Stephenson 2-18 vs Phoenix A: which is bigger, star or black hole?

Phoenix A is bigger by both size and mass: even at its lowest published mass of about 10 billion Suns, its event horizon would be about 395 astronomical units (AU) across, roughly 20–30 times the diameter of Stephenson 2-18, a red supergiant whose radius is estimated at about 1,500–2,150 times the Sun’s (roughly 7–10 AU). In mass the gap is far larger—hundreds of millions of times or more.

The comparison only works if like is compared with like. A star’s size is the radius of its glowing gas surface. A black hole’s size is usually given as its event-horizon radius, which depends only on its mass and is not a material surface.

Key facts: Stephenson 2-18 vs Phoenix A

• Stephenson 2-18: a red supergiant star in our own galaxy, about 5.5–5.8 kiloparsecs (18,000–19,000 light-years) away if it belongs to the Stephenson 2 star cluster.
• Phoenix A: the supermassive black hole in the central galaxy of the Phoenix galaxy cluster, whose light has travelled about 5.7–5.9 billion years to reach us.
• Size: star radius about 7–10 AU; black-hole horizon radius about 197–1,974 AU for masses of 10–100 billion Suns.
• Mass: the star’s mass has not been measured; Phoenix A’s published estimates range from about 1 × 10¹⁰ to possibly more than 1 × 10¹¹ solar masses.
• Uncertainty: both objects’ key numbers are model-dependent. The star’s radius depends on an uncertain distance, and Phoenix A has never been weighed directly.

How big is Stephenson 2-18?

Astronomers cannot photograph Stephenson 2-18’s surface. They calculate its radius from two quantities: its luminosity (total power output) and its surface temperature. The Stefan–Boltzmann law links them, so for a star R/R☉ = √(L/L☉) × (T☉/T)², where the Sun’s effective temperature T☉ is 5,772 K.

The widely quoted radius of about 2,150 solar radii traces to a 2012 study by Fok and colleagues. Their dust-model fit gave Stephenson 2-18 (listed as St2-18) a temperature of 3,200 K and log L = 5.64, about 440,000 times the Sun’s luminosity, at an adopted distance of 5.5 kpc. Worked through: √440,000 ≈ 663, and (5,772/3,200)² ≈ 3.25, so R ≈ 663 × 3.25 ≈ 2,150 solar radii.

A 2026 ALMA study by Siebert and colleagues, posted as a preprint, refitted the star’s spectral energy distribution and obtained a luminosity of (3.21 ± 1.15) × 10⁵ L☉ at 3,200 K, with the uncertainty including a ±1 kpc distance error. SciQuest’s calculation from those values gives a radius of about 1,840 solar radii, with a range of roughly 1,480–2,150.

Radii calculated by SciQuest from the published luminosities and temperatures using T☉ = 5,772 K and R☉ = 695,700 km (NASA Sun Fact Sheet); 1 AU = 149,597,870.7 km.

There is a larger caveat. Both estimates assume the star belongs to the Stephenson 2 cluster. A 2020 study by Humphreys and colleagues classed it as a non-member, and the 2026 paper notes it may be a foreground giant, partly because its velocity differs from the cluster average. If it is closer than assumed, it is less luminous and smaller. Placed at the Sun’s position, a 7–10 AU star would extend beyond Jupiter’s orbit (5.2 AU) to about Saturn’s (9.6 AU).

How big is Phoenix A’s event horizon?

For a non-spinning black hole, the event-horizon radius is r = 2GM/c², about 2.95 km for every solar mass. At 1 × 10¹⁰ solar masses that is 2.95 × 10¹⁰ km, or about 197 AU; at 1 × 10¹¹ it is about 1,974 AU.

Phoenix A’s mass is the weak link. A 2025 JWST study argued for about 10 billion solar masses but called it weakly constrained; a 2012 NASA Chandra release quoted about 20 billion without stating a method; and a 2016 modelling study said it “might exceed” 100 billion. None is a direct measurement. Our Phoenix A black hole profile explains where each figure comes from.

Stephenson 2-18 vs Phoenix A: side-by-side comparison

Source note: star values from Fok et al. (2012) and Siebert et al. (2026); Phoenix A values from Reefe et al. (2025), NASA’s Chandra X-ray Center (2012) and Brockamp et al. (2016). Horizon sizes calculated by SciQuest.

Divide the diameters and Phoenix A’s horizon is about 20–30 times wider than the star at the 10-billion-Sun estimate, and about 200–290 times wider at 100 billion. Even the smallest plausible Phoenix A would swallow Stephenson 2-18 many times over.

How much more massive is Phoenix A than Stephenson 2-18?

No one has weighed Stephenson 2-18. The 2026 study notes that red supergiants in the Stephenson 2 cluster are thought to have begun life with roughly 12–25 solar masses, but that estimate may not apply if this star is a foreground object. Using 25 solar masses as a generous figure, a 10-billion-solar-mass Phoenix A would be about 400 million times more massive. At 100 billion, the ratio is about 4 billion.

Earlier versions of this page gave the star a mass of “12 to 100 times” the Sun’s; we could not find a published basis for that range.

Is the Phoenix A galaxy also bigger?

Yes, by an enormous margin, but that is a different comparison. “Phoenix A” refers to the black hole, not to its host galaxy or the Phoenix galaxy cluster. The host galaxy contains billions of stars and is vastly larger than either the star or the black hole’s horizon. Comparing a single star with a whole galaxy would not be a like-for-like test.

Why both objects matter

Stephenson 2-18 is a test case for how large and luminous red supergiants can become, and for how distance errors inflate stellar size records. Red supergiants are late stages in the lives of massive stars; our guide to the stages of a star’s life cycle explains what comes next. Phoenix A sits at the centre of one of the most extreme galaxy clusters known, where rapid cooling, a starburst and black-hole jets interact.

For the largest stars, see our WOH G64 vs Stephenson 2-18 comparison. For the largest black holes, see our guide to the biggest black hole in the universe.

What scientists don’t know yet

• Whether Stephenson 2-18 belongs to the Stephenson 2 cluster. Its distance, luminosity and radius all depend on the answer.
• Stephenson 2-18’s mass, which has not been measured.
• Phoenix A’s mass, and therefore its horizon size, which could differ by a factor of ten between published estimates.

Related reading

• What is the largest star known?
• Phoenix A vs TON 618 vs Holmberg 15A: which black hole is heaviest?
• How big is TON 618?

Update note: This article was substantially revised on October 12, 2026, to compare the star’s physical radius with the black hole’s event-horizon radius instead of mixing size measures; correct the event-horizon diameter (previously given inconsistently as 366 billion miles and 366 billion kilometres); clarify the Phoenix A distance; correct the claim that it is the most massive black hole known; replace an unsupported stellar mass range; and add newer radius estimates, the cluster-membership caveat and worked calculations. It was drafted with AI assistance and checked against the sources listed below.

Sources

• Fok et al., Maser observations of Westerlund 1 and comprehensive considerations on maser properties of red supergiants associated with massive clusters, The Astrophysical Journal 760, 65 (2012)
• Siebert et al., The molecular circumstellar environments of red supergiants in Galactic open clusters, preprint submitted to Astronomy & Astrophysics (2026)
• Reefe et al., Cold gas and star formation in the Phoenix cluster with JWST, accepted by The Astrophysical Journal (2025)
• NASA Chandra X-ray Center, Phoenix Cluster Sets Record Pace at Forming Stars (August 15, 2012)
• Brockamp et al., Unveiling Gargantua: a new search strategy for the most massive central cluster black holes, Astronomy & Astrophysics 585, A153 (2016)
• NASA NSSDCA, Sun Fact Sheet and Planetary Fact Sheet
• IAU, 2012 Resolution B2 on the astronomical unit