Humanity has long gazed at the night sky, captivated by distant cosmic objects. Among the most intriguing inquiries is the quest to identify the “biggest” star in the universe. Understanding this requires navigating immense distances and diverse stellar properties.
Defining “Biggest” in the Cosmos
The term “biggest” for stars primarily refers to either stellar radius (physical size) or stellar mass (total matter). These two characteristics do not always correlate directly. A star exceptionally large in physical size may not be the most massive, and a highly massive star might be relatively compact.
For instance, some stars expand to enormous sizes in their later evolutionary stages, becoming diffuse giants with relatively low densities, even if their initial mass was not extraordinarily high. Other stars, born with immense amounts of matter, remain comparatively smaller in radius due to their extreme gravitational compression. This distinction is crucial, as the “biggest” star by radius is often a very different object from the “biggest” star by mass, leading to different contenders for the title.
The Current Size Record Holder
Stephenson 2-18 is recognized as one of the largest stars by stellar radius. This red supergiant or red hypergiant is located approximately 18,900 to 20,000 light-years from Earth in the constellation Scutum. Estimates place its radius at approximately 2,150 times that of our Sun. To provide perspective, if Stephenson 2-18 were placed at the center of our solar system, its outer layers would extend past the orbit of Saturn. The star is also highly luminous, shining with an energy output hundreds of thousands of times that of the Sun.
Other Stellar Behemoths
While Stephenson 2-18 holds the record for physical size, other stars are considered behemoths for different reasons. UY Scuti was previously considered the largest star by radius, with estimates around 1,700 times the Sun’s radius. Newer measurements, however, suggest a smaller radius of approximately 755 to 909 solar radii, due to updated distance calculations. This red supergiant is also incredibly luminous, radiating energy more than 86,000 times that of the Sun.
In terms of mass, the star R136a1 is currently the most massive known star. Located in the Tarantula Nebula within the Large Magellanic Cloud, this Wolf-Rayet star has an estimated mass of 170 to 300 times that of our Sun. Despite its extreme mass, R136a1 is relatively compact, with a radius only about 30 times that of the Sun. It is also one of the hottest and most luminous stars, emitting millions of times more energy than the Sun.
How Stars Are Measured
Astronomers employ several sophisticated methods to determine the size and mass of distant stars. One fundamental technique for measuring stellar distances is parallax. As Earth orbits the Sun, nearby stars appear to shift slightly against the background of more distant stars. By measuring this apparent shift, known as the parallax angle, astronomers can use trigonometry to calculate the star’s distance. Accurate distance measurements are crucial because a star’s apparent size and brightness depend heavily on how far away it is.
Once the distance is known, angular diameter measurements can be converted into true physical size. Stellar interferometry is a powerful technique that combines light from multiple telescopes to achieve very high angular resolution, allowing astronomers to measure the tiny apparent diameters of stars. This method is particularly effective for large, nearby stars.
For binary star systems, where two stars orbit a common center of mass, astronomers can measure their orbital periods and velocities to determine their individual masses using gravitational laws. For single stars, mass is often estimated through stellar modeling, which uses a star’s observed luminosity, temperature, and spectral characteristics to infer its internal structure and mass based on theoretical predictions of stellar evolution.
These complex measurements, especially for very distant or obscured objects, often have associated uncertainties. This is why the “biggest star” record can be debated and updated as technology and understanding advance.