What’s hotter than the hottest stars in the Universe?

Stars are what illuminate the depths of space.

This wide-field image from the ESA’s Euclid mission centers on galaxy cluster Abell 2390, but shows a large number of foreground stars from our own Milky Way, extragalactic objects unassociated with the cluster, as well as the galaxy cluster itself. In all of these points of light, starlight is what causes this illumination.

Nearly all luminous radiation is starlight: emitted from plasma-rich stellar photospheres.

This graphic compares a Sun-like star with a red dwarf, a typical brown dwarf, an ultra-cool brown dwarf, and a planet like Jupiter. Only about 5% of all stars are like the Sun or more massive; K-type stars represent 15% of all stars, while red dwarfs represent 75-80% of all stars. Brown dwarfs, although they are failed stars, may be just as common as red dwarfs are, but are cooler and lower in mass.

Stars’ typical surface temperatures are no lower than ~2700 K.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. In terms of size, the smallest M-class stars are still about 12% the diameter of the Sun, but the largest main sequence stars can be dozens of times the Sun’s size, with evolved red supergiants (not shown) reaching hundreds or even 1000+ times the size of the Sun. A star’s (main sequence) lifetime, color, temperature, and luminosity are all primarily determined by a single property: mass.

The most massive main-sequence stars cap out with exterior temperatures of ~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 hottest young white dwarf surfaces reach ~150,000 K.

When red giants eject their outer layers, their central cores contract into white dwarfs, reaching surface temperatures of ~150,000 K.

The evolution of a solar-mass star on the Hertzsprung-Russell (color-magnitude) diagram from its pre-main-sequence phase to the end of fusion. Every star of every mass will follow a different curve, but the Sun is only a star once it begins hydrogen burning, and ceases to be a star once helium burning is completed. Stars on the upper-left of the diagram are more massive, hotter, and more luminous than our Sun, but are also the shortest-lived.

Highly evolved Wolf-Rayet stars have the hottest photospheres.

The extremely high-excitation nebula shown here is powered by an extremely rare binary star system: a Wolf-Rayet star orbiting an O-star. The stellar winds coming off of the central Wolf-Rayet member are between 10,000,000 and 1,000,000,000 times as powerful as our solar wind, and illuminated at a temperature of 120,000 degrees. (The green supernova remnant off-center is unrelated.) Systems like this are estimated, at most, to represent 0.00003% of the stars in the Universe but could lead to supernovae if the conditions are right.

Surrounded by ejecta and fusing heavy elements internally, they can achieve temperatures of ~210,000 K.

The Wolf-Rayet star WR 102 is the hottest star known, at 210,000 K. In this infrared composite from WISE and Spitzer, it’s barely visible, as almost all of its energy is in shorter-wavelength light than those instruments can detect. The blown-off, ionized hydrogen, however, stands out spectacularly, and reveals a series of shells to its structure.

But there are places in the Universe where even greater temperatures are attained.

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. Neutron star surfaces are at similar temperatures to white dwarf interiors: typically at several hundred thousand kelvin.

Young neutron star surfaces, like the Crab pulsar, radiate at ~600,000 K.

The Crab pulsar, like all pulsars, is an example of a neutron star corpse. The gas and matter surrounding it is quite common, and is capable of providing fuel for the pulsing behavior of these neutron stars. Matter-antimatter pairs, as well as high-energy particles, are produced in copious amounts by neutron stars: enough to explain the positrons that strike Earth from a variety of cosmic sources. The neutron star originally reached a temperature of ~1 trillion K, but even now, it’s already cooled to “only” about 600,000 K.

X-ray emitting gas clouds can surpass those temperatures: reaching up to perhaps 100,000,000 K.

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.

The interiors of stars and stellar remnants are often hotter still.

This cutaway showcases the various regions of the surface and interior of the Sun, including the core, which is the only location where nuclear fusion occurs. As time goes on and hydrogen is consumed, the helium-containing region in the core expands and the maximum temperature increases, causing the Sun’s energy output to increase. The balance between the inward-pulling gravity and the outward-pushing radiation pressure is what determines the size and stability of a star, while the core temperature determines the rate of fusion and which elements can fuse inside.

The hottest stellar cores can exceed 300,000,000 K, causing electron-positron pair production and photodisintegration effects.

This diagram illustrates the pair production process that astronomers once thought triggered the hypernova event known as SN 2006gy. At core temperatures cresting past 300,000,000 K, high-enough-energy photons are produced, which create electron/positron pairs, which can then cause a pressure drop and a runaway reaction that destroys the star. This event is known as a pair-instability supernova. Peak luminosities of a hypernova, also known as a superluminous supernova, are many times greater than that of any other, ‘normal’ supernova.

Neutron star interiors reach ~10¹² (one trillion) K: sufficiently hot to create quark-gluon plasmas.

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.

But supermassive black holes create the highest-energy phenomena of all.

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, Alcyoneus, like many active galaxies, also radiates in the high-energy portion of the spectrum as well.

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 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.

Only in the Big Bang itself were hotter conditions ever created.

In the earliest stages of the hot Big Bang, there were no bound structures that could form, only a “primordial soup” of matter particles, antimatter particles, and bosons like the photon. This hot, dense, and rapidly expanding state represents the most extreme conditions ever achieved in the Universe, but they were fleeting: the Universe quickly cools off.

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