Comparing our Sun to the largest stars in the cosmos immediately highlights the immense scale of the universe. Our everyday experience offers no true frame of reference for the vastness of astronomical objects, making stellar sizes challenging to grasp. While the Sun appears large and powerful in our sky, it is merely one star among billions, and its size is far from the maximum attainable by celestial bodies. Comparing our home star to cosmic giants requires embracing a new perspective.
Defining Our Baseline: The Sun’s Place in the Cosmos
The Sun serves as the fundamental yardstick for measuring other stars, a unit known as the solar radius (\(R_\odot\)). Its average equatorial radius is approximately 695,508 kilometers, a distance so great that over one million Earths could be packed inside its volume.
Astronomically, the Sun is classified as a G2V star, signifying it is a main-sequence star currently fusing hydrogen into helium in its core. The “V” in this classification indicates it is a dwarf star, despite its enormous size compared to a planet. The Sun is in its stable middle age, which dictates its relatively moderate size and yellow color. Compared to the extremes of stellar evolution, the Sun is considered quite average when measured against massive blue stars or bloated red giants.
The Record Holders: Identifying the Biggest Stars
The title of the largest star known is not permanently fixed, as measurements are constantly being refined for these distant and variable objects. Based on current estimates, the red hypergiant Stephenson 2-18 (S2-18) is considered the strongest candidate for the largest star discovered so far. This colossal star is located nearly 20,000 light-years away within the constellation Scutum, and its estimated radius is a staggering 2,150 times that of the Sun, or \(2,150\ R_\odot\).
This immense size is characteristic of red supergiant and hypergiant stars, which represent a late stage in the life of the most massive stars. When these stars exhaust the hydrogen fuel in their cores, they expand dramatically, swelling their outer layers to phenomenal volumes. The volume of Stephenson 2-18 is estimated to be nearly 10 billion times that of the Sun. For context, UY Scuti is often cited as the record holder, though newer data suggests it may be significantly smaller than its older estimate of \(1,708\ R_\odot\).
Grasping the Scale: Visualizing the Comparison
The numerical difference between the Sun’s \(1\ R_\odot\) and Stephenson 2-18’s \(2,150\ R_\odot\) is too vast for intuitive comprehension. Relatable analogies are required to visualize this difference.
Scaling Analogy
If the Sun were scaled down to the size of a standard basketball, the red hypergiant would have a scaled diameter of approximately 516 meters. This means the largest star in this model would be roughly as tall as the Willis Tower in Chicago.
Solar System Replacement
If the Sun were instantly replaced by Stephenson 2-18, the star’s surface would extend far beyond the orbits of the inner planets. Its photosphere would engulf Mercury, Venus, Earth, and Mars, continuing outward past the asteroid belt. Based on the \(2,150\ R_\odot\) estimate, the star’s outer edge would completely swallow the gas giant Jupiter and extend past the orbit of Saturn. This demonstrates that the largest star contains a significant portion of our entire solar system within its boundaries.
Circumnavigation Time
Another way to comprehend the difference is to imagine flying a commercial jet, traveling at an average cruising speed of 900 kilometers per hour, around the equator of both stars. A complete circumnavigation of the Sun would take the jet approximately 202 days. To complete a single trip around the equator of Stephenson 2-18, the same jet would have to fly continuously for nearly 1,200 years. This time difference illustrates the unimaginable disparity between the size of our solar star and the gargantuan stellar record holders.