Stars shine by Nuclear fusion that is the conversion of Hydrogen into Helium.
This requires a star to have a mass of 0.08 of our Sun (about 84 Jovian Units)
There is a type of star not massive enough to do this.
These stars are known as brown dwarfs.
They shine in the Infra-red by deuterium fusion and by gravitational heating.
The First one Gliese 229B was found in the constellation Lepus in 1995. Since then various infra-red telescopes have located hundreds of other brown dwarfs. A brown dwarf will have a temperature in the range 800 to 1800 °K.

Mass is the main thing which separates brown dwarfs from gas giant planets.
Below 13 Jovian Units¹ an object does not have the mass to be a brown dwarf and some objects above this mass lack the proper makeup to reach the temperature of a brown dwarf².
There has even been cases of a gas giant rotating around a brown dwarf³ in the constellation centaurus.