Dwarf star

A dwarf star is a star of relatively small size and low luminosity. Most main sequence stars are dwarf stars. The term was originally coined in 1906 when the Danish astronomer Ejnar Hertzsprung noticed that the reddest stars—classified as K and M in the Harvard scheme could be divided into two distinct groups. They are either much brighter than the Sun, or much fainter. To distinguish these groups, he called them "giant" and "dwarf" stars,^[1] the dwarf stars being fainter and the giants being brighter than the Sun. Most stars are currently classified under the Morgan Keenan System using the letters O, B, A, F, G, K, and M, a sequence from the hottest: O type, to the coolest: M type. The scope of the term "dwarf" was later expanded to include the following:

• Dwarf star alone generally refers to any main-sequence star, a star of luminosity class V: main-sequence stars (dwarfs). Example: Achernar (B6Vep)^[2]
  • Red dwarfs are low-mass main-sequence stars.
  • Yellow dwarfs are main-sequence (dwarf) stars with masses comparable to that of the Sun.
  • Orange dwarfs are K-type main-sequence stars.
• A blue dwarf is a hypothesized class of very-low-mass stars that increase in temperature as they near the end of their main-sequence lifetime.
• A white dwarf is a star composed of electron-degenerate matter, thought to be the final stage in the evolution of stars not massive enough to collapse into a neutron star or undergo a Type II supernova—stars less massive than roughly 9 solar masses.
  • A black dwarf is a white dwarf that has cooled sufficiently such that it no longer emits any visible light.
• A brown dwarf is a substellar object not massive enough to ever fuse hydrogen into helium, but still massive enough to fuse deuterium—less than about 0.08 solar masses and more than about 13 Jupiter masses.

See also

• Sub-brown dwarf
• Subdwarf star

References

Star
Evolution	Formation Pre-main-sequence star Main sequence Red giant branch Horizontal branch Asymptotic giant branch Dredge-up Instability strip Red clump PG1159 star Mira variable Planetary nebula Protoplanetary nebula Luminous red nova Luminous blue variable Wolf–Rayet star Stellar population Supernova Supernova impostor Hypernova Hertzsprung–Russell diagram Color–color diagram
Protostars	Molecular cloud H II region Bok globule Young stellar object Herbig–Haro object Hayashi track Henyey track Orion T Tauri FU Orionis Herbig Ae/Be
Luminosity class	Subdwarf Dwarf Blue Red White Yellow Black Brown Subgiant Giant Blue Red Bright giant Supergiant Blue Red Yellow Hypergiant Yellow Blue straggler Variable star
Spectral classification	O B A F G K M Be OB Subdwarf O Subdwarf B Late-type Chemically peculiar Am Ap/Bp Oscillating Barium Carbon CH CN Extreme helium Lambda Boötis Lead Mercury-manganese S Shell Technetium
Remnants	White dwarf Black dwarf Helium planet Neutron star Pulsar Binary X-ray X-ray binary X-ray burster Magnetar Stellar black hole Compact star Quark Strange Preon Planck Exotic Stellar core: EF Eridani B
Theoretical stars	Dark-matter star Dark-energy star Black star Gravastar Frozen star Q star Quasi-star Thorne–Żytkow object Iron star Blitzar
Nucleosynthesis	Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Carbon burning Neon burning Oxygen burning Alpha process Silicon burning S-process R-process Fusor Nova Remnants
Structure	Core Convection zone Microturbulence Oscillations Radiation zone Atmosphere Photosphere Starspot Chromosphere Corona Stellar wind Bubble Asteroseismology Eddington luminosity Kelvin–Helmholtz mechanism
Properties	Designation Dynamics Effective temperature Kinematics Magnetic field Magnitude Absolute Mass Metallicity Rotation UBV color Variability
Star systems	Binary Contact Common envelope Multiple Accretion disk Star cluster Open cluster Globular cluster Super star cluster Planetary system Earth's Solar System
Earth-centric observation of	Pole star Circumpolar star Magnitude Apparent Extinction Photographic Color Radial velocity Proper motion Parallax Photometric-standard star
Lists	Star names Arabic Chinese Extremes Most massive Highest temperature Largest volume Smallest volume Brightest Historical Most luminous Nearest Nearest bright Stars with exoplanets Brown dwarfs White dwarfs Milky Way novae Notable supernovae Supernova candidates Supernova remnants Planetary nebulae Timeline of stellar astronomy
Related articles	Substellar object Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Supercluster Helioseismology Guest star Constellation Asterism Gravity Intergalactic star Infrared dark cloud Starfield
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