Editing Stellar classification (section)

====Class Y====
{{See also|Y dwarf}}
[[File:WISE 1828+2650 Brown dwarf.jpg|thumb|Artist's impression of a Y-dwarf]]

Brown dwarfs of spectral class&nbsp;Y are cooler than those of spectral class&nbsp;T and have qualitatively different spectra from them. A total of 17&nbsp;objects have been placed in class&nbsp;Y as of August 2013.<ref>{{cite journal |bibcode=2013ApJ...776..128K |arxiv=1308.5372 |title=Discovery of the Y1 Dwarf WISE J064723.23-623235.5 |journal=The Astrophysical Journal |volume=776 |issue=2 |pages=128 |last1=Kirkpatrick |first1=J. Davy |last2=Cushing |first2=Michael C. |last3=Gelino |first3=Christopher R. |last4=Beichman |first4=Charles A. |last5=Tinney |first5=C. G. |last6=Faherty |first6=Jacqueline K. |author6-link=Jackie Faherty|last7=Schneider |first7=Adam |last8=Mace |first8=Gregory N. |year=2013 |doi=10.1088/0004-637X/776/2/128|s2cid=6230841 }}</ref> Although such dwarfs have been modelled<ref>{{cite journal |arxiv=astro-ph/0607305 |title=Y-Spectral class for Ultra-Cool Dwarfs |journal=Monthly Notices of the Royal Astronomical Society|volume=371 |pages=1722–1730 |first1=N. R. |last1=Deacon |first2=N. C. |last2=Hambly |year=2006 |doi=10.1111/j.1365-2966.2006.10795.x |doi-access=free |s2cid=14081778 }}</ref> and detected within forty light-years by the [[Wide-field Infrared Survey Explorer]] (WISE)<ref name=nasa1010>{{Cite web|url=https://science.nasa.gov/science-news/science-at-nasa/2011/23aug_coldeststars/|title=Discovered: Stars as Cool as the Human Body &#124; Science Mission Directorate|website=science.nasa.gov|access-date=12 July 2017|archive-date=7 October 2011|archive-url=https://web.archive.org/web/20111007124027/http://science.nasa.gov/science-news/science-at-nasa/2011/23aug_coldeststars/|url-status=dead}}</ref><ref name="stars-cooler-than-body">{{cite web |last=Wehner |first=Mike |url=http://ca.news.yahoo.com/blogs/technology-blog/nasa-spots-chilled-stars-cooler-human-body-004551421.html |title=NASA spots chilled-out stars cooler than the human body &#124; Technology News Blog – Yahoo! News Canada |publisher=Ca.news.yahoo.com |date=24 August 2011 |access-date=22 May 2012}}</ref><ref name="cool-y-dwarf">{{Cite magazine|url=https://www.wired.com/2011/08/y-dwarf-stars/|title=NASA Satellite Finds Coldest, Darkest Stars Yet|magazine=Wired|first=Danielle|last=Venton|date=23 August 2011|via=www.wired.com}}</ref><ref>{{Cite web|url=https://www.nasa.gov/mission_pages/WISE/news/wise20110823.html|title=NASA - NASA'S Wise Mission Discovers Coolest Class of Stars|website=www.nasa.gov|access-date=1 November 2019|archive-date=14 February 2021|archive-url=https://web.archive.org/web/20210214132907/https://www.nasa.gov/mission_pages/WISE/news/wise20110823.html|url-status=dead}}</ref><ref>{{cite journal |doi=10.1051/0004-6361:200810038 |bibcode=2009A&A...493.1149Z |title=The minimum Jeans mass, brown dwarf companion IMF, and predictions for detection of Y-type dwarfs |journal=Astronomy and Astrophysics |volume=493 |issue=3 |pages=1149–1154 |year=2009 |last1=Zuckerman |first1=B. |last2=Song |first2=I. |arxiv=0811.0429|s2cid=18147550 }}</ref> there is no well-defined spectral sequence yet and no prototypes. Nevertheless, several objects have been proposed as spectral classes Y0, Y1, and Y2.<ref name=ydwarfsurvey>{{cite journal |arxiv=1309.1422 |doi=10.1126/science.1241917 |pmid=24009359 |title=Distances, Luminosities, and Temperatures of the Coldest Known Substellar Objects |journal=Science |volume=341 |issue=6153 |pages=1492–5 |year=2013 |last1=Dupuy |first1=T. J. |last2=Kraus |first2=A. L. |bibcode=2013Sci...341.1492D|s2cid=30379513 }}</ref>

The spectra of these prospective Y objects display absorption around 1.55&nbsp;[[micrometers]].<ref name=four600k>{{cite journal |doi=10.1088/0004-637X/695/2/1517 |last1=Leggett |first1=Sandy K. |last2=Cushing |first2=Michael C. |last3=Saumon |first3=Didier |last4=Marley |first4=Mark S. |last5=Roellig |first5=Thomas L. |last6=Warren |first6=Stephen J. |last7=Burningham |first7=Ben |last8=Jones |first8=Hugh R. A. |last9=Kirkpatrick |first9=J. Davy |last10=Lodieu |first10=Nicolas |last11=Lucas |first11=Philip W. |last12=Mainzer |first12=Amy K. |last13=Martín |first13=Eduardo L. |last14=McCaughrean |first14=Mark J. |last15=Pinfield |first15=David J. |last16=Sloan |first16=Gregory C. |last17=Smart |first17=Richard L. |last18=Tamura |first18=Motohide |last19=Van Cleve |first19=Jeffrey E. |title=The Physical Properties of Four ~600 K T Dwarfs |date=2009 |journal=[[The Astrophysical Journal]] |volume=695 |issue=2 |pages=1517–1526 |arxiv=0901.4093 |bibcode=2009ApJ...695.1517L |s2cid=44050900 }}</ref> Delorme et al. have suggested that this feature is due to absorption from [[ammonia]], and that this should be taken as the indicative feature for the T-Y transition.<ref name=four600k/><ref name=tytrans>{{cite journal |doi=10.1051/0004-6361:20079317 |bibcode=2008A&A...482..961D |title=CFBDS J005910.90-011401.3: Reaching the T-Y brown dwarf transition? |journal=Astronomy and Astrophysics |volume=482 |issue=3 |pages=961–971 |year=2008 |last1=Delorme |first1=Philippe |last2=Delfosse |first2=Xavier |last3=Albert |first3=Loïc |last4=Artigau |first4=Étienne |last5=Forveille |first5=Thierry |last6=Reylé |first6=Céline |last7=Allard |first7=France |last8=Homeier |first8=Derek |last9=Robin |first9=Annie C. |last10=Willott |first10=Chris J. |last11=Liu |first11=Michael C. |last12=Dupuy |first12=Trent J. |arxiv=0802.4387 |s2cid=847552 }}</ref> In fact, this ammonia-absorption feature is the main criterion that has been adopted to define this class.<ref name=ydwarfsurvey/> However, this feature is difficult to distinguish from absorption by [[water]] and [[methane]],<ref name=four600k/> and other authors have stated that the assignment of class Y0 is premature.<ref>{{cite journal |doi=10.1111/j.1365-2966.2008.13885.x |bibcode=2008MNRAS.391..320B |title=Exploring the substellar temperature regime down to ~550 K |journal=Monthly Notices of the Royal Astronomical Society |volume=391 |issue=1 |pages=320–333 |year=2008 |last1=Burningham |first1=Ben |last2=Pinfield |first2=D. J. |last3=Leggett |first3=S. K. |last4=Tamura |first4=M. |last5=Lucas |first5=P. W. |last6=Homeier |first6=D. |last7=Day-Jones |first7=A. |last8=Jones |first8=H. R. A. |last9=Clarke |first9=J. R. A.|last10=Ishii|first10=M. |last11=Kuzuhara |first11=M. |last12=Lodieu |first12=N. |last13=Zapatero-Osorio |first13=María Rosa |last14=Venemans |first14=B. P. |last15=Mortlock |first15=D. J. |last16=Barrado y Navascués |first16=D. |last17=Martin |first17=Eduardo L. |last18=Magazzù |first18=Antonio |doi-access=free |arxiv=0806.0067|s2cid=1438322 }}</ref>

The latest brown dwarf proposed for the Y spectral type, [[WISE 1828+2650]], is a >&nbsp;Y2&nbsp;dwarf with an effective temperature originally estimated around 300&nbsp;[[kelvin|K]], the temperature of the human body.<ref name="stars-cooler-than-body"/><ref name="cool-y-dwarf"/><ref name=eso1110>[[European Southern Observatory]]. [http://www.eso.org/public/news/eso1110/ "A Very Cool Pair of Brown Dwarfs"], 23 March 2011</ref> [[Parallax]] measurements have, however, since shown that its luminosity is inconsistent with it being colder than ~400&nbsp;K. The coolest Y dwarf currently known is [[WISE 0855−0714]] with an approximate temperature of 250&nbsp;K, and a mass just seven times that of Jupiter.<ref name="Luhman2016">{{cite journal |title=The Spectral Energy Distribution of the Coldest Known Brown Dwarf |arxiv=1605.06655 |first1=Kevin L. |last1=Luhman |first2=Taran L. |last2=Esplin |date=May 2016 |doi=10.3847/0004-6256/152/3/78 |volume=152 |issue=3 |journal=The Astronomical Journal |page=78 |bibcode=2016AJ....152...78L|s2cid=118577918 |doi-access=free }}</ref>

The mass range for Y&nbsp;dwarfs is 9–25&nbsp;[[Jupiter]] masses, but young objects might reach below one Jupiter mass (although they cool to become planets), which means that Y class objects straddle the 13&nbsp;Jupiter mass [[deuterium]]-fusion limit that marks the current [[International Astronomical Union|IAU]] division between brown dwarfs and planets.<ref name=ydwarfsurvey/>