Every twinkling point of light in the night sky is a star, a colossal sphere of plasma generating heat and light through nuclear fusion. While our Sun is medium-sized, the universe contains stellar bodies that dwarf it. Determining the absolute largest star is challenging for astronomers due to the vast distances and measurement uncertainties. The star currently holding the record for the largest physical size is a red hypergiant, a stellar giant nearing the end of its dramatic life cycle.
The Largest Known Star
The star widely recognized as the largest by physical size is Stephenson 2-18. This red hypergiant resides approximately 20,000 light-years away within the constellation Scutum. It is a rare, short-lived star that has expanded dramatically after exhausting the hydrogen fuel in its core. Its estimated radius is about 2,150 times that of the Sun, though this figure is subject to debate among astronomers due to the inherent difficulty of measuring such distant objects.
Despite its colossal size, Stephenson 2-18 has a relatively cool surface temperature of around 3,200 Kelvin, which is significantly cooler than our Sun’s 5,778 Kelvin. This low temperature causes the star to emit a reddish glow, a characteristic common to these aged, expanded stars. Its total energy output, or luminosity, is enormous, radiating energy at a rate of roughly 440,000 times that of the Sun. This massive luminosity, combined with its cool temperature, accounts for its extraordinary size, as the star’s outer layers have puffed up to an extreme degree.
The star’s estimated mass is far less certain than its size, but it is thought to be many tens of times the mass of the Sun. Stars of this magnitude burn through their fuel at an incredibly rapid pace, ensuring that their lifespan is measured in mere millions of years, rather than the billions of years our Sun will live.
Visualizing Hypergiant Scale
Grasping the sheer immensity of Stephenson 2-18 requires comparing its dimensions to objects we are familiar with in our own solar system. If the Sun were replaced by this hypergiant, its photosphere, the star’s visible surface, would extend far beyond the orbits of Mars and Jupiter. The star’s boundary would likely engulf the orbit of Saturn, which is nearly 1.4 billion kilometers from the Sun.
The volume of space contained within Stephenson 2-18 is so vast that it could theoretically hold approximately 10 billion stars the size of our Sun. To put its size into a time-based perspective, consider a beam of light traveling around the star’s equator. While a light beam circles the Sun’s equator in about 14.5 seconds, it would take nearly nine hours to complete a single trip around Stephenson 2-18. The star’s radius of 2,150 solar radii translates to a diameter of over 3 billion kilometers. These comparisons illustrate the extreme physical size possible in stellar evolution.
How Astronomers Measure Stellar Size
The radius of a star as distant as Stephenson 2-18 cannot be measured directly with a ruler or simple angular observation. Instead, astronomers rely on a combination of fundamental physics and observational techniques to derive its size. One primary method involves the Stefan–Boltzmann Law, which relates a star’s total energy output, or luminosity, to its surface temperature and its physical size.
Astronomers first determine a star’s luminosity by measuring its apparent brightness and then calculating its distance. The distance is often found using stellar parallax, which measures the tiny shift in a star’s position against background objects as the Earth orbits the Sun. A more distant star will show a smaller parallax angle, providing the necessary data to accurately gauge its true distance.
Once the luminosity and distance are known, the star’s effective temperature is determined by analyzing its light spectrum, which reveals the star’s color and spectral type. With the luminosity and the effective temperature established, the Stefan–Boltzmann equation allows scientists to calculate the star’s radius. The uncertainties in both the distance measurement and the temperature estimate are what lead to the range of published sizes for the largest hypergiants.
For closer, very large stars, astronomers sometimes employ a technique called interferometry. This technique combines the light collected by multiple telescopes to achieve the angular resolution of a much larger single telescope. This can allow for a direct measurement of the star’s angular diameter, which, when combined with the distance, yields a more precise physical radius.