The Sun serves as the standard reference point for measuring the size of other stars across the cosmos. Classified as a G-type main-sequence star, our star is currently in a stable, middle-aged phase, fusing hydrogen into helium in its core. This spectral classification places the Sun in a wide category of stars that are far from the largest in the universe. The cosmos contains stars that dwarf the Sun by hundreds or even thousands of times its diameter. Understanding these stellar giants requires looking at the different stages of a star’s life cycle.
The Stellar Evolutionary Ladder
Stellar evolution dictates a star’s size based on its age and mass. Stars significantly larger than the Sun are typically in late-stage stellar life, which begins when the hydrogen fuel in the core is depleted. The core contracts and heats up, causing hydrogen fusion to start in a shell surrounding the core.
The energy released by this shell burning pushes the star’s outer layers outward, dramatically increasing its volume and surface area. Stars with a mass similar to the Sun enter the Red Giant phase, swelling up to 200 times the Sun’s current radius. If the Sun were to undergo this change, its expanded outer atmosphere would likely engulf the orbits of Mercury, Venus, and possibly Earth.
More massive stars, typically those born with eight or more solar masses, evolve into Red Supergiants, representing an even greater leap in scale. Betelgeuse in the Orion constellation is a prime example, with a diameter estimated to be around 640 times that of the Sun. Were Betelgeuse placed at the center of our solar system, its photosphere would extend beyond the orbit of Mars and potentially reach Jupiter. These stages demonstrate that size is more about the expansive, low-density outer layers of a star nearing the end of its life than initial mass.
The Absolute Titans
The stars with the largest known physical diameters belong to the Hypergiant class. These objects push the theoretical limits of stellar size, exhibiting radii that exceed even the largest Red Supergiants. The current record holders are often Red Hypergiants, whose vast, tenuous outer envelopes result from extreme luminosity and rapid mass loss.
Among the largest confirmed stars is UY Scuti, a pulsating variable Red Supergiant located in the constellation Scutum. Its estimated radius is difficult to pinpoint precisely due to its distance and variable nature, but measurements place it between 900 and 1,700 times the radius of the Sun. If UY Scuti replaced our Sun, its photosphere would extend well past the orbit of Jupiter.
Another contender is Stephenson 2-18, an extremely luminous Red Hypergiant. Its radius is estimated to be approximately 2,150 times that of the Sun, making it the largest star currently known by volume. Nearly ten billion Suns could theoretically fit inside the volume of Stephenson 2-18. These stars are so large that their gravity is barely able to hold their outer layers, leading to continuous mass loss into space.
How Astronomers Measure Cosmic Scale
Determining the size of these distant, expanded stellar envelopes presents a substantial challenge for astronomers. The first step in measuring a star’s size is accurately establishing its distance from Earth, a process that relies on the method of stellar parallax for relatively nearby objects. Parallax involves observing the star’s apparent shift in position against distant background objects as Earth moves around the Sun over six months.
Once a star’s distance is known, its radius is estimated indirectly by combining measurements of luminosity and surface temperature. The star’s temperature is determined by analyzing its light spectrum, and luminosity is calculated from its observed brightness and distance. Since luminosity is proportional to the star’s surface area, this relationship allows scientists to calculate the radius.
These measurements, particularly for the largest hypergiants, carry uncertainties because the outer layers are diffuse, irregular, and often obscured by thick clouds of gas and dust. This makes defining a precise “edge” or diameter challenging and leads to ongoing revisions in the estimated sizes of record-holding stars like UY Scuti and Stephenson 2-18.