Editing Polaris (section)

==Stellar system==
[[File:Polaris alpha ursae minoris.jpg|thumb|left|upright=1.2|Polaris components as seen by the [[Hubble Space Telescope]]]]
Polaris Aa is an [[stellar evolution|evolved]] [[yellow supergiant]] of [[stellar classification|spectral type]] F7Ib with 5.4 [[solar mass]]es ({{Solar mass|link=y}}). It is the first classical [[Cepheid]] to have a mass determined from its orbit. The two smaller companions are Polaris B, a {{Solar mass|1.39}} F3 [[main sequence|main-sequence]] star orbiting at a distance of {{val|2400|ul=astronomical units|fmt=commas}} (AU),<ref name=Wielen>{{cite journal|bibcode=2000A&A...360..399W|arxiv = astro-ph/0002406 |title = Polaris: Astrometric orbit, position, and proper motion |journal = Astronomy and Astrophysics |volume = 360 |pages = 399–410 [400–402, 406] |last1 = Wielen |first1 = R. |last2 = Jahreiß |first2 = H. |last3 = Dettbarn |first3 = C. |last4 = Lenhardt |first4 = H. |last5 = Schwan |first5 = H. |year = 2000 }}</ref> and Polaris Ab (or P), a very close F6 main-sequence star with a mass of {{Solar mass|1.26}}.<ref name=hst/> Polaris B can be resolved with a modest telescope. William Herschel discovered the star in August 1779 using a [[reflecting telescope]] of his own,<ref>{{cite book
 | title=An Anthology of Visual Double Stars
 | display-authors=1 | first1=Bob | last1=Argyle
 | first2=Mike | last2=Swan | first3=Andrew | last3=James
 | date=August 29, 2019 | page=265 | isbn=9781108601702 
 | publisher=Cambridge University Press
 | url=https://books.google.com/books?id=jSmqDwAAQBAJ&pg=PT265
}}</ref> one of the best telescopes of the time. In January 2006, [[NASA]] released images, from the [[Hubble Space Telescope|Hubble telescope]], that showed the three members of the Polaris ternary system.<ref>{{cite web|url=https://hubblesite.org/contents/news-releases/2006/news-2006-02.html |title=There's More to the North Star Than Meets the Eye |publisher=Hubblesite.org |date=2006-01-09 |access-date=2020-02-27}}</ref><ref name=evans/>

The variable [[radial velocity]] of Polaris A was reported by [[William Wallace Campbell|W. W. Campbell]] in 1899, which suggested this star is a binary system.<ref>{{cite journal
 | title=On the variable velocity of Polaris in the line of sight
 | last=Campbell | first=W. W.
 | journal=Publications of the Astronomical Society of the Pacific
 | volume=11 | pages=195–199| date=October 1899
 | doi=10.1086/121339 | bibcode=1899PASP...11..195C
| s2cid=122429136 }}</ref> Since Polaris A is a known cepheid variable, [[Joseph Haines Moore|J. H. Moore]] in 1927 demonstrated that the changes in velocity along the line of sight were due to a combination of the four-day [[stellar pulsation|pulsation]] period combined with a much longer [[orbital period]] and a large [[orbital eccentricity|eccentricity]] of around 0.6.<ref>{{cite journal
 | title=Note on the Longitude of the Lick Observatory
 | last=Moore | first=J. H.
 | journal=Publications of the Astronomical Society of the Pacific
 | volume=39 | issue=230 | page=249 | date=August 1927
 | doi=10.1086/123734 | bibcode=1927PASP...39..249M
| s2cid=119469812 | doi-access=free }}</ref> Moore published preliminary [[orbital elements]] of the system in 1929, giving an [[orbital period]] of about 29.7 years with an eccentricity of 0.63. This period was confirmed by [[proper motion]] studies performed by [[Boris Gerasimovich|B. P. Gerasimovič]] in 1939.<ref name=Roemer1965>{{cite journal
 | title=Orbital Motion of Alpha Ursae Minoris from Radial Velocities
 | last=Roemer | first=Elizabeth
 | journal=Astrophysical Journal
 | volume=141 | page=1415 | date=May 1965
 | doi=10.1086/148230 | bibcode=1965ApJ...141.1415R
| doi-access=free}}</ref>

As part of her doctoral thesis, in 1955 [[Elizabeth Roemer|E. Roemer]] used radial velocity data to derive an orbital period of 30.46&nbsp;y for the Polaris A system, with an eccentricity of 0.64.<ref>{{cite journal
 | title=Parallax and orbital motion of spectroscopic binary Polaris from photographs taken with the 24-inch Sproul refractor.
 | last=Wyller | first=A. A.
 | journal=Astronomical Journal
 | volume=62 | pages=389–393 | date=December 1957
 | doi=10.1086/107559 | bibcode=1957AJ.....62..389W }}</ref> [[Karl Walter Kamper|K. W. Kamper]] in 1996 produced refined elements with a period of {{val|29.59|0.02|u=years}} and an eccentricity of {{val|0.608|0.005}}.<ref name=Kamper1996>{{cite journal
 | title=Polaris Today
 | last=Kamper | first=Karl W.
 | journal=Journal of the Royal Astronomical Society of Canada
 | volume=90 | page=140 | date=June 1996
 | bibcode=1996JRASC..90..140K }}</ref> In 2019, a study by R. I. Anderson gave a period of {{val|29.32|0.11|u=years}} with an eccentricity of {{val|0.620|0.008}}.<ref name=Anderson2019>{{cite journal
 | title=Probing Polaris' puzzling radial velocity signals. Pulsational (in-)stability, orbital motion, and bisector variations
 | last=Anderson | first=R. I.
 | journal=Astronomy & Astrophysics
 | volume=623 | id=A146 | pages=17 | date=March 2019
 | doi=10.1051/0004-6361/201834703 | arxiv=1902.08031
 | bibcode=2019A&A...623A.146A | s2cid=119467242 }}</ref>

There were once thought to be two more widely separated components—Polaris C and Polaris D—but these have been shown not to be physically associated with the Polaris system.<ref name=Wielen/><ref>{{cite journal|bibcode=2010AJ....139.1968E|title=Chandra Observation of Polaris: Census of Low-mass Companions|journal=The Astronomical Journal|volume=139|issue=5|pages=1968|last1=Evans|first1=Nancy Remage|last2=Guinan|first2=Edward|last3=Engle|first3=Scott|last4=Wolk|first4=Scott J.|last5=Schlegel|first5=Eric|last6=Mason|first6=Brian D.|last7=Karovska|first7=Margarita|last8=Spitzbart|first8=Bradley|year=2010|doi=10.1088/0004-6256/139/5/1968|doi-access=free}}</ref>