Stars come in a wide range of colors and temperatures. The color of a star depends on its surface temperature – the hotter the star, the bluer it appears. The coolest stars have surface temperatures under 3,500 Kelvin and appear reddish in color, while the hottest stars can reach over 40,000 Kelvin and take on a brilliant blue-white hue. In this article, we’ll take a look at what determines a star’s color and temperature, and explore examples of the hottest and coolest stars discovered so far.
What Determines a Star’s Color and Temperature?
A star’s color and temperature are primarily determined by its mass and age. When stars are born, gravity causes hydrogen gas to accumulate and compress in stellar nurseries called nebulae. Eventually when enough mass accumulates, nuclear fusion is triggered and the protostar begins to shine. More massive stars have stronger gravity and higher core pressures and temperatures, so they fuse hydrogen into helium at a faster rate than smaller, less massive stars.
This means more massive stars consume their nuclear fuel faster, evolving more quickly through their lifecycles. Large, hot, blue stars may only live for a few million years before going supernova, while small, cool, red dwarf stars can survive for hundreds of billions of years. A star’s mass and rate of fusion also determine its luminosity – more massive stars are intrinsically brighter.
As stars age and fusion materials are depleted in their cores, their temperatures and luminosities decrease. So very young stars tend to be hotter and bluer, while older stars trend cooler and redder. However, the relationship between mass, temperature, and color is complex. Let’s look at examples of the coolest and hottest star types discovered so far.
The Coolest Stars – Red and Brown Dwarfs
The coolest stars known are the red dwarfs, with surface temperatures under 4,000 Kelvin. Despite their cool temperature, red dwarfs are still hot enough at their cores for hydrogen fusion to occur. Prominent examples of red dwarfs include Proxima Centauri (3,050 K), TRAPPIST-1 (2,550 K), and LHS 3447 (2,030 K) – some of the smallest stars known with radii less than a tenth that of our Sun.
|Star Name||Spectral Type||Temperature|
|Proxima Centauri||M5.5Ve red dwarf||3,050 K|
|TRAPPIST-1||M8V red dwarf||2,550 K|
|LHS 3447||M9V red dwarf||2,030 K|
Even cooler than red dwarfs are the brown dwarfs, with temperatures under 2,000 K. Brown dwarfs are not massive enough to sustain hydrogen fusion, so they can’t be properly called stars. However, they are star-like in appearance and originate from the same gas clouds as true stars. Some examples of brown dwarfs include WISE 0855−0714 (250 K), WD 0806-661B (1,500 K), and WISE J085510.83-071442.5 (225-260 K) – the coldest brown dwarf known.
The Hottest Stars – Blue Giants and Hypergiants
At the other end of the temperature scale, the hottest stars are the blue giants and hypergiants. These behemoths are tens to hundreds of times more massive than our Sun, with surface temperatures exceeding 30,000 K.
Some examples include Rigel (12,100 K) and Deneb (8,500 K), two of the brightest stars in our night sky. The hottest known stars are even more extreme, like Eta Carinae A (40,000 K), HD 93129A (50,000 K), and WR 102ka (200,000 K) – one of the hottest stars ever observed.
|Star Name||Spectral Type||Temperature|
|Rigel||B8Ia blue supergiant||12,100 K|
|Deneb||A2Ia blue supergiant||8,500 K|
|Eta Carinae A||Luminous Blue Variable star||40,000 K|
|HD 93129A||O2If* blue hypergiant||50,000 K|
|WR 102ka||Wolf-Rayet star||200,000 K|
These blazing hot stars emit most of their radiation in the ultraviolet and appear strikingly blue or blue-white. Their extreme temperatures are a result of their huge masses and luminosities that keep their cores fused at incredibly high rates. However, they burn through their nuclear fuel quickly, surviving only a few million years at most before experiencing cataclysmic deaths as supernovae or gamma ray bursts.
Intermediate Temperature Stars
Between the coolest red dwarfs and hottest blue giants lie stars with intermediate temperatures similar to our Sun. Our Sun is a G-type main sequence star with a surface temperature of about 5,800 K, giving it a white hue with a yellow tinge. Other stars around this temperature include Vega (A-type star, 9,600 K) and Capella (G-type giant star, 5,300 K) which appear white with bluish or yellowish tints.
While our Sun and stars like it are relatively ordinary in size, temperature, and color, their stable lifespans of billions of years make them hot enough for life-supporting planetary systems to evolve. More massive blue stars and smaller red dwarfs might harbor planets, but their stellar lifespans are likely too short or variable for advanced life. Stars with intermediate temperatures seem to provide the best incubators for life as we know it.
The Relationship Between Color, Temperature, and Stellar Classification
Across the range of stellar temperatures, from coolest to hottest, stars follow a classification sequence invented in the early 1900s as the Harvard Spectral Classification. From coolest to hottest, the sequence runs: M, K, G, F, A, B, O, and reflects the dominate wavelengths of light emitted by the stars. Each class is then divided into 10 subtypes numbered 0 to 9 based on finer differences in their spectra.
Our Sun is a G2 star – midway between K and F classes, and halfway along their sequence of 10 subtypes. As mentioned, hotter blue stars like Rigel are classified B8, while cool red dwarfs like Proxima Centauri come in as M5.5. This spectral classification neatly orders stars by the relationship between their colors and surface temperatures.
|Spectral Type||Temperature Range||Color|
|M||2,000 – 3,500 K||Red|
|K||3,500 – 5,000 K||Orange|
|G||5,000 – 6,000 K||Yellow|
|F||6,000 – 7,500 K||Yellow-white|
|A||7,500 – 10,000 K||White|
|B||10,000 – 30,000 K||Blue-white|
|O||Over 30,000 K||Blue|
This sequence demonstrates the strong link between a star’s surface temperature and the color it emits and appears to our eyes. While other smaller factors can influence a star’s exact color, this relationship generally holds across the wide spectrum of stellar types and temperatures that exist.
From red dwarfs to blue giants, stars exhibit an enormous range of surface temperatures spanning thousands of degrees. The coolest red dwarfs fuse hydrogen at just over 2,000 Kelvin, while the hottest blue hypergiants blaze at over 50,000 Kelvin. A star’s color is intimately linked to its temperature, with redder stars being cooler and bluer stars being hotter.
Our Sun and other G-type yellow stars strike the ideal balance between cool stability and hot intensity to potentially serve as hosts to habitable planets and life as we know it. The study of stellar colors and temperatures provides insight into the lifecycles and evolution of stars across the universe. Continuing to catalog and analyze the coldest brown dwarfs to the most blistering blue giants deepens our understanding of these celestial bodies that make life on our planet possible.