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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.
wolf rayet wr 31a
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, many suspect that this star will explode in a supernova much like WR 124, enriching the surrounding interstellar medium with new, heavy elements. It cannot be predicted which evolved, massive star in our galaxy will be the Milky Way's next supernova.
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 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.
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. This reaction set occurs in the interiors of all young, hydrogen-rich stars, regardless of mass.
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, and demonstrates that it was created in the process of stellar nucleosynthesis.
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 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

Helium fusion then begins, injecting even more energy.

As the Sun becomes a true red giant, expanding to over 100 times its current size as its interior contracts and heats up to fuse helium, the Earth itself may be swallowed or engulfed, but will definitely be roasted as never before. The Sun’s outer layers will swell, but the exact details of its evolution, and how those changes will affect the orbits of the planets, still have large uncertainties in them. Mercury and Venus will definitely be swallowed by the Sun, but Earth will be very close to the border of survival/engulfment.
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. Stars on the upper-left of the diagram are more massive, hotter, and more luminous than our Sun, but are also the shortest-lived.
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 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)

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 surrounding nebula M1-67, as imaged by Hubble, both owe their origin to the same originally massive star that blew off its hydrogen-rich 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. Could a star like this, rather than Betelgeuse, be our galaxy’s next naked-eye supernova? Only time will tell.
Credit: ESA/Hubble & NASA; Acknowledgement: Judy Schmidt (geckzilla.com)

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. The fate of this star: supernova, white dwarf, or a direct collapse black hole, is not yet determined.
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 but could lead to supernovae if the conditions are right.
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, and reveals a series of shells to its structure.
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 pulsar wind nebula exhibits spectacular details that can only be revealed, at present, with the power of NASA’s Chandra X-ray observatory.
Credit: NASA/CXC/CfA/P. Slane et al.

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

The remnant of SN 1987A, located in the Large Magellanic Cloud some 165,000 light years away. It was the closest observed supernova to Earth in more than three centuries, and reached a maximum magnitude of +2.8, clearly visible to the naked eye and significantly brighter than the host galaxy containing it.
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 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

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)

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


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