Editing Red dwarf (section)

==Definition==
The term "red dwarf" when used to refer to a star does not have a strict definition. One of the earliest uses of the term was in 1915, used simply to contrast "red" dwarf stars from hotter "blue" dwarf stars.<ref name=lindemann>{{cite journal|bibcode=1915Obs....38..299L|title=The age of the Earth|journal=The Observatory|volume=38|pages=299|last1=Lindemann|first1=F. A.|year=1915}}</ref> It became established use, although the definition remained vague.<ref name=edgeworth>{{cite journal|bibcode=1946Natur.157..481E|title=Red Dwarf Stars|journal=Nature|volume=157|issue=3989|pages=481|last1=Edgeworth|first1=K. E.|year=1946|doi=10.1038/157481d0|s2cid=4106298|doi-access=free}}</ref> In terms of which spectral types qualify as red dwarfs, different researchers picked different limits, for example K8–M5<ref name=dyer>{{cite journal|bibcode=1956AJ.....61..228D|title=An analysis of the space motions of red dwarf stars|journal=Astronomical Journal|volume=61|pages=228|last1=Dyer|first1=Edward R.|year=1956|doi=10.1086/107332}}</ref> or "later than K5".<ref name=mumford>{{cite journal|bibcode=1956AJ.....61..224M|title=The motions and distribution of dwarf M stars|journal=Astronomical Journal|volume=61|pages=224|last1=Mumford|first1=George S.|year=1956|doi=10.1086/107331|doi-access=free}}</ref>  ''Dwarf M star'', abbreviated dM, was also used, but sometimes it also included stars of spectral type K.<ref name=Vyssotsky>{{cite journal|bibcode=1956AJ.....61..201V|title=Dwarf M stars found spectrophotometrically|journal=Astronomical Journal|volume=61|pages=201|last1=Vyssotsky|first1=A. N.|year=1956|doi=10.1086/107328}}</ref>

In modern usage, the definition of a ''red dwarf'' still varies. When explicitly defined, it typically includes [[Late-type star|late K-]] and early to mid-M-class stars,<ref name=engle>{{cite journal|bibcode=2011ASPC..451..285E|arxiv=1111.2872|title=Red Dwarf Stars: Ages, Rotation, Magnetic Dynamo Activity and the Habitability of Hosted Planets|journal=9th Pacific Rim Conference on Stellar Astrophysics. Proceedings of a Conference Held at Lijiang|volume=451|pages=285|last1=Engle|first1=S. G.|last2=Guinan|first2=E. F.|year=2011}}</ref> but in many cases it is restricted just to M-class stars.<ref name=heath>{{cite journal|doi=10.1023/A:1006596718708|pmid=10472629|year=1999|last1=Heath|first1=Martin J.|title=Habitability of planets around red dwarf stars|journal=Origins of Life and Evolution of the Biosphere|volume=29|issue=4|pages=405–24|last2=Doyle|first2=Laurance R.|last3=Joshi|first3=Manoj M.|last4=Haberle|first4=Robert M.|bibcode=1999OLEB...29..405H|s2cid=12329736|doi-access=free}}</ref><ref name=farihi>{{cite journal|bibcode=2006ApJ...646..480F|arxiv=astro-ph/0603747|title=White Dwarf-Red Dwarf Systems Resolved with the Hubble Space Telescope. I. First Results|journal=The Astrophysical Journal|volume=646|issue=1|pages=480–492|last1=Farihi|first1=J.|last2=Hoard|first2=D. W.|last3=Wachter|first3=S.|year=2006|doi=10.1086/504683|s2cid=16750158}}</ref>  In some cases all K stars are included as red dwarfs,<ref name=petterson>{{cite journal|bibcode=1989A&A...217..187P|title=A spectroscopic survey of red dwarf flare stars|journal=Astronomy and Astrophysics|volume=217|pages=187|last1=Pettersen|first1=B. R.|last2=Hawley|first2=S. L.|year=1989}}</ref> and occasionally even earlier stars.<ref name=alekseev>{{cite journal|bibcode=2002A&A...396..203A|title=Starspots and active regions on the emission red dwarf star LQ Hydrae|journal=Astronomy and Astrophysics|volume=396|pages=203–211|last1=Alekseev|first1=I. Yu.|last2=Kozlova|first2=O. V.|year=2002|doi=10.1051/0004-6361:20021424|doi-access=free}}</ref>

The most recent surveys place the coolest true main-sequence stars into spectral types L2 or L3. At the same time, many objects cooler than about M6 or M7 are brown dwarfs, insufficiently massive to sustain [[Isotopes of hydrogen#Hydrogen-1 (Protium)|hydrogen-1]] fusion.<ref name=dietrich/> This gives a significant overlap in spectral types for red and brown dwarfs. Objects in that spectral range can be difficult to categorize.