-like star. Here, NGC 7027 has just recently crossed that threshold, and is still rapidly expanding. At just ~ 0.1-to-0.2 light-years across, it is one of the smallest and youngest planetary nebulae known.

(Credit: NASA, ESA, and J. Kastner (RIT))

When electrons cascade down their energy levels, they give off a variety of emission signatures.

The Large Magellanic Cloud is home to the closest supernova of the last century. The pink regions here are not artificial, but are signals of ionized hydrogen and active star formation, likely triggered by gravitational interactions and tidal forces. The pink regions specifically arise when electrons fall back onto ionized hydrogen nuclei, and transition from the n = 3 to the n = 2 energy level, producing photons of precisely 656.3 nm.

(Credit: Jesús Peláez Aguado)

At a few thousand degrees, hydrogen ionizes, turning nebulae pink with emission lines.

Around a variety of stellar corpses and dying stars, doubly-ionized oxygen atoms produce a characteristic green glow, as electrons cascade down the various energy levels when heated to temperatures exceeding ~ 50,000 K. Here, the planetary nebula IC 1295 shines brilliantly.

(Credit: ESO)

Above ~ 50,000 K, around dying stars, doubly ionized oxygen glows an eerie green.


This image from NASA’s Chandra X-ray Observatory shows the location of various elements in the Cassiopeia A supernova remnant including silicon (red), sulfur (yellow), calcium (green) and iron (purple). Each of these elements produces X-rays within narrow energy ranges, allowing maps of their location to be created.

(Credit: NASA / CXC / SAO)

Colliding galaxies heat gas further, leading to X-ray emissions.

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. The X-rays come in two varieties, soft (lower-energy) and hard (higher-energy), where galaxy collisions can create temperatures exceeding several hundreds of thousands of degrees.

(Credit: NASA, ESA, D. Harvey (auscole Polytechnique Fédérale de Lausanne, Switzerland; University of Edinburgh, UK), R. Massey (Durham University, UK), T. Kitching (University College London, UK), and A. Taylor and E. Tittley (University of Edinburgh, UK))

But radiating neutron stars and black holes can shape entire galaxies.


The radio features shown here, in orange, highlight the giant radio galaxy Alcyoneus, as well as the central black hole, its jets, and the lobes at either end. This feature is the largest known in the Universe to correspond to a single galaxy, and makes Alcyoneus the largest known galaxy in the Universe at present. Although only radio and infrared features are shown here, it radiates in the high-energy portion of the spectrum as well.

(Credit: MSSL Oei et al., Astronomy & Astrophysics, 2022)

Producing gamma-ray photons, the highest-energy there are, even the Large Hadron Collider can not compete.

Fermi’s view of the gamma-ray sky reveals the emission from our own galaxy, from extragalactic objects, from pulsars, and, as highlighted here, from supernova remnants as well.

(Credit: NASA / DOE / Fermi LAT Collaboration)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

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