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Starts With A Bang

From the hottest to the coldest places in the Universe

We can’t go back to the Big Bang, nor ahead to the heat death of the Universe. Nevertheless, here are today’s natural temperature extremes.
crab pulsar remnant
A combination of X-ray, optical, and infrared data reveal the central pulsar at the core of the Crab Nebula, including the winds and outflows that the pulsars carry in the surrounding matter. The central bright purplish-white spot is, indeed, the Crab pulsar, which itself spins at about 30 times per second. The material shown here spans about 5 light-years in extent, originating from a star that went supernova about 1,000 years ago, teaching us that the typical speed of the ejecta is around 1,500 km/s. The neutron star originally reached a temperature of ~1 trillion K, but even now, it's already cooled to "only" about 600,000 K.
Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA-JPL-Caltech
Key Takeaways
  • Although the start of the hot Big Bang was the hottest the Universe ever achieved, some things, in our late-time Universe, still get extremely hot.
  • Similarly, although the far future of the Universe will see everything approach absolute zero, nothing has gotten there yet, and “cold things” aren’t completely heat-free.
  • From the hottest places to the coldest places in today’s modern Universe, here are the extremes, along with everything in between.

The visible Universe is full of temperature extremes.

centaurus A x-ray
The galaxy Centaurus A is the closest example of an active galaxy to Earth, with its high-energy jets caused by electromagnetic acceleration around the central black hole. The extent of its jets are far smaller than the jets that Chandra has observed around Pictor A, which themselves are much smaller than the jets of Alcyoneus, which are still smaller than jets found in massive galaxy clusters. This picture, alone, illustrates temperatures ranging from ~10 K to as high as several millions of K, and relativistic jets that are even physically larger than the stellar extent of the galaxy itself.
Credit: X-ray: NASA/CXC/CfA/R.Kraft et al Radio: NSF/VLA/Univ. of Hertfordshire/M.Hardcastle et al. Optical: ESO/VLT/ISAAC/M.Rejkuba et al.

It’s true: the past was hotter and the future will be colder.

space expanding
A visual history of the expanding Universe includes the hot, dense state known as the Big Bang and the growth and formation of structure subsequently. The full suite of data, including the observations of the light elements and the cosmic microwave background, leaves only the Big Bang as a valid explanation for all we see. As the Universe expands, it also cools, enabling ions, neutral atoms, and eventually molecules, gas clouds, stars, and finally galaxies to form. Without the Higgs giving mass to the particles in the Universe at a very early, hot stage, none of this would have been possible.
Credit: NASA/CXC/M. Weiss

But even today, incredibly hot and cold extremes are ubiquitous.

active supermassive black hole quasar
This illustration of a radio-loud quasar that is embedded within a star-forming galaxy gives a close-up look of how giant radio galaxies are expected to emerge. At the center of an active galaxy with a supermassive black hole, jets are emitted that slam into the larger galactic halo, energizing the gas and plasma and causing radio emissions in the form of jets close by the black hole, and then plumes and/or lobes farther away. Both supermassive and stellar-mass black holes have overwhelming evidence supporting their existence, but supermassive black holes may heat matter to the highest temperatures of all, accelerating particles to even beyond the GZK cutoff set by particle physics.
Credit: ESA/C. Carreau

The hottest environments exist around natural particle accelerators: supermassive black holes.

Alcyoneus
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: M.S.S.L. Oei et al., Astronomy & Astrophysics, 2022

When active, their accelerated particles maximally achieve ~1020 eV energies, implying ~1024 K temperatures.

cosmic rays
These graphs show the spectrum of cosmic rays as a function of energy from the Pierre Auger Observatory. You can clearly see that the function is more-or-less smooth until an energy of ~5 x 10^19 eV, corresponding to the GZK cutoff. Above that, particles still exist, but are less abundant, likely due to their nature as heavier atomic nuclei. It is plausible that active, supermassive black holes are the generators of these highest-energy cosmic rays, but identifying individual cosmic ray sources with known supermassive black holes does not lead to a very compelling correlation.
Credit: Pierre Auger Collaboration, Phys. Rev. Lett., 2020

Neutron star interiors come next, where quark-gluon plasmas peak at T ~ 1012 K.

neutron star interiors
A white dwarf, a neutron star, or even a strange quark star are all still made of fermions. The Pauli degeneracy pressure helps hold up the stellar remnant against gravitational collapse, preventing a black hole from forming. Inside the most massive neutron stars, an exotic form of matter, a quark-gluon plasma, is thought to exist, with temperatures rising up to ~1 trillion (10^12) K inside.
Credit: NASA/CXC/M.Weiss

The centers of massive stars reach 108-109 K, necessary to fuse heavy elements.

supergiant star
The Sun, when it becomes a red giant, will become similar in size to Arcturus. Antares is more of a supergiant star and is much larger than our Sun (or any Sun-like stars) will ever become. Even though red giants put out far more energy than our Sun, they are cooler and radiate at a lower temperature at their surfaces. Inside their cores, where helium fusion occurs, temperatures can rise into the tens of millions of K.
Credit: Sakurambo/SkateBiker at English Wikipedia

The hottest gas/plasma clouds reach several million degrees.

ophiuchus x-ray largest explosion cavity
Evidence for the biggest explosion seen in the Universe comes from a combination of X-ray data from Chandra and XMM-Newton. The eruption is generated by a black hole located in the cluster’s central galaxy, which has blasted out jets and carved a large cavity in the surrounding hot gas. Researchers estimate this explosion released five times more energy than the previous record holder and hundreds of thousands of times more than a typical galaxy cluster. The X-ray emitting gas can reach temperatures ranging from millions up to even ~100 million K.
Credit: X-ray: Chandra: NASA/CXC/NRL/S. Giacintucci, et al., XMM-Newton: ESA/XMM-Newton; Radio: NCRA/TIFR/GMRT; Infrared: 2MASS/UMass/IPAC-Caltech/NASA/NSF

Neutron star surfaces and white dwarf interiors are slightly cooler: from 105-106 K.

neutron star magnetic field
This computer simulation of a neutron star shows charged particles being whipped around by a neutron star’s extraordinarily strong electric and magnetic fields. It is possible that a neutron star has formed within the remnant of SN 1987A, but the region is still too dusty and gas-rich for the “pulses” to seep out.
Credit: NASA’s Goddard Space Flight Center

Next, giant planet interiors and white dwarf surfaces measure 8,000-50,000 K.

planetary nebula
When our Sun runs out of fuel, it will become a red giant, followed by a planetary nebula with a white dwarf at the center. The Cat’s Eye Nebula is a visually spectacular example of this potential fate, with the intricate, layered, asymmetrical shape of this particular one suggesting a binary companion. At the center, a young white dwarf heats up as it contracts, reaching temperatures tens of thousands of Kelvin hotter than the surface of the red giant that spawned it. The outer shells of gas are mostly hydrogen, which gets returned to the interstellar medium at the end of a Sun-like star’s life.
Credit: Nordic Optical Telescope and Romano Corradi (Isaac Newton Group of Telescopes, Spain)

Stellar surfaces are relatively cooler: 2700 K and up.

This illustration shows some of the largest stars in the Universe, along with the orbits of Saturn (brown ellipse) and Neptune (blue ellipse) for comparison. The stars, from left to right, are the largest blue hypergiant, yellow hypergiant, orange hypergiant, and then the largest two stars of all: the red hypergiants UY Scuti and Stephenson 2-18. The largest stars are approximately 2,000 times the diameter of our Sun, but the temperatures at the surfaces of these stars range from only a few thousand K all the way up to Wolf-Rayet stars, with temperatures of ~200,000 K.
Credit: SkyFlubbler/Wikimedia Commons

Brown dwarfs and hot planets achieve ~500-2000+ K.

largest planet
An artist’s illustration of a world that would be classified as a rocky super-Earth. If you’re hot enough to boil off the atmosphere of a large planet, you can wind up with a rocky super-Earth: a stripped planetary core. The temperatures will be so high that you’ll roast your planet. If you’re more than about 30% larger in radius than Earth and aren’t too close to your parent star, you’ll collect a large envelope of volatile gases, and be more like Neptune than Earth.
Credit: ESA/ATG medialab

Planetary bodies range from thousands down to tens of degrees, determined by their orbital distances.

solar system worlds
By size, it’s clear that the gas giant worlds vastly outstrip any of the terrestrial planets. In terms of temperature, the distance from the parent star is the overwhelming factor for a planet’s temperature so long as it doesn’t produce much of its own internal heat. In our Solar System, a Pluto-like object is at ~40 K, while Venus is the hottest planet at ~700+ K.
Credit: CactiStaccingCrane/Wikimedia Commons

In interstellar space, temperatures are merely 10-30 K.

eagle nebula stars
The Eagle Nebula, famed for its ongoing star formation, contains a large number of Bok globules, or dark nebulae, which have not yet evaporated and are working to collapse and form new stars before they disappear entirely. These cold, dark places in space, particularly when no star-formation has occurred inside of them, can frequently achieve temperatures ranging from 10-30 K, making them some of the coldest locations inside a galaxy.
Credit: ESA/Hubble and NASA

Deep, intergalactic space achieves 2.725 K: heated only by the CMB.

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. As each new year passes, the CMB cools down further by about 0.2 nanokelvin, and in several billion years, will become so redshifted that it will possess radio, rather than microwave, frequencies.
Credit: Earth: NASA/BlueEarth; Milky Way: ESO/S. Brunier; CMB: NASA/WMAP

But rapidly expanding gases achieve the coldest natural temperatures.

planetary nebula
The Egg Nebula, as imaged here by Hubble, is a preplanetary nebula, as its outer layers have not yet been heated to sufficient temperatures by the central, contracting star to become fully ionized. Many of the giant stars visible today will evolve into a nebula like this before shedding their outer layers completely and dying in a white dwarf/planetary nebula combination. As the central star loses mass, the outermost objects in that stellar system, such as the analogue of our Oort cloud and Kuiper belt, become ejected.
Credit: NASA and the Hubble Heritage Team (STScI/AURA), Hubble Space Telescope/ACS

Preplanetary nebulae, like the Boomerang Nebula, achieve temperatures from 0.5-1.0 K.

coldest place in the Universe
A color-coded temperature map of the Boomerang Nebula and the areas around it. The blue areas, which have expanded the most, are the coolest and lowest in temperature, and some locations around the Boomerang Nebula range from 0.5-1.0 K: the coldest natural temperatures ever seen.
Credit: ESO/NTT telescope; R. Sahai (JPL)/L. Nyman (ESO)

Today, only laboratory experiments achieve colder conditions.

axion admx experiment
This photograph shows the ADMX detector being extracted from the surrounding apparatus that creates a large magnetic field to induce axion-photon conversions. The mist is a result of the cryogenically cooled insert interfacing with the warm, humid air. Laboratory experiments can achieve ~nanokelvin or even ~picokelvin temperatures: far colder than anything found in the natural Universe.
Credit: Rakshya Khatiwada, University of Washington

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


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