From the hottest to the coldest places in the Universe

The visible Universe is full of temperature extremes.

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 found in massive galaxy clusters. This picture, alone, illustrates temperatures ranging from ~10 K to as high as several millions of K.

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

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. Early on, the highest temperature conditions of all-time were achieved; in the far future, everything will eventually cool off toward absolute zero.

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

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.

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

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.

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

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 generally thought that active, supermassive black holes are the generators of these highest-energy cosmic rays, which can correspond to reaching temperatures of 10^22-10^24 K.

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

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.

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

The Sun, when it becomes a red giant, will become similar inside 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 carbon-and-heavier-element fusion occurs, temperatures can exceed several hundred million K.

The hottest gas/plasma clouds reach several million degrees.

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.

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

This computer simulation of a neutron star shows charged particles being whipped around by a neutron star’s extraordinarily strong electric and magnetic fields. The fastest-spinning neutron star we’ve ever discovered is a pulsar that revolves 766 times per second: faster than our Sun would spin if we collapsed it down to the size of a neutron star. Irrespective of their spin rates, neutron stars may be the densest physical objects nature can create without progressing to create a singularity, and typically have surface temperatures of several hundreds of thousands of degrees.

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

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.

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.

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

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.

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

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.

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

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.

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

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.

But rapidly expanding gases achieve the coldest natural temperatures.

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. Although similar in many ways to the Boomerang Nebula, it is at a significantly higher temperature at the moment, although it may cool off further as the outer gas layers expand over the next few thousand years.

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

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.

Today, only laboratory experiments achieve colder conditions.

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.

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