There are about 1080 particles in the Universe. Most of them are concentrated in stars though some can be found in interstellar and intergalactic space. At the centers of stars, particle densities are sufficiently high to allow nuclear reactions to take place. The energy liberated by these reactions heats the gas that makes up the star to the point where the gas pressure balances the gravitational pressure leading to hydrostatic equilibrium and long-term stability. Stars like the Sun are stable for about 1010 years, a large fraction of the age of the Universe.

The radiation produced by the nuclear reactions achieves a near equilibrium with the gas particles in the stellar interior, leading to local (but not global) thermodynamic equilibrium. The presence of local thermodynamic equilibrium (LTE) means that radiation inside the star is close to that of a black body at the local temperature. The absence of global equilibrium allows the radiation to leak out through the surface of the star and into space. Thus, stars are objects which attain hydrostatic equilibrium and radiate as near-black bodies for most of their lives.

The early Universe, just after the Big Bang, was much like the interior of the star. The radiation from that epoch has the characteristics of black-body radiation and we see it nowadays as the 2.7 K cosmic background radiation. The early Universe also experienced a period of thermonuclear reactions when most of the hydrogen and helium was produced.