The Largest Known Star

WHAT IS STEPHENSON 2-18?

Stephenson 2-18 is a red supergiant star associated with the Stephenson 2 stellar cluster in the constellation Scutum. It is frequently listed among the largest known stars by radius. Estimates of its physical size are highly uncertain because its distance, cluster membership and atmospheric properties are difficult to determine precisely.

HOW LARGE IS IT?

Some estimates place Stephenson 2-18 at roughly 2,000 times the radius of the Sun, with values around 2,150 solar radii often quoted. If such an estimate were correct, its photosphere would extend enormously beyond the orbit of Mars if the star replaced the Sun. However, the exact radius should not be treated as a precisely measured value.

WHY IS ITS SIZE UNCERTAIN?

Determining the radius of a distant red supergiant is difficult. Astronomers must estimate its distance, temperature, luminosity and atmospheric properties. Stephenson 2-18 is especially challenging because its relationship with the Stephenson 2 cluster is not completely certain. For this reason, astronomers should describe its extreme size as an estimate rather than an exact measurement.

WHAT WILL HAPPEN TO THE STAR?

A massive red supergiant is in a late stage of stellar evolution. Eventually, its core can collapse when it can no longer support itself against gravity. Depending on the star's final core mass, the collapse could produce a powerful core-collapse supernova and leave behind a neutron star or black hole.

KEY IDEAS

RED SUPERGIANT

Stephenson 2-18 is classified as an extremely large red supergiant.

~2,000 SUN RADII

Some estimates give a radius thousands of times larger than the Sun's.

SCUTUM

The star is located in the direction of the constellation Scutum.

EXTREME DISTANCE

Its estimated distance is on the order of ten thousand light-years.

COOL SURFACE

Despite its enormous size, its surface is much cooler than the Sun's.

NOT CERTAINLY #1

Its exact ranking among the largest stars depends on uncertain measurements and definitions.

DID YOU KNOW?

If Stephenson 2-18 really has a radius of about 2,150 Suns, its surface would extend far beyond the orbit of Mars if it replaced our Sun.

HOW ASTRONOMERS ESTIMATE STELLAR SIZE

Stellar radius is not usually measured by simply seeing a sharp surface. Astronomers combine observations such as luminosity, temperature, distance, spectra and, for some stars, interferometric measurements to estimate size. Extended atmospheres and surrounding material can make the boundary of a giant star difficult to define precisely.

For that reason, “largest known star” should be treated as a measurement-dependent record rather than a permanent title. Improved distances and stellar models can change estimated radii and rankings.