Proxima Centauri is the star closest to our solar system, yet its size often surprises people who assume its proximity means it is similar to the Sun. At a distance of just over four light-years, this star is an object of intense astronomical study, particularly due to the exoplanets found orbiting it. Despite being our closest stellar neighbor, Proxima Centauri is significantly smaller and fainter than the Sun.
Stellar Classification and Type
The reason Proxima Centauri is so small is directly related to its stellar classification as a red dwarf. Specifically, it is categorized as an M5.5Ve star, meaning it is a main-sequence star that is both cool and low in mass. Red dwarfs are the most common type of star found throughout the Milky Way galaxy.
These stars are characterized by their relatively low surface temperatures, which give them a faint, reddish hue compared to the Sun’s yellow-white light. The Sun is classified as a G-type star, which is much hotter and more luminous. Proxima Centauri’s low-mass nature dictates its modest size and low light output.
Measuring Its Physical Dimensions
Proxima Centauri’s radius is a mere fraction of the Sun’s, making the comparison quite dramatic. Measurements indicate that its diameter is approximately one-seventh, or about 14%, of the diameter of the Sun. This means that if the Sun were the size of a basketball, Proxima Centauri would be roughly the size of a golf ball.
To put its size into a more relatable context, Proxima Centauri is only about 50% larger than the planet Jupiter. If the Sun were suddenly replaced by Proxima Centauri, the star would barely be larger than the orbit of Mercury.
Because it is so small, Proxima Centauri is incredibly dim, shining with only about 0.16% of the Sun’s total luminosity. Its light output is heavily concentrated in the infrared spectrum, which is why it is far too faint to be seen with the naked eye from Earth, despite being the closest star. This low luminosity is a direct consequence of its restricted dimensions and cooler temperature.
Mass and Density
While Proxima Centauri is small in volume, its mass tells a different story about its substance. The star contains about 12.5% of the Sun’s total mass. This mass is substantial enough to sustain nuclear fusion in its core, which is the defining feature of a star.
The combination of low volume and significant mass results in an extremely high density. Proxima Centauri is estimated to be about 40 times denser than the Sun. This high density is a defining feature of red dwarfs, contrasting sharply with the Sun’s lower average density.
The compressed internal structure of Proxima Centauri allows for a process called full convection, where matter circulates throughout the entire star. This constant churning means that the hydrogen fuel in the core is continuously replenished from the outer layers. As a result, red dwarfs burn their fuel slowly and efficiently, giving them incredibly long lifespans, estimated to be trillions of years, far outlasting the Sun.