Stars are luminous celestial bodies made of hot gases, primarily hydrogen and helium, held together by their own gravity. They generate immense energy through nuclear fusion in their cores, radiating light and heat. Our Sun is one such star.
The Sun’s Size in Perspective
The Sun, measuring about 864,000 miles (1.4 million km) across, is enormous. Over a million Earths could fit inside its volume. Despite its impressive size, the Sun is considered an average-sized star when compared to the vast diversity of stars across the universe.
It is classified as a G-type main-sequence star, meaning it is in a stable phase of its life, fusing hydrogen into helium in its core. This “average” classification means the Sun is neither among the smallest nor the largest stars discovered. It sits in the middle range of stellar sizes and brightness.
Imagine stars ranging from the size of a small city to those that could swallow entire solar systems. The Sun is a common type in the galaxy.
Stars Smaller Than the Sun
Many stars are significantly smaller than the Sun. The most common type in our Milky Way galaxy is the red dwarf, with masses ranging from about 0.08 to 0.6 times that of the Sun. These stars are tiny, with some having radii as small as 9% of the Sun’s.
Red dwarfs are also much cooler and dimmer than the Sun, with surface temperatures between 2,000 and 3,500 Kelvin. Their low mass leads to a slower rate of nuclear fusion, granting them incredibly long lifespans. Proxima Centauri, the closest star to our solar system, is a well-known example of a red dwarf.
Stars Larger Than the Sun
Conversely, stars exist that dwarf our Sun. These include red giants, supergiants, and hypergiants, which represent later stages in the life cycles of more massive stars. Red giant stars can swell to hundreds of times the Sun’s diameter.
Red supergiants like Betelgeuse can be over 900 to 1,000 times the Sun’s radius. If Betelgeuse were placed at the center of our solar system, its outer layers would extend past the orbit of Mars, encompassing Mercury, Venus, and Earth. Antares, another red supergiant, is about 680 times larger than the Sun and would extend past Mars or even Jupiter’s orbit.
Even larger are red hypergiants, such as UY Scuti, with an estimated radius of 1,700 to 1,800 times that of the Sun. If UY Scuti replaced our Sun, its immense size would extend beyond Jupiter’s orbit, nearly reaching Saturn. These colossal stars are rare but demonstrate the upper limits of stellar size.
How We Know Star Sizes
Astronomers determine the sizes of distant stars through observations and physical principles. A fundamental method involves measuring a star’s apparent brightness and its distance from Earth. Distance is often found using parallax, observing the slight shift in a star’s position as Earth orbits the Sun.
Once the distance is known, astronomers can calculate the star’s true luminosity, or the total energy it emits. By analyzing a star’s spectrum, they can also determine its surface temperature. With luminosity and temperature, the star’s radius can be calculated.
Direct imaging of a star’s disk is only possible for a very small number of extremely large and relatively close stars. For the vast majority of stars, these indirect methods, based on light properties and distances, provide the most reliable way to understand their sizes.