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)

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 them all.

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.

Deep inside the Sun’s core, where temperatures rise above ~4 million K, nuclear fusion occurs between subatomic particles. This produces photons, particles and antiparticles, and neutrinos, the last of which carries a little more than 1% of the Sun’s total energy output away. (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. (Credit: NASA/SDO)