Here's why:

* Temperature: Red dwarfs are the coolest and smallest type of main sequence stars. Their surface temperatures range from about 2,400 to 3,500 Kelvin (K), compared to our Sun's 5,778 K.

* Color: Their low temperatures cause them to emit a reddish hue, hence the name "red dwarf."

* Fuel Efficiency: While they are much cooler than our Sun, red dwarfs are incredibly long-lived. They burn their hydrogen fuel very slowly and efficiently. They are estimated to have lifespans of trillions of years, compared to the Sun's 10 billion years.

Note: There are also other types of stars that are relatively cool, but red dwarfs are the most common and the coolest.