Stars come in a wide range of colors that are determined by their surface temperature. The hottest stars tend to appear bluish-white or white, while the coolest stars are red. This relationship between temperature and color is referred to as a star’s spectral type.
What determines a star’s color?
A star’s color is primarily a result of its surface temperature. Stars are made up mostly of hydrogen and helium gas. When the gas is heated to high temperatures, the atoms excite electrons to higher energy levels. As the electrons fall back down to lower energy levels, they emit photons of light at specific wavelengths.
The hotter a star is, the more high energy photons it emits. Blue light has a shorter wavelength than red light. So hotter stars that emit more blue light appear bluish-white, while cooler stars that emit more red light appear reddish.
What is the coolest spectral type of stars?
Stars are categorized into spectral types based on their surface temperature and color. From hottest to coolest, the main spectral types are:
- O – Bluish-white
- B – Bluish-white
- A – White
- F – Yellowish-white
- G – Yellow
- K – Orange
- M – Reddish
The M spectral type contains the coolest stars with surface temperatures under 3,500 Kelvin. These red dwarf stars have a reddish color.
What causes the colors of the coolest M type stars?
Red dwarf stars have such low temperatures that their emitted light is shifted towards the red end of the visible spectrum. Their cool atmospheres cannot heat up enough to emit large amounts of bluer wavelengths of light.
However, red dwarfs do still emit some blue and green light. So they appear more orange-red than blood red. The exact shades of M dwarfs range from deep orange-red to pale orange-red.
The coolest red dwarfs with the lowest mass and temperatures can also exhibit large stellar flares. These flares emit more blue light, making the star temporarily appear more whitish during the flare.
Examples of cool red M dwarf stars
Here are some well-known examples of red M dwarf stars:
| Star name | Spectral type | Temperature |
|---|---|---|
| Proxima Centauri | M5.5Ve | 3,050 K |
| Barnard’s Star | M4Ve | 3,134 K |
| Wolf 359 | M6Ve | 2,800 K |
Proxima Centauri is the closest star to our solar system at 4.2 light years away. As an M5.5 red dwarf, it has a temperature of 3,050 K and magnitude 10.5, giving it a dim, red glow.
Conclusion
The coolest stars categorized as spectral type M are red dwarfs with temperatures below 3,500 K. Their reddish color is caused by the shift of emitted light toward longer red wavelengths due to their cool atmospheres. Red dwarfs include the closest stars to our solar system like Proxima Centauri and Barnard’s Star, which emit a faint red-orange glow.
