The universe operates on a scale far beyond our daily experience, especially when comparing our modest Sun to the largest known stars. Our Sun is a common yellow dwarf that has sustained life for billions of years. Astronomers continually discover celestial bodies that redefine what “big” truly means. These stellar behemoths dwarf the Sun and our entire solar system, existing as transient spectacles of immense size and power.
Clarifying the Metric: Mass vs. Radius
The question of “what is the biggest star” is complicated because stars are measured by two different metrics: mass and physical size. Stellar mass refers to the total amount of material a star contains, which determines its lifespan and ultimate fate. For example, the most massive star currently known, R136a1, holds over 300 times the mass of the Sun but is relatively compact.
The record for “biggest” is determined by a star’s physical volume or radius. Stars that have exhausted their core hydrogen fuel expand dramatically, becoming incredibly diffuse. These evolved stars, known as red hypergiants, are physically enormous but contain far less mass than the most massive stars. Therefore, the star that is the most voluminous is not necessarily the heaviest.
The Largest Known Star by Volume
The star currently holding the title for the largest estimated radius is Stephenson 2-18 (St2-18). This Red Hypergiant is located approximately 20,000 light-years away in the constellation Scutum. Its maximum estimated size places it at a staggering 2,150 times the radius of the Sun. This measurement is derived from its high luminosity and relatively cool surface temperature, which suggests a colossal physical size.
The great distance and thick layers of dust surrounding hypergiants introduce significant uncertainty into measuring their true size. The star’s visible surface, or photosphere, is highly diffuse and constantly changing, making precise calculation difficult. Due to these challenges, some astronomers consider more reliably measured Red Hypergiants, such as UY Scuti or WOH G64 (with radii closer to 1,500 solar radii), to be the largest confirmed stars. St2-18’s estimate represents the upper limit of stellar size observed in the Milky Way galaxy.
Comparing the Hypergiant to Our Solar System
Translating the size of Stephenson 2-18 into a relatable scale requires placing it hypothetically at the center of our Solar System. If St2-18 replaced the Sun, its outer atmosphere would extend far beyond the orbits of the inner planets, instantly engulfing Mercury, Venus, Earth, and Mars.
The star’s size would also stretch well into the outer solar system, swallowing the gas giant Jupiter. Its outer boundary would nearly reach the orbit of Saturn, which circles the Sun at about 9.5 astronomical units (AU). At its maximum estimated radius of 2,150 times the radius of the Sun, light would take nearly nine hours to travel once around its circumference, compared to only 14.5 seconds around the Sun.
The Life Cycle of Massive Stars
The colossal size of a red hypergiant is a brief, transient phase in the life of an extremely massive star, typically one that began with over 25 solar masses. These stars burn through their hydrogen fuel at an astonishing rate via the CNO cycle, leading to lifespans that last only a few million years. Once the core hydrogen is exhausted, the core contracts, triggering the outer layers to swell and cool dramatically, transforming the star into a red hypergiant.
The star’s enormous size and luminosity cause it to push against the theoretical stability limit, where the outward pressure of radiation nearly overcomes the inward pull of gravity. This condition creates instability in the star’s outer envelope, leading to pulsations and a violent stellar wind. These winds cause the star to shed massive amounts of material into space, sometimes losing the equivalent of one solar mass every few thousand years. This mass loss continues as the star fuses progressively heavier elements in concentric “onion skin” layers around its core. The process ends when the core forms inert iron, which cannot be fused to release energy, leading to a Type II Supernova explosion and the formation of either a neutron star or a black hole.