Red dwarfs are the coldest and smallest type of stars. They have surface temperatures less than 3,500 Kelvin and are very dim. Red dwarfs make up about 75% of all the stars in the Milky Way galaxy, but they are too faint to be seen with the naked eye.

Brown dwarfs are even colder than red dwarfs, with surface temperatures below 2,700 Kelvin. They are not hot enough to sustain nuclear fusion in their cores, so they are not true stars. Brown dwarfs are very rare, and only a few hundred are known to exist in the Milky Way.

Here are some examples of cold stars:

* Proxima Centauri is the closest star to the Sun. It is a red dwarf with a surface temperature of 2,700 Kelvin.

* TRAPPIST-1 is a system of seven Earth-sized planets that orbit an ultra-cool red dwarf star. The star is about the size of Jupiter and has a surface temperature of 2,500 Kelvin.

* WISE 0855–0714 is a brown dwarf that is located about 7 light-years from Earth. It has a surface temperature of 225 Kelvin and is about the same size as Jupiter.