Here's why:

* Size: Red dwarfs are significantly smaller than our Sun. They can be as small as just 10% the Sun's diameter.

* Mass: They are also much less massive, typically having only 8% of the Sun's mass.

* Temperature: They are the coolest stars, with surface temperatures around 2,500-3,500 degrees Celsius (4,500-6,300 degrees Fahrenheit).

* Lifespan: They burn their fuel very slowly and have incredibly long lifespans, potentially lasting trillions of years.

It's worth noting that there's a debate about whether even smaller, "brown dwarfs" qualify as stars. These objects are too small to sustain nuclear fusion in their cores, which is the defining characteristic of a star.