The Sun is the most important star for life on Earth, providing constant light and warmth. However, compared to the countless other stars in the Milky Way galaxy, the Sun is neither exceptionally large nor tiny. To determine how many stars are truly bigger than our Sun, we must examine the stellar population as a whole and understand how astronomers define and measure stellar dimensions.
Defining Stellar Size and the Measurement Challenge
When astronomers talk about a star’s size, they primarily refer to its radius, which measures the physical extent of the star, often expressed in units of the Sun’s radius. Determining this dimension for distant objects presents a significant technical hurdle because stars appear as mere pinpricks of light, even through the most powerful telescopes.
For most stars, the radius is calculated using the star’s luminosity and its surface temperature. This method relies on the Stefan-Boltzmann law, which relates a star’s energy output to its temperature and surface area. Surface temperature is inferred from the star’s color and spectrum, while luminosity is calculated from the star’s apparent brightness and its distance, determined through parallax.
Only a few hundred relatively nearby, large stars can have their size measured more directly using techniques like interferometry or lunar occultation. Interferometry combines the light collected by multiple telescopes to achieve the angular resolution needed to resolve the star’s disc. These direct measurements are invaluable for calibrating the more common indirect calculations based on temperature and brightness.
The Sun’s Position in the Stellar Population
The Sun is classified as a G-type main-sequence star, meaning it is currently fusing hydrogen in its core. Approximately 90% of stars spend their existence on this main sequence, and their size is closely tied to their mass. Crucially, the distribution of stellar masses is heavily skewed toward the low end.
This distribution is described by the Initial Mass Function (IMF), which shows that small stars are overwhelmingly more common than large stars. Stars significantly smaller than the Sun, known as M-type stars or Red Dwarfs, make up an estimated 75-80% of the entire stellar population. These stars are less massive and have a smaller radius than the Sun, making them the most numerous in the galaxy.
The Sun is more massive and larger than approximately 85% to 95% of the stars in the Milky Way. While the Sun is often called an “average” star, it is actually quite large and bright compared to the common low-mass, dim Red Dwarfs. Therefore, the stars that are larger than the Sun—the F, A, B, and O-type stars—constitute a relatively small fraction, estimated to be less than 5% to 10% of the total stellar count.
Extreme Examples: The Giants and Supergiants
When considering the most colossal stars in the universe, the question shifts from “how many” to “how big.” Stars that dwarf the Sun are typically evolved stars that have exhausted the hydrogen fuel in their core and have expanded dramatically. These are known as giants, supergiants, and hypergiants, and they are extremely rare compared to the general stellar population.
Red Giants and Supergiants can expand to hundreds or even over a thousand times the radius of the Sun. This immense swelling is caused by the outer layers of the star puffing up as the core contracts and begins fusing helium or other heavier elements. Their sheer size is often accompanied by a relatively low density, making them more like vast, glowing spheres of gas.
The red supergiant Betelgeuse in the constellation Orion is a well-known example of this stellar gigantism. Its radius is estimated to be around 764 times that of the Sun. If Betelgeuse were placed at the center of our solar system, its outer layers would extend past the orbits of Mercury, Venus, Earth, and Mars, nearly reaching the orbit of Jupiter.
Stars like UY Scuti, a red hypergiant, represent the upper limit of known stellar size, with an estimated radius over 1,700 times that of the Sun. If this star replaced our Sun, its envelope would extend past the orbit of Jupiter and possibly even Saturn. These colossal objects are short-lived in astronomical terms and are a testament to the extreme scales found in the universe.