Yellow dwarfs are stars much like our Sun, with similar temperatures, masses and long, stable lifespans. Despite their familiar name, however, these stars are neither necessarily yellow nor particularly small. Our Sun is the best-known example, but billions of similar stars are scattered throughout the Milky Way.
Our Sun Is a Yellow Dwarf
Yellow dwarfs are stars broadly similar to the Sun in their physical properties. They typically have around 0.8 to 1.2 times the Sun’s mass and surface temperatures of roughly 5,300 to 6,000 K (5,000–5,700°C / 9,100–10,300°F). The Sun, with a surface temperature of about 5,800 K and a mass of one solar mass, sits comfortably within this range and is the yellow dwarf we can study in by far the greatest detail.
What Are G-Type Main-Sequence Stars?
Yellow dwarfs belong to the class of stars astronomers call G-type main-sequence stars. Within this classification, the Sun is a G2V star. The G identifies its spectral class, which is based largely on surface temperature and the features seen in its spectrum, while the 2 places it toward the hotter end of the G-class range. The V is its luminosity class and indicates that it is a main-sequence star, steadily fusing hydrogen into helium in its core. Other yellow dwarfs have classifications such as G0V or G8V, but all belong to the same broad family of G-type main-sequence stars.
Yellow Dwarfs Aren’t Really Yellow
Despite their name, yellow dwarf stars are not actually yellow in the way the term suggests. G-type stars such as the Sun emit light across the entire visible spectrum. Although their light is strongest around the green part of the spectrum, the combination of red, green and blue light makes them appear essentially white to our eyes.
From Earth, however, the Sun can appear quite different. Our atmosphere scatters some wavelengths of sunlight more strongly than others, so the Sun can appear yellow, orange or red, especially when it is low in the sky. Seen from space, however, the Sun’s natural visible-light color is white.
They Aren’t as Small as Their Name Suggests
Despite the name, yellow dwarfs are not especially small stars. They are larger than red dwarfs, but much smaller than giant and supergiant stars, which can reach tens or even hundreds of times the Sun’s diameter.
Brighter Than Most Stars in the Milky Way
The word “dwarf” can make yellow dwarfs sound unusually small or faint, but stars like the Sun are actually brighter than most stars in the Milky Way. That is because the galaxy is dominated by much smaller and dimmer red dwarfs, which make up roughly three-quarters of its stars. Yellow dwarfs are still modest compared with massive blue stars and giants, but among the enormous population of ordinary main-sequence stars, they outshine the majority.
Yellow Dwarfs Can Remain Stable for Billions of Years
Yellow dwarfs spend most of their lives in a remarkably stable state. Gravity constantly pulls the star inward, while pressure generated by its hot interior pushes outward, creating a balance known as hydrostatic equilibrium. Nuclear fusion helps maintain this balance as hydrogen is gradually converted into helium in the core. For a star like the Sun, this main-sequence phase lasts roughly 10 billion years, providing a remarkably long period of relatively steady energy output before major changes begin.
They Will Eventually Become Red Giants
Yellow dwarfs do not remain on the main sequence forever. Once a Sun-like star exhausts the hydrogen in its core, its internal structure begins to change and its outer layers eventually expand enormously, transforming it into a red giant. The Sun is expected to enter this stage in about 5 billion years. Later, it will shed its outer layers into space, creating a planetary nebula, while the exposed core contracts into a dense white dwarf roughly the size of Earth. Unlike more massive stars, yellow dwarfs do not end their lives in supernova explosions.
Yellow Dwarfs Are Surprisingly Uncommon
The Sun may seem like an ordinary star, but yellow dwarfs are actually surprisingly uncommon. G-type main-sequence stars are estimated to account for only around 6% of the Milky Way’s stellar population, far fewer than the red dwarfs that dominate our galaxy. Yet in a galaxy containing hundreds of billions of stars, that small fraction still amounts to billions of yellow dwarfs. Astronomically, then, the Sun is just one member of a vast population of broadly similar stars. Other well-known examples include Alpha Centauri A and Tau Ceti, but the Sun remains the yellow dwarf we know best — and the benchmark against which astronomers measure and understand stars throughout the universe.