Dwarf star in the context of Main-sequence stars

In astrophysics, the main sequence is a classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band. Stars spend the majority of their lives on the main sequence, during which core hydrogen burning is dominant. These main-sequence stars, or sometimes interchangeably dwarf stars, are the most numerous true stars in the universe and include the Sun. Color-magnitude plots are known as Hertzsprung–Russell diagrams after Ejnar Hertzsprung and Henry Norris Russell.

When a gaseous nebula undergoes sufficient gravitational collapse, the high pressure and temperature concentrated at the core will trigger the nuclear fusion of hydrogen into helium (see stars). The thermal energy from this process radiates out from the hot, dense core, generating a strong pressure gradient. It is this pressure gradient that counters the star's collapse under gravity, maintaining the star in a state of hydrostatic equilibrium. The star's position on the main sequence is determined primarily by the mass, but also by age and chemical composition. As a result, radiation is not the only method of energy transfer in stars. Convection plays a role in the movement of energy, particularly in the cores of stars greater than 1.3 to 1.5 times the Sun's mass, again depending on age and chemical composition.

A giant star has a substantially larger radius and luminosity than a main-sequence (or dwarf) star of the same surface temperature. They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type (namely K and M) by Ejnar Hertzsprung in 1905 or 1906.

Giant stars have radii up to a few hundred times the Sun and luminosities over 10 times that of the Sun. Stars still more luminous than giants are referred to as supergiants and hypergiants.

An ultra-cool dwarf is a stellar or sub-stellar object that has an effective temperature lower than 2,700 K (2,430 °C; 4,400 °F). This category of dwarf stars was introduced in 1997 by J. Davy Kirkpatrick, Todd J. Henry, and Michael J. Irwin. It originally included very low mass M-dwarf stars with spectral types of M7 but was later expanded to encompass stars ranging from the coldest known to brown dwarfs as cool as spectral type T6.5. Altogether, ultra-cool dwarfs represent about 15% of the astronomical objects in the stellar neighborhood of the Sun. One of the best known examples is TRAPPIST-1.

Models of the formation of planets suggest that due to their low masses and the small size of their proto-planetary disks, these stars could host a relatively abundant population of terrestrial planets ranging from Mercury-sized to Earth-sized bodies, rather than a population of super-Earths and Jupiter-massed planets. The discovery of the TRAPPIST-1 planetary system, consisting of seven Earth-sized planets, would appear to validate this accretion model.