The universe contains objects of staggering size, but few inspire as much awe as the largest stars. These brilliant celestial bodies, massive spheres of plasma powered by nuclear fusion, vary enormously in their physical dimensions. Astronomers measure size by the star’s physical radius, which can balloon to incomprehensible proportions during its lifetime. Understanding these cosmic behemoths requires exploring the extreme limits of stellar physics.
Establishing Scale: Comparing Star Sizes
To grasp the true size of the largest stars, we must first establish a cosmic baseline, which is our own Sun. The Sun, an average-sized star, defines one solar radius (R☉), approximately 432,000 miles. A common star like Sirius is only about 1.7 R☉, but stars show immense scale when they evolve into Red Giants, such as Arcturus, which expands to over 25 R☉.
The largest stars belong to a class called Hypergiants, which dwarf even the largest Red Giants. If one of these Hypergiants were placed at the center of our solar system, its outer edge would extend far past the orbits of the inner planets. Traveling around their circumference at the speed of light would take many hours, compared to the 14.5 seconds it takes to circle the Sun.
The Physical Constraints on Stellar Mass
A star’s size is ultimately governed by a delicate balance between two opposing forces: the inward pull of gravity and the outward pressure generated by fusion. The intense light and heat produced by nuclear reactions in the core create a powerful outward force known as radiation pressure. This pressure is what prevents the star from collapsing under its own immense weight.
The point at which the outward radiation pressure perfectly balances the inward force of gravity is described by the Eddington Limit. Stars that approach or exceed this theoretical luminosity limit become highly unstable. The tremendous radiation pressure begins to push the star’s outer layers into space, causing rapid and dramatic mass loss. This physical restraint prevents stars from growing infinitely massive.
This physical principle sets a practical upper limit on how much mass a star can stably hold. The theoretical maximum mass for a single star is generally considered to be in the range of 150 to 300 solar masses. Stars born with mass exceeding this limit are so luminous and unstable that they shed their extra material almost immediately. This mass limit then translates into the extreme physical sizes observed in the largest stars, as high-mass stars will swell dramatically as they age.
The Biggest Stars Discovered
The largest stars by physical radius are classified as Red Hypergiants, a rare and short-lived type of star. The current record holder is often cited as Stephenson 2-18, a Red Hypergiant located thousands of light-years away in the constellation Scutum. Its estimated radius is an astounding 2,150 times that of the Sun. If this star replaced the Sun, its photosphere, the visible surface, would extend well past the orbit of Saturn.
Precisely measuring the size of these remote, unstable giants is extremely challenging, and their estimated sizes are frequently revised. The previous record holder, UY Scuti, was once estimated to be over 1,700 R☉, but recent data suggests its size is likely much smaller, perhaps closer to 900 R☉. Distinguishing the actual stellar surface from the surrounding, dense dust and gas ejected by the star’s instability is a major hurdle for astronomers.
Another contender, WOH G64 in the Large Magellanic Cloud, has a more reliably determined radius of about 1,540 R☉. The difficulty in measurement stems from the fact that Red Hypergiants are variable stars, meaning their size fluctuates over time as they shed and re-accrete their outer layers. Their extreme instability, a direct result of pushing against the Eddington Limit, makes any precise measurement challenging. The largest stars by radius are not always the most massive.
The Violent End of Hypergiant Stars
The same extreme mass that allows a star to become a Red Hypergiant also guarantees it a short and spectacular life. Stars that start with masses greater than eight to ten times the Sun’s mass burn through their nuclear fuel at an incredibly fast rate. While our Sun will live for approximately 10 billion years, a massive Hypergiant might only survive for a few million years.
Once the star has exhausted the fuel in its core, the outward pressure ceases, and gravity takes over in a catastrophic manner. The core collapses rapidly, triggering one of the most powerful explosions in the universe: a Type II Supernova. This explosion briefly outshines entire galaxies and blasts the star’s outer material into interstellar space.
The ultimate remnant of this violent stellar death depends on the original mass of the star’s core. If the core’s remaining mass is sufficiently high, nothing can stop the continued gravitational collapse. The immense gravity compresses the core into a stellar-mass black hole. Less massive cores will collapse into an incredibly dense object known as a neutron star.