A star is a massive, luminous ball of plasma held together by its own gravity. The question of the “biggest star ever discovered” is complex because “bigness” can refer to either the star’s physical size (radius) or its total mass. Since distances to these celestial objects are vast, directly measuring their dimensions is difficult. Astronomers must rely on indirect methods to determine the physical size and total mass of immense stars.
Measuring Stellar Dimensions
Measuring a star’s physical size requires more than simply pointing a telescope at it, as most stars appear as mere points of light. The primary technique relies on combining the star’s brightness, temperature, and distance from Earth. Astronomers first determine the star’s distance, often using stellar parallax, which measures the tiny apparent shift in a star’s position as the Earth orbits the Sun.
Once the distance is known, the star’s true luminosity can be calculated from its apparent brightness. The star’s surface temperature is determined by analyzing its color spectrum; hotter stars appear bluer and cooler stars appear redder. These three pieces of information—distance, luminosity, and temperature—are linked by the Stefan-Boltzmann law.
This law states that a star’s total energy output (luminosity) is proportional to its surface area and the fourth power of its temperature. By mathematically rearranging this formula, astronomers can infer the star’s radius. The resulting stellar radius is then expressed in units of Solar Radii (\(R_{\odot}\)), comparing the star’s size directly to that of our Sun.
Identifying the Largest Known Star
The largest star known by physical size (radius) is generally considered to be UY Scuti, a red hypergiant located in the constellation Scutum. This star is estimated to have a radius of approximately 1,700 times that of the Sun. However, recent measurements suggest a range of 900 to 1,700 Solar Radii due to its variable nature. UY Scuti is a pulsating variable star, meaning its brightness and size fluctuate over a period of about 740 days, making precise measurement challenging.
To grasp the scale of UY Scuti, imagine placing it at the center of our solar system in place of the Sun. Its outer photosphere would extend far past the orbit of Jupiter, potentially even past the orbit of Saturn. This immense size means its volume could contain nearly five billion objects the size of our Sun. UY Scuti reached this size because it is a highly evolved star, having exhausted the hydrogen fuel in its core and expanded into the late stage of its life cycle as a red hypergiant.
Differentiating Size and Mass
Despite UY Scuti holding the record for physical size, it is not the most massive star known, illustrating the difference between stellar radius and stellar mass. A star’s mass is the total amount of matter it contains, typically measured in Solar Masses (\(M_{\odot}\)). While UY Scuti is enormous in volume, its density is extremely low, giving it a relatively modest mass of around 7 to 10 \(M_{\odot}\).
The most massive star currently known is R136a1, a hot, bright star located in the Tarantula Nebula within the Large Magellanic Cloud. R136a1 has a mass estimated to be around 290 times that of the Sun. This star is a Wolf-Rayet star, a type of extremely hot blue supergiant, and its high temperature and internal pressure keep it relatively compact.
R136a1’s radius is only about 40 times the size of the Sun, making it physically much smaller than UY Scuti. The distinction is that the largest stars by radius are inflated, cooler, and less dense. Conversely, the most massive stars are hot, dense, and physically smaller giants or supergiants. This difference highlights that the answer to “What is the biggest star?” depends entirely on whether one means the largest by volume or the heaviest by mass.