It is frequently applied to the coolest objects, including K - and M - dwarfs, which are true stars, and brown dwarfs, often referred to as «failed stars» because they do not
sustain hydrogen fusion in their cores.
Brown dwarfs are not considered stars because they are too small to fuse hydrogen in their cores — they don't have the gravitational oomph in their core to
sustain hydrogen fusion, but, depending on how massive they are, they do have enough mass to sporadically fuse elements like lithium and deuterium.
And in the past several years, disks seemed equally likely around brown dwarfs: gaseous balls less than 75 times the mass of Jupiter, unable to ignite
sustained hydrogen fusion.
Not exact matches
With no nuclear
fusion to
sustain them, they collapse into Earth - size balls of tightly bound carbon and oxygen nuclei with an outer layer of
hydrogen plasma (disrupted atoms).
But Michael Skrutskie, a University of Virginia astronomer and a member of the WISE science team, is especially interested in the satellite's ability to pick out previously unknown brown dwarfs, objects larger than planets but too small to
sustain nuclear
fusion of
hydrogen.
The outward pressure from
fusion counteracts the inward pressure from gravity and an equilibrium point was reached where they are equal and opposite in force and the sun has a relatively stable diameter that will persist until it starts running out of
hydrogen fuel to
sustain the outward pressure.