Editing Dwarf star

{{short description|Star of relatively small size and low luminosity}}
{{confused|compact star}}

[[File:H-R diagram.svg|thumb|The [[Hertzsprung–Russell diagram]] showing the location of [[main sequence]] dwarf stars and [[white dwarfs]]]]
A '''dwarf star''' is a [[star]] of relatively small size and low [[luminosity]]. Most [[main sequence stars]] are dwarf stars. The meaning of the word "dwarf" was later extended to some star-sized objects that are not stars, and compact stellar remnants that are no longer stars.

==History==
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 [[Stellar classification#Harvard spectral classification|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 star|giant]]" and "dwarf" stars,<ref name=Brown-Pais-Pippard-1995/> 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 star|type&nbsp;O]]'', to the coolest: ''[[Red dwarf|type&nbsp;M]]''.

With the development of [[infrared astronomy]] in the late 20th&nbsp;century the Morgan Keenan system was extended to cooler types&nbsp;L and T, all of which are "dwarfs" but not all of which are  stars as such.

==Current uses of the term "dwarf"==
[[File: LAWD 37 (heic2301a).jpg|thumb|[[LAWD 37]], a [[white dwarf]] star that has had its mass measured by [[microlensing]]]]
The scope of the term "dwarf" at present includes the following:

* ''Dwarf star'' with no other qualification generally refers to a [[main sequence|main-sequence]] star, a star of [[Stellar classification#Yerkes spectral classification|luminosity class]] V: [[main sequence|main-sequence]] stars (dwarfs). Example: [[Achernar]] (B6Vep)<ref name=Nazé-2009-11/>
** ''[[Red dwarf]]s'' are low-mass main-sequence stars.
** ''[[G-type main-sequence star|Yellow dwarfs]]'' are main-sequence (dwarf) stars with masses comparable to that of the [[Sun]].
** ''[[Orange dwarf]]s'' are [[K-type main-sequence star]]s.
** Blue ([[O-type star|type&nbsp;O]] and [[B-type main-sequence star|type&nbsp;B]]) [[main sequence]] stars are so large that they are difficult to distinguish from blue [[giant star]]s, either in size or brightness, and because of the cognitive dissonance, the word "dwarf" is avoided when referring to them.                                                      
* A ''[[Blue dwarf (red dwarf stage)|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. It is believed that the universe is not old enough for any red dwarf to have yet reached the so-called "blue" stage, which is actually more of a medium white.
* A ''[[white dwarf]]'' is the remains of a dead star, composed of [[electron-degenerate matter]]. It is thought to be the final stage in the evolution of stars not massive enough to collapse into a [[neutron star]] or [[black hole]] – stars less massive than roughly 9&nbsp;{{solar mass|link=y}}.
** A ''[[black dwarf]]'' is theorized as a white dwarf that has cooled sufficiently that it no longer emits any visible light. It is believed that the universe is not old enough for any white dwarf to have yet cooled to "black".
* 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&nbsp;&nbsp;{{solar mass|link=y}} and more than about 13&nbsp;Jupiter masses.

==See also==
* [[Chandrasekhar limit]]
* [[Dwarf planet]]
* [[Stellar classification]]
* [[Sub-brown dwarf]]
* [[Subdwarf]]
* [[Ultra-cool dwarf]]

==References==
{{reflist|25em|refs=

<ref name=Brown-Pais-Pippard-1995>
{{cite book
 | editor1-first=Laurie M. | editor1-last=Brown
 | editor2-first=Abraham   | editor2-last=Pais     | editor2-link=Abraham Pais
 | editor3-first=A.B.      | editor3-last=Pippard  | editor3-link=A. B. Pippard
 | year=1995
 | title=Twentieth Century Physics
 | page=1696
 | location=[[Bristol]], UK; New York, NY
 | publisher=[[Institute of Physics]], [[American Institute of Physics]]
 | isbn=0-7503-0310-7 |oclc=33102501
}}
</ref>

<ref name=Nazé-2009-11>{{cite journal
 |last1=Nazé
 |first1=Y.
 |date=November 2009
 |title=Hot stars observed by XMM-Newton. I. The catalog and the properties of OB stars
 |journal=Astronomy and Astrophysics
 |volume=506
 |issue=2
 |pages=1055–1064
 |doi=10.1051/0004-6361/200912659
 |bibcode=2009A&A...506.1055N
 |arxiv=0908.1461
 |s2cid=17317459
 |url=http://orbi.ulg.ac.be/jspui/handle/2268/6224
 }}{{Dead link|date=February 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

}}

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[[Category:Star types]]