What is the term for celestial objects that lack sufficient mass to initiate nuclear fusion?

The term for celestial objects that lack sufficient mass to initiate nuclear fusion is brown dwarfs. These objects are often referred to as "failed stars" because they have some characteristics of stars but do not possess enough mass to sustain the nuclear fusion reactions that power true stars like our Sun.

Brown dwarfs have a mass range between the heaviest gas giant planets and the lightest stars, specifically between approximately 13 and 80 times the mass of Jupiter. Below this threshold, they cannot reach the temperatures and pressures required for hydrogen fusion in their cores. Instead, they primarily emit light in the infrared spectrum due to the heat generated from their formation and the gradual cooling over time.

In contrast, red giants, white dwarfs, and neutron stars represent evolutionary stages of stars that have undergone nuclear fusion, thus differentiating them from brown dwarfs. Red giants are stars in the later stages of stellar evolution, capable of fusing elements heavier than hydrogen. White dwarfs are the remnants of low to medium mass stars after they have exhausted their nuclear fuel. Neutron stars are the collapsed cores of massive stars that have exploded in supernova events, exhibiting extreme densities and different physical characteristics altogether.