The size of the universe makes it difficult to grasp the scale of the objects within it. We often use comparison to understand this vastness, contrasting the largest celestial bodies with the one we know best. Our own Sun provides the only familiar stellar reference point, serving as the necessary baseline for measuring the cosmos. Comparing our star to the largest known stars helps us appreciate the scale of stellar giants scattered across the galaxy.
The Sun as a Cosmic Yardstick
The Sun, a medium-sized star, is a furnace of plasma that dominates our local solar system. Its mean radius measures approximately 696,000 kilometers, or roughly 432,000 miles. This size means its diameter is about 109 times greater than that of Earth.
The Sun’s volume is large enough that approximately 1.3 million Earths could fit inside it. Its mass is 333,000 times that of Earth, establishing the Sun as a powerful star. However, in the galactic context, our star is merely an average main-sequence star used as a measuring stick against stellar behemoths.
Identifying the Largest Known Stars
The stars that hold the title for largest size are not the most massive, but rather the most physically expansive, belonging to the class of Red Supergiants or Red Hypergiants. These stars have swelled significantly in their advanced evolutionary stages, leading to their colossal dimensions. The current leading candidate for the largest known star is UY Scuti, a variable Red Hypergiant located in the constellation Scutum.
UY Scuti’s radius is estimated to be approximately 1,700 times that of the Sun. This value carries a significant margin of error due to the star’s distance and its diffuse outer atmosphere. Other prominent Red Supergiants, such as Stephenson 2-18 and the former record holder VY Canis Majoris, also represent the upper limits of stellar size, with estimates often exceeding 1,500 solar radii.
This immense scale means that the volume of a star like UY Scuti is staggering compared to our Sun. If the radius is 1,700 times greater, the volume is over 4.9 billion times larger than the Sun’s volume. The difference is a magnitude that spans billions.
Visualizing the Scale Difference
The numerical comparison of 1,700 solar radii quickly becomes an abstract concept, requiring a shift in perspective to appreciate. A helpful way to visualize this scale is to imagine replacing our Sun with UY Scuti at the center of the solar system.
If UY Scuti replaced the Sun, the star’s outer atmosphere would extend well beyond the orbits of the inner planets. The star would engulf Mercury, Venus, Earth, and Mars. Its surface would likely reach past the orbit of Jupiter, and potentially even Saturn.
For a physical analogy, if the Sun were shrunk down to the size of a standard basketball, Earth would be a tiny sesame seed orbiting it. On this same scale, UY Scuti would become a sphere occupying the space of a large sports stadium or a small city. This comparison demonstrates the qualitative difference between a common star and a stellar giant.
Stellar Immensity Explained
The reason for this expansion lies in the late stages of stellar evolution, specifically for stars that begin their lives with a high mass, typically 25 times that of the Sun or more. These stars burn through their nuclear fuel quickly compared to smaller stars. Once the hydrogen fuel in the core is exhausted, the core contracts under gravity and heats up dramatically.
This core heating ignites a shell of hydrogen fusion surrounding the core, pushing the star’s outer layers outward with immense radiation pressure. As the outer layers swell and diffuse outward, they cool significantly. This cooling gives the star its characteristic reddish hue and its classification as a Red Supergiant or Hypergiant. The resulting stellar body is a low-density, highly luminous giant that has traded extreme mass for extreme volume.