The Science of Star Colors
Stars display a variety of colors, from brilliant blue to deep red, directly related to their surface temperature: hottest stars appear blue, while coolest stars are red. This phenomenon is a consequence of a fundamental physical principle known as blackbody radiation, which describes how objects emit light based on their temperature.
A star acts as an approximate “blackbody,” meaning it absorbs all incident radiation and then emits its own thermal radiation. Hotter objects emit more energy at shorter wavelengths, which correspond to the blue and ultraviolet parts of the spectrum. Conversely, cooler objects emit more energy at longer wavelengths, falling into the red and infrared regions. For example, stars with surface temperatures exceeding 40,000 Kelvin tend to appear blue, while those around 2,000 Kelvin appear red. Our Sun, with a surface temperature of about 6,000 Kelvin, emits most strongly in the yellow-green part of the spectrum, appearing yellow to our eyes. This direct relationship makes a star’s color a reliable indicator of its surface temperature.
Exploring the Coolest Stars
The stars that radiate primarily in red wavelengths are classified as M-type stars, representing the coolest stars that undergo sustained nuclear fusion. These stars typically exhibit surface temperatures ranging from approximately 2,000 Kelvin to 3,900 Kelvin. Red dwarfs are the most common type of M-type star and are also the smallest main-sequence stars. They possess masses between 0.08 and 0.6 times that of our Sun, and their radii are typically a fraction of the Sun’s diameter.
Red dwarfs have very low luminosity, often emitting as little as 0.0001 to 0.07 times the light of the Sun. Their internal structure allows for a slow and efficient burning of their hydrogen fuel, granting them extraordinarily long lifespans that can extend for trillions of years. These characteristics make red dwarfs the most abundant type of star in the Milky Way galaxy, estimated to comprise about 75% of the stellar population. Despite their prevalence, individual red dwarfs are often too dim to be observed without a telescope.
Beyond red dwarfs, red giant stars also appear red due to their relatively cool surface temperatures, typically around 3,000 to 5,000 Kelvin. However, red giants are evolved stars that have expanded significantly after exhausting the hydrogen fuel in their cores, making them much larger and more luminous than red dwarfs. While they share a similar color, their evolutionary stage distinguishes them from red dwarf stars.
Even cooler celestial objects exist, known as brown dwarfs, which bridge the gap between planets and stars. These “failed stars” are not massive enough to sustain the nuclear fusion of hydrogen into helium like true stars. Brown dwarfs typically have masses between 13 and 80 times that of Jupiter. Their surface temperatures range from about 2,100 Kelvin down to below 600 Kelvin for the oldest and smallest. Consequently, brown dwarfs emit most of their radiation in the infrared part of the spectrum, appearing deep red to the human eye.