A star is a celestial body that generates intense light and heat through nuclear fusion in its core. Astronomers classify these luminous spheres primarily by their surface temperature, which correlates with their mass and lifespan, using a system that assigns letters from O (hottest) down to M (coolest). Understanding the “most common” stars requires differentiating between those that are bright and easily visible and those that are numerically abundant throughout the galaxy. The stars that dominate the total stellar count are often too faint to be seen without a telescope, while the brightest stars represent a tiny fraction of the overall stellar population.
The Stellar Census: Why Red Dwarfs Dominate
The most common type of star in the Milky Way galaxy is the Red Dwarf. These dim, low-mass stars are estimated to account for approximately 73% to 80% of all stars in the galaxy’s stellar population. Red Dwarfs are defined by their extremely low mass, which typically ranges from 0.08 to about 0.6 times the mass of our Sun.
Their low mass means that nuclear fusion occurs at an exceedingly slow rate, resulting in very low luminosity, often less than 0.1% that of the Sun. This low energy output gives them a cool, reddish hue, with surface temperatures generally falling between 2,100 and 3,900 Kelvin. The most distinctive feature of these stars is their incredible longevity, with lifespans that can extend from hundreds of billions to trillions of years.
Lower-mass Red Dwarfs are fully convective, meaning that helium ash does not build up in the core. Instead, the star’s material slowly cycles, allowing all of its hydrogen fuel to be consumed over time, unlike more massive stars that only burn the hydrogen in their core. Their high abundance is directly linked to the amount of material needed to form them, as they require far less gas and dust than any other star type.
Mid-Range Main Sequence Stars
Main sequence stars are those actively fusing hydrogen in their cores. These stars are classified in a temperature sequence from the hottest O and B types, through A, F, G, and K types. The Sun is a G-type star, falling in the mid-range of this spectrum with a yellowish-white color and a surface temperature of about 5,800 Kelvin.
While G-type stars like the Sun are common compared to the massive O and B-type stars, they are still rare compared to Red Dwarfs, representing only about 6% to 7.6% of the Milky Way’s population. K-type stars, which are slightly cooler and dimmer than the Sun, are significantly more numerous than G-types, making up around 12% to 13% of all stars. O-type stars are the rarest, making up less than 0.0001% of all main sequence stars.
The lower numbers for these brighter, higher-mass stars are due to a trade-off: higher mass correlates with a much higher rate of fuel consumption. An O-type star, which is significantly more massive than the Sun, can burn through its fuel and end its life in only a few million years. Stars like the Sun have a lifespan of about 10 billion years, while the slightly smaller K-type stars can last for 15 to 45 billion years.
Common Stellar Remnants
White Dwarfs are the most common of the “dead” stars, representing the typical final state for any star that begins its life with up to about eight times the mass of the Sun. These compact objects form when a star like the Sun sheds its outer layers to become a planetary nebula, leaving behind a dense, hot core.
The core is no longer supported by the outward pressure of nuclear fusion, but rather by electron degeneracy pressure. The resulting White Dwarf is roughly the size of Earth but contains a mass comparable to the Sun, giving it an extreme density. Most White Dwarfs consist primarily of a carbon and oxygen core. They do not generate energy internally and instead shine only from residual heat, which they slowly radiate into space over trillions of years.