Stephenson 2-18 is the kind of star that breaks your sense of scale. It’s a red hypergiant in the constellation Scutum, about 18,900 light-years from Earth, and is roughly 2,150 times the diameter of our Sun. That makes it the current top candidate for the biggest star known to humans. The measurement comes with a warning label - giant stars don’t have sharp edges, and their size is hard to pin down precisely.
If you replaced our Sun with Stephenson 2-18, the surface of the star would extend out past the orbit of Saturn. Mercury, Venus, Earth, Mars, Jupiter, and the entire asteroid belt would all be inside the star. To circle it once at the speed of light would take about nine hours - circling our Sun takes about 15 seconds.
Stephenson 2-18 took the crown from another red supergiant, UY Scuti, which used to hold the title at “1,700 times the Sun.” Newer distance measurements from the Gaia space telescope revised UY Scuti’s size way down, and Stephenson 2-18 ended up on top. These stars are so puffed up that their outer layers are thinner than the air around you - blazing in the final stage before they collapse and explode as supernovas.
