Drop the universe’s largest star into our solar system, and it would swallow every planet out to Saturn. That monster is Stephenson 2-18, burning 19,000 light-years away in the constellation Scutum.
Embedded within the open cluster Stephenson 2, this red hypergiant has an estimated radius roughly 2,150 times that of the Sun—about 1.5 billion kilometers. By volume, it could swallow nearly ten billion Suns. Yet in astrophysics, "biggest" does not mean "heaviest." Stephenson 2-18 is an aging star nearing the end of its life, swollen into a red hypergiant after exhausting the hydrogen fuel in its core. While it likely began with 30 to 50 times the Sun's mass, violent stellar winds have blown away much of its outer envelope over time. As a result, its outer atmosphere is remarkably diffuse—thousands of times less dense than the air in Earth's atmosphere, behaving more like a glowing, heated vacuum than a solid celestial body. The most massive star ever found, R136a1, packs up to 200 solar masses into a radius only about 40 times that of the Sun.
Holding the crown for the largest star is also notoriously precarious. Red hypergiants lack a definite physical edge; they are turbulent, pulsating cauldrons enveloped by expanding clouds of dust and gas. Moreover, determining a star's radius requires knowing its true luminosity, which hinges directly on its distance from Earth. For years, popular astronomy ranked UY Scuti as the largest star at roughly 1,700 solar radii. That changed when data from the European Space Agency's Gaia observatory revealed UY Scuti was much closer to Earth than earlier estimates suggested, slashing its calculated radius to around 755 solar radii. Because Stephenson 2-18 sits near the absolute theoretical limit of stellar size, future observations could easily revise its measurements as well.