​The supergiant WOH G64 is a very cool, extreme red supergiant (RSG) with an effective temperature near 3,400 K and a radius of roughly 1,500–1,700 times the Sun’s radius, making it among the largest known stars. It is in the Large Magellanic Cloud (LMC), about 163,000 light‑years away, and is one of the largest known stars by radius. Around 280,000–300,000 times the solar luminosity, with an estimated initial mass of about 25–30,M_\odot, so it is a probable supernova‑progenitor.
 
WOH G64 sheds mass at an exceptionally high rate, embedded in a thick, asymmetric dust torus and surrounded by nitrogen‑rich nebular gas moving faster than the star itself. Long‑term monitoring shows that after appearing as a canonical red supergiant for decades, WOH G64 has dramatically changed since the 2010s: its spectrum shifted from cool absorption to more emission‑dominated features, and it has been argued to have transitioned into (or toward) a yellow hypergiant phase, possibly as part of a massive symbiotic binary system with a hot B‑type companion.
 
It sits at the extreme upper‑right of the Hertzsprung–Russell diagram and challenges standard RSG models because of its low temperature, huge radius, and intense mass loss. The system is now viewed as a rare laboratory for observing late‑stage, massive‑star evolution and binary interaction, possibly on the verge of a core‑collapse supernova or a major eruptive episode.
 
However, its exact evolutionary route is complicated by extreme mass loss and possible binary interaction. The default pathway as a massive star, WOH G64 has already burned hydrogen, helium, and heavier elements in its core, and now sits as a red supergiant with a cool, inflated envelope and a hot, dense core. Standard massive‑star models expect that once the core builds up an iron‑group core, fusion stops, gravity wins, and the core collapses in seconds to form a neutron star or black hole, ejecting the outer layers in a Type II‑P or similar core‑collapse supernova.
 
Over the last 30 years, WOH G64 has shown sudden photometric and spectroscopic changes: its variability shifted from semi‑regular to irregular, its spectrum became more emission‑line dominated, and it appears to have become warmer and less extended, consistent with a transition toward a yellow hypergiant (YHG)‑like or Be‑type state. This behavior is interpreted as either a pre‑supernova superwind phase expelling the outer envelope in a final, violent mass‑loss episode, or a binary‑driven evolution in which a hidden B‑type companion has stripped part of the RSG envelope, leading to a massive symbiotic system (RSG/YHG + hot companion).
 
In either scenario, the core mass and composition are what ultimately decide the explosion, but the surface evolution is messier. Instead of a canonical cool RSG, WOH G64 might explode while in a warm, inflated, or binary‑altered state (YHG‑like with strong mass loss plus a companion). The intense mass loss implies the hydrogen envelope could be significantly reduced or even partially removed before core collapse, potentially producing a stripped‑envelope or hybrid supernova (e.g., Type IIb or IIL/IIn) rather than a classic hydrogen‑rich Type II‑P.
 
In short, the nominal path is core‑collapse of a formerly red supergiant, but WOH G64’s recent transition and likely binary nature may turn it into a peculiar, stripped‑envelope or symbiotic‑system supernova progenitor, offering a rare real‑time probe of how some of the most massive stars die.
 
References
Levesque, E.M., Massey, P., Plez, B. & Knut, A.G. (2009, June). The Physical Properties of the Red Supergiant WHO G64: The Largest Known Star? The Astronomical Journal.
 
Munoz-Sanchez, G., Kalitsounaki, M., de Wit, S., Antoniadis, K., Bonanos, A.Z., et al. (2026, February 23). The Dramatic Transition of the Extreme Red Supergiant WHO G64 to a Yellow Hypergiant. Nature Astronomy.
 
Ohnaka, K. & van Loon, J. (2026, February 7). A Giant Star is Changing Before Our Eyes and Astronomers are Watching in Real Time. Phys.Org.
 
(2026, April 1). WHO G64. In Wikipedia: https://en.wikipedia.org/wiki/WOH_G64

The center of the Milky Way hosts Sagittarius A (Sgr A), a supermassive black hole with about 4 million solar masses. This conclusion stems from decades of observations, including star orbits like S2’s, which trace a compact massive object, and the 2022 Event Horizon Telescope image showing its shadow. Recent hypotheses propose a dense fermionic dark matter core instead. A February 2026 study suggests this core could mimic black hole effects on star motions, rotation curves, and even the shadow image through extreme light bending. Scientific consensus still favours the black hole. These dark matter ideas remain speculative, requiring future tests like sharper photon ring imaging or precision GRAVITY/Gaia data to distinguish them.
 
Stars like S2 orbit Sgr A* in tight, relativistic paths, completing cycles every 16 years at speeds up to 3% of light speed. Their trajectories reveal a 4.3 million solar mass object confined to a radius smaller than our solar system, consistent only with a black hole. Further, the 2022 EHT image shows Sgr A*‘s shadow, a dark central region ringed by glowing plasma, matching general relativity predictions for a black hole’s event horizon and photon ring. The GRAVITY/VLT observations in 2018 detected gas clumps at 30% light speed and bright flares from hot electrons near the event horizon, aligning with black hole accretion models. And the Chandra X-ray data confirms a jet-like structure impacting surrounding gas, with spectra typical of supermassive black hole outflows; magnetic fields spiral gas inward, keeping emissions low.
 
However, a February 2026 study proposes that a dense core of fermionic dark matter, rather than Sagittarius A, powers the Milky Way’s center. The lead author Lic. Valentina Crespi from the Institute of Astrophysics La Plata, and collaborators, including Dr. Carlos R. Argüelles, model this as a compact core mimicking black hole gravity for S-stars and G-sources, surrounded by a halo explaining the galaxy’s rotation curve slowdown (Keplerian decline) per GAIA DR3 data. This was published in Monthly Notices of the Royal Astronomical Society (DOI: 10.1093/mnras/staf1854). It unifies central orbits, large-scale dynamics, and the EHT shadow image via strong light bending, without photon rings unique to black holes.
 
Key Model Features
Fermionic dark matter (light subatomic particles) forms a continuous core-halo structure, unlike traditional cold dark matter’s extended tails;
Fits inner relativistic star speeds (thousands km/s) and outer halo motions with ordinary matter components; and
Builds on 2024 Pelle et al. work showing accretion disk shadows match EHT observations.
 
Future GRAVITY/VLT data and photon ring searches could distinguish it from black hole models, as current data cannot yet rule either out.
 
References
Chu, J. (2022, May 12). Astronomers Snap First-Ever Image of Supermassive Black Hole Sagittarius A*. MIT Department of Physics.
 
Mohon, L. (2013, November 290). NASA’s Chandra Helps Confirm Evidence of Jet in Milky Way’s Black Hole. NASA.
 
Tonkin, S. (2026, February 5). Dark Matter, not a Black Hole, Could Power Milky Way’s Heart. Royal Astronomical Society.
 
(2026, January 14). What is the Center of Our Galaxy Like? NASA. https://science.nasa.gov/mission/webb/science-overview/science-explainers/what-is-the-center-of-our-galaxy-like/
 
(2026, April 5). Galactic Center. In Wikipedia, https://en.wikipedia.org/wiki/Galactic_Center
 
(2026, April 15). Sagittarius A*. In Wikipedia, https://en.wikipedia.org/wiki/Sagittarius_A*