Stars in the universe vary widely in size, ranging from tiny neutron stars just a few kilometers wide to giants that dwarf our solar system. This disparity prompts questions about the limit of stellar size and how our Sun compares to the largest known stars. Comparing our Sun to these cosmic titans requires understanding magnitudes far beyond everyday comprehension. Astronomers use the Sun as a standard measure to quantify the true scale of the most expansive objects in the galaxy.
Understanding Our Reference Point: The Sun’s Scale
To appreciate the dimensions of the largest stars, we must first establish the scale of our Sun, which serves as the unit of measure. Our Sun is a medium-sized star, yet its radius is approximately 695,700 kilometers. This means its diameter is more than 109 times greater than that of Earth.
If the Sun were hollow, it could contain about 1.3 million Earths inside its volume. This enormous size makes it the gravitational anchor and dominant object in our solar system. Solar Volume is the three-dimensional benchmark used to express the capacity of larger celestial objects, which are measured in multiples of the Sun’s radius.
Identifying the Current Record Holder for Size
The title of the largest star is not permanently fixed and remains subject to ongoing refinement, complicated by the nature of these objects. The current leading candidate is Stephenson 2-18, a red hypergiant located thousands of light-years away in the Scutum constellation. This star is estimated to have a radius over 2,150 times that of the Sun, though this number carries a high degree of uncertainty.
Measuring the precise boundary of a hypergiant is difficult because they lack a sharp, solid surface. Instead, their outer layers are diffuse and constantly being shed into space, making the effective radius an estimate based on thermal emissions and distance. If Stephenson 2-18 were placed at the center of our solar system, its outer layers would extend past the orbit of Jupiter and potentially close to Saturn’s orbit.
Quantifying the Scale: The Volume Comparison
The number of Suns that can fit inside the biggest star is determined by comparing the volumes of the two spheres. Since volume scales with the cube of the radius, a star 2,150 times larger in radius than the Sun has a volume calculated by raising 2,150 to the third power. This calculation yields approximately 9.9 billion.
This means that nearly 10 billion objects the size of our Sun could theoretically be contained within the volume of Stephenson 2-18. For comparison, if UY Scuti were used with a conservative radius estimate of 900 Solar Radii, its volume would hold about 729 million Suns. This comparison measures volume, not mass or density, as the material within a red hypergiant is vastly less dense than the Sun’s material.
The Science Behind Stellar Hyper-Gigantism
The size of hypergiants results from a specific and rapid phase in the life cycle of the most massive stars. These stars begin as hot, blue O-type stars, burning through hydrogen fuel quickly. They possess initial masses many times that of the Sun, which dictates their eventual fate.
Once the core depletes its hydrogen, it contracts and heats up, initiating the fusion of heavier elements, primarily helium. This change causes the star’s outer layers to swell dramatically, forming the red supergiant or hypergiant structure. The outward pressure generated by the intense radiation from core fusion is so powerful that it pushes the stellar material to vast distances.
The maximum size limit is a balancing act between gravity and the outward pressure from fusion and radiation. These stars are inherently unstable, shedding mass rapidly in powerful stellar winds that contribute to their diffuse boundaries. This process ensures a short, violent lifespan before they eventually collapse and explode in a supernova.