What’s hotter than the hottest stars in the Universe? | by Ethan Siegel | Starts With A Bang! | Jan, 2025

MarylandDigitalNews.comSCIENCE

posted on Jan. 20, 2025 at 12:21 pm

The galaxy Centaurus A is the closest example of an active galaxy to Earth, with its high-energy jets caused by electromagnetic acceleration around the central black hole. The extent of its jets are far smaller than the jets that Chandra has observed around Pictor A, which themselves are much smaller than the jets of Alcyoneus, which are still smaller than jets found in the newly discovered Porphyrion. This picture, alone, illustrates temperatures ranging from ~10 K to as high as several millions of K: hotter than the surfaces of the hottest stars. (Credit: X-ray: NASA/CXC/CfA/R.Kraft et al Radio: NSF/VLA/Univ. of Hertfordshire/M.Hardcastle et al. Optical: ESO/VLT/ISAAC/M.Rejkuba et al.)

Here in our Universe, stars shine brightly, providing light and heat to planets, moons, and more. But some objects get even hotter, by far.

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. (Credit: ESA/Euclid/Euclid Consortium/NASA; Processing: J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi)

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. (Credit: MPIA/V. Joergens)

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. (Credit: LucasVB/Wikimedia Commons; Annotations: E. Siegel)

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. (Credit: Nordic Optical Telescope and Romano Corradi (Isaac Newton Group of Telescopes, Spain))

MarylandDigitalNews.comJanuary 20, 2025

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