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Starts With A Bang

How hot are the hottest stars in the Universe?

At their cores, stars can reach many millions or even billions of degrees. But even that doesn't touch the hottest of all.
This Wolf–Rayet star is known as WR 31a, located about 30,000 light-years away in the constellation of Carina. The outer nebula is expelled hydrogen and helium, while the central star burns at over 100,000 K. In the relatively near future, this star will explode in a supernova, enriching the surrounding interstellar medium with new, heavy elements.
(Credit: ESA/Hubble & NASA; Acknowledgement: Judy Schmidt)
Key Takeaways
  • If you were looking for the hottest stars, you might think to look to the brightest, most massive, most luminous stars of all.
  • Sure, it turns out they're hot: much hotter than stars like the Sun, from their cores to the edges of their photospheres.
  • But they're still not the hottest stars of all. Which ones are? The answer will thoroughly surprise you.

Surprise! The biggest, most massive stars aren’t always the hottest.

Although its neighbor, Messier 42, gets all the attention, Messier 43 lies just across a dust lane and continues the great nebula, illuminated largely by a single star that shines hundreds of thousands of times brighter than our own Sun. Located between 1000 and 1500 light-years away, this is part of the same molecular cloud complex as the main Orion Nebula.
(Credits: Yuri Beletsky (Carnegie Las Campanas Observatory) and Igor Chilingarian (Harvard-Smithsonian CfA))

To first become a star, your core must cross a critical temperature threshold: ~4,000,000 K.

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, the helium-rich core will contract and heat up, enabling the fusion of helium into carbon. However, additional nuclear states for a carbon-12 nucleus beyond the ground state is required for the necessary reactions to occur.
(Credit: Wikimedia Commons/KelvinSong)

Such temperatures are required to initiate core fusion of hydrogen into helium.

The most straightforward and lowest-energy version of the proton-proton chain, which produces helium-4 from initial hydrogen fuel. Note that only the fusion of deuterium and a proton produces helium from hydrogen; all other reactions either produce hydrogen or make helium from other isotopes of helium.
(Credit: Sarang/Wikimedia Commons)

However, the surrounding layers diffuse heat, capping photosphere temperatures at ~50,000 K.

Solar coronal loops, such as those observed by NASA’s Solar Dynamics Observatory (SDO) satellite here in 2014, follow the path of the magnetic field on the Sun. Although the Sun’s core may reach temperatures of ~15 million K, the edge of the photosphere hangs out at a relatively paltry ~5700 to ~6000 K, with cooler temperatures found toward the outermost regions of the photosphere and hotter temperatures found closer to the interior. Magnetohydrodynamics, or MHD, describes the interplay of the surface magnetic fields with interior processes in stars like the Sun.
(Credit: NASA/SDO)

Higher temperatures require additional evolutionary steps.

The prediction of the Hoyle State and the discovery of the triple-alpha process is perhaps the most stunningly successful use of anthropic reasoning in scientific history. This process is what explains the creation of the majority of carbon that’s found in our modern-day Universe.
(Credit: E. Siegel/Beyond the Galaxy)

Your star’s core contracts and heats up upon exhausting its hydrogen.

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.
(Credit: Sakurambo at English Wikipedia)

Helium fusion then begins, injecting even more energy.

As the Sun becomes a true red giant, the Earth itself may be swallowed or engulfed, but will definitely be roasted as never before. The Sun’s outer layers will swell to more than 100 times their present diameter, but the exact details of its evolution, and how those changes will affect the orbits of the planets, still have large uncertainties in them.
(Credit: Fsgregs/Wikimedia Commons)

However, “red giant” stars are quite cool, expanding to lower their surface temperatures.

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.
(Credit: szczureq/Wikimedia Commons)

Most red giants blow their outer layers away, revealing a heated, contracted core.

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 red giant that spawned it.
(Credit: Nordic Optical Telescope and Romano Corradi (Isaac Newton Group of Telescopes, Spain))

With white dwarf surfaces reaching ~150,000 K, they surpass even blue supergiants.

The largest group of newborn stars in our Local Group of galaxies, cluster R136, contains the most massive stars we’ve ever discovered: over 250 times the mass of our Sun for the largest. The brightest of the stars found here are more than 8,000,000 times as luminous as our Sun. And yet, these stars only achieve temperatures of up to ~50,000 K, with white dwarfs, Wolf-Rayet stars, and neutron stars all getting hotter.
(Credit: NASA, ESA, and P. Crowther (University of Sheffield))

The highest stellar temperatures, however, are achieved by Wolf-Rayet stars.

The Wolf-Rayet star WR 124 and the nebula M1-67, which surrounds it, both owe their origin to the same originally massive star that blew off its outer layers. The central star is now far hotter than what came before, as Wolf-Rayet stars typically have temperatures between 100,000 and 200,000 K, with some stars cresting even higher.
(Credit: ESA/Hubble & NASA; Acknowledgement: Judy Schmidt (

Destined for cataclysmic supernovae, Wolf-Rayet stars are fusing the heaviest elements.

Imaged in the same colors that Hubble’s narrowband photography would reveal, this image shows NGC 6888: the Crescent Nebula. Also known as Caldwell 27 and Sharpless 105, this is an emission nebula in the Cygnus constellation, formed by a fast stellar wind from a single Wolf-Rayet star.
(Credit: J-P Metsävainio (Astro Anarchy))

They’re highly evolved, luminous, and surrounded by ejecta.

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.
(Credit: ESO)

The hottest one measures ~210,000 K; the hottest “true” star.

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. The blown-off, ionized hydrogen, however, stands out spectacularly.
(Credit: Judy Schmidt; data from WISE, Spitzer/MIPS1 and IRAC4)

The remnant cores of supernovae can form neutron stars: the hottest objects of all.

A small, dense object only twelve miles in diameter is responsible for this X-ray nebula that spans ~150 light-years. This pulsar is spinning around almost 7 times a second and has a magnetic field at its surface estimated to be 15 trillion times stronger than the Earth’s magnetic field. This combination of rapid rotation and ultra-strong magnetic field drives an energetic wind of electrons and ions, ultimately creating the elaborate nebula seen by NASA’s Chandra.
(Credit: NASA/CXC/CfA/P. Slane et al.)

With initial interior temperatures cresting ~1 trillion K, they radiate heat quickly.

The remnant of supernova 1987a, located in the Large Magellanic Cloud some 165,000 light years away, is revealed in this Hubble image. It was the closest observed supernova to Earth in more than three centuries, and has the hottest known object, at its surface, currently known in the Milky Way. It’s surface temperature now is estimated at around ~600,000 K.
(Credit: ESA/Hubble & NASA)

After mere years, their surfaces cool to ~600,000 K.

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 care in the surrounding matter. The central bright purplish-white spot is, indeed, the Crab pulsar, which itself spins at about 30 times per second. A next-generation X-ray observatory could reveal features, across new energies and at superior resolutions, that we can only dream about today.
(Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA-JPL-Caltech)

Despite all we’ve discovered, neutron stars remain the hottest and densest singularity-free objects known.

The two best-fit models of the map of the neutron star J0030+0451, constructed by the two independent teams who used the NICER data, show that either two or three ‘hot spots’ can be fitted to the data, but that the legacy idea of a simple, bipolar field cannot accommodate what NICER has seen. This neutron star measures just ~12 km across, and are both the densest non-singular objects in the Universe and also the hottest at their surfaces.
(Credit: NASA, NICER, GSFC’s CI Lab)

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