The color of stars depends on their surface temperature. Stars emit light across the electromagnetic spectrum, but the peak wavelength of light they emit corresponds to their temperature. Hotter stars appear bluish-white while cooler stars appear reddish-orange. So are the coolest stars blue or white? Let’s take a closer look at stellar classification and star colors.
Stellar Classification
Astronomers classify stars by their spectra into categories O, B, A, F, G, K, and M. This is known as the Harvard spectral classification system, or more commonly, the OBAFGKM sequence.
O stars are the hottest with surface temperatures over 30,000 K. They appear bluish-white. B stars are next with temperatures from 10,000 K to 30,000 K. They too look blueish-white.
A stars have temperatures between 7,500 K and 10,000 K and are white in color. F stars follow from 6,000 K to 7,500 K with a yellowish-white hue. Our Sun is a G star with a temperature around 5,800 K and yellow color.
K stars have temperatures from 3,500 K to 5,000 K and orange color. Finally, M stars are the coolest with temperatures below 3,500 K and deep red color.
So in general, the sequence goes from hot, blueish stars to cool, reddish ones:
O -> B -> A -> F -> G -> K -> M
But when astronomers refer to “cool stars”, they typically mean K and M dwarf stars with temperatures under 5,000 K.
K Dwarf Stars
K dwarf stars have temperatures between 3,500-5,000 K. Examples of K dwarfs include Epsilon Indi, 61 Cygni, and Tau Ceti. They have an orange or reddish-orange hue.
Some of the earliest K dwarfs discovered in the 19th century were mistaken for red giant stars due to their orange-red appearance. But spectroscopy revealed they are actually main sequence stars cooler than the Sun.
K dwarfs comprise about 12% of stars in our Milky Way galaxy and over 75% of stars in the solar neighborhood. Due to their low mass and luminosity, K dwarfs burn through their fuel very slowly and can live for up to 70 billion years, much longer than the 10 billion year lifetime of a star like our Sun.
So K dwarf stars are definitely not blue. Their orange-red color corresponds to the “cooler” end of the stellar temperature sequence.
M Dwarf Stars
M dwarf stars have surface temperatures below 3,500 K. Well-known examples include Barnard’s Star, Wolf 359, Proxima Centauri, and TRAPPIST-1. Their emissions peak in infrared, so M dwarfs have a deep red color.
With temperatures between 2,400-3,500 K, M dwarf stars emit very little visible light. But they still shine in infrared, which allows us to detect and analyze them.
M dwarfs are by far the most common type of star, making up 75-80% of all stars in the Milky Way. Their low mass and luminosity means M dwarfs will burn for trillions of years, far longer than the current age of the universe.
So M dwarfs are also definitively not blue. As the coolest stellar class, they emit very strongly in the red and infrared parts of the spectrum but hardly any blue or visible light.
Why Stars Have Different Colors
A star’s color depends on its surface temperature according to blackbody radiation principles. Hotter objects emit bluer, shorter wavelengths while cooler objects give off redder, longer wavelengths.
This is why O, B, and A stars appear blueish-white – their high temperatures (>7,500 K) mean they emit strongly in the blue region of the visible spectrum.
Our yellow Sun sits at an intermediate temperature of 5,800 K. Cooler K dwarfs and even colder M dwarfs radiate most strongly at red and infrared wavelengths, giving them an orange-red hue.
Stellar classification is based on this relationship between a star’s surface temperature and peak emission color. As astronomers measure more star temperatures through spectroscopy, the OBAFGKM system helps organize them into a meaningful sequence.
Differences in Star Size
In addition to color, stellar classes differ greatly in size. The table below summarizes key properties:
| Class | Temperature | Color | Radius | Mass |
|---|---|---|---|---|
| O | > 30,000 K | Blue | 6-20 solar radii | 16-120 solar masses |
| B | 10,000 – 30,000 K | Blue-white | 1.5-8 solar radii | 2.1-16 solar masses |
| A | 7,500 – 10,000 K | White | 1.4-2 solar radii | 1.4-2.1 solar masses |
| F | 6,000 – 7,500 K | Yellow-white | 1.15-1.4 solar radii | 1.04-1.4 solar masses |
| G | 5,300 – 6,000 K | Yellow | 0.96-1.15 solar radii | 0.8-1.04 solar masses |
| K | 3,500 – 5,000 K | Orange | 0.7-0.96 solar radii | 0.45-0.8 solar masses |
| M | Red |
Hot O and B stars are supergiants many times larger than our Sun. K and M main sequence stars are much smaller and fainter than the Sun. So stellar class also correlates with size – hot stars are physically much larger than cool stars.
Stellar Evolution
A star’s temperature and color evolve over its lifetime as it fuses fuel. Stars are born from collapsing clouds of gas. At this early stage, protostars heat up but are still cool red objects.
After ignition, a new star will sit on the main sequence, stably fusing hydrogen into helium for billions of years. Our Sun is currently in this midlife phase.
Eventually, stars turn off the main sequence and become red giants as they fuse heavier elements. They end life as hot blue white dwarfs or explode in spectacular supernovas.
So a single star can change color from red to yellow to blue as it ages. But its stellar class stays the same since that only depends on initial mass and composition. The classes help distinguish different types of stars at similar life stages.
Finding Exoplanets Around Cool Stars
One reason astronomers study K and M dwarf stars is to search for planets. Dwarf stars’ small size and luminosity makes it easier to detect planetary transits.
When an orbiting planet crosses in front of the star, it slightly dims the star’s light. This periodic dip can reveal an exoplanet. Thousands of planets have been found around M dwarfs using the transit technique.
TRAPPIST-1, an ultracool M dwarf star, hosts 7 temperate Earth-sized planets. Their transits were measured by the TRAPPIST telescope and Spitzer Space Telescope. TRAPPIST-1 shows that even dim, cool red stars can harbor potentially habitable worlds.
Upcoming missions like James Webb and PLATO will continue surveying K and M dwarfs for exoplanets. Their cool temperatures and long lifetimes make dwarf stars prime targets in the search for life in our galaxy.
Conclusion
In summary, the coolest category of stars are K and M dwarf stars. Despite the term “cool”, they do not emit blue light but instead shine reddish to deep red. Hot O and B stars are blueish-white while intermediate A, F, and G types are white to yellow.
A star’s color depends on its surface temperature and peak radiation according to blackbody principles. Cooler stars emit longer wavelengths in the red and infrared, appearing orange to red. Hotter stars give off more blue light.
Stellar classification organizes stars by temperature and helps explain why they have different observable properties like color. As astronomy progresses, the OBAFGKM system remains a valuable tool for understanding distinctions between star types and searching for worlds beyond our Solar System.