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Starts With A Bang

The hottest stars in the Universe are all missing one key ingredient

Want to be hotter? Add more mass. Want to go even hotter than that? Lose almost all of it.

“A candidate is not going to suddenly change once they get into office. Just the opposite, in fact. Because the minute that individual takes that oath, they are under the hottest, harshest light there is. And there is no way to hide who they really are.” –Michelle Obama

In astronomy, there’s a simple formula for stars: add more mass, and your star becomes brighter, bluer, and hotter.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. The overwhelming majority (75%) of stars today are M-class stars, with only 1-in-800 being massive enough for a supernova. Yet as hot as O-stars get, they’re not the hottest stars in the entire Universe. Image credit: Wikimedia Commons user LucasVB, additions by E. Siegel.

This pattern hold from stars just a few percent the mass of the Sun to over 200 times as massive.

The giant star-forming region 30 doradus in the gas-rich Tarantula nebula. The most massive stars known to humanity can be found in the central cluster highlighted at right, with R136a1 coming in at ~260 solar masses. Image credit: ESO/P. Crowther/C.J. Evans.

But there’s a limit to the temperature these stars achieve, even the most massive ones.

O-class stars are the hottest main-sequence stars, but by expelling their outer hydrogen layers, as this illustration shows, they can achieve even greater temperatures. The star illustrated here, WR 122, is the first Wolf-Rayet star to be found with a disk. Image credit: NASA, ESA, and G. Bacon (STScI); Science Credit: NASA, ESA, and J. Mauerhan.

If you want to go hotter, you need something extra: to lose your hydrogen.

The Crescent Nebula in Cygnus is powered by the central massive star, WR 136, where the hydrogen expelled during the red giant phase is shocked into a visible bubble by the hot star at the center. Image credit: Wikimedia Commons user Hewholooks.

As the most massive stars evolve, they burn through their core’s fuel, expanding into a red giant and fusing helium.

The anatomy of a very massive star throughout its life, culminating in a Type II Supernova when the core runs out of nuclear fuel. This is how fusion works if a star holds onto its outer, hydrogen envelope, but a small percentage of massive stars do not, becoming Wolf-Rayet stars. Image credit: Nicole Rager Fuller/NSF.

Normally, this progresses into even heavier elements: carbon fusion, then oxygen, and so on.

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. Image credit: ESA/Hubble & NASA; Acknowledgement: Judy Schmidt (

But in a special stellar class — Wolf-Rayet — the outer hydrogen layers get blown off, leaving only heavier elements behind.

The unusual hot massive young star WR 22 is silhouetted against a portion of the Carina nebula here, and exhibits signs of highly, multiply ionized heavy elements like carbon and nitrogen. Image credit: ESO.

With strongly, multiply ionized atoms of carbon, nitrogen, and oxygen in their atmospheres, these stars are the hottest known.

The extremely high-excitation nebula shown here is powered by a 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. Image credit: ESO.

Largely at only be 10-to-20 times the Sun’s mass, they burn at up to 200,000 K, emitting hundreds of thousands of times the light of the Sun.

The nova of the star GK Persei, shown here in an X-ray (blue), radio (pink), and optical (yellow) composite, contains Wolf-Rayet elements in its spectrum, indicating that perhaps it had a Wolf-Rayet progenitor. Image credit: X-ray: NASA/CXC/RIKEN/D.Takei et al; Optical: NASA/STScI; Radio: NRAO/VLA.

Only a few of them are visible to the naked eye, as most of this energetic radiation is ultraviolet, not visible.

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. Image credit: Judy Schmidt, based on data from WISE and Spitzer/MIPS1 and IRAC4.

Only ~1,000 Wolf-Rayet stars populate the entire Local Group.

Mostly Mute Monday tells the story of an astronomical objects, class, or phenomenon in images, visuals, and no more than 200 words.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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