Editing Polaris (section)

===Variability===
[[File:AlphaUMiLightCurve.png|thumb|left|A [[light curve]] for Polaris, plotted from ''[[Transiting Exoplanet Survey Satellite|TESS]]'' data<ref name=MAST>{{cite web |title=MAST: Barbara A. Mikulski Archive for Space Telescopes |url=https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html |publisher=Space Telescope Science Institute |access-date=8 December 2021}}</ref>]]
Polaris Aa, the supergiant primary component, is a low-amplitude [[Population I]] [[classical Cepheid variable]], although it was once thought to be a [[type II Cepheid]] due to its high [[galactic latitude]]. Cepheids constitute an important [[standard candle]] for determining distance, so Polaris, as the closest such star,<ref name=Anderson2019/> is heavily studied. The [[Variable star|variability]] of Polaris had been suspected since 1852; this variation was confirmed by [[Ejnar Hertzsprung]] in 1911.<ref>{{cite journal
 | last1=Hertzsprung | first1=Ejnar
 | title=Nachweis der Veränderlichkeit von α Ursae Minoris
 | journal=Astronomische Nachrichten | volume=189 | page=89
 | date=August 1911 | language=de
 | doi=10.1002/asna.19111890602 | bibcode=1911AN....189...89H
 | issue=6 | url=https://zenodo.org/record/1424878
 }}</ref>

The range of brightness of Polaris is given as 1.86–2.13,<ref name=gcvs/> but the amplitude has changed since discovery. Prior to 1963, the amplitude was over 0.1 magnitude and was very gradually decreasing. After 1966, it very rapidly decreased until it was less than 0.05 magnitude; since then, it has erratically varied near that range. It has been reported that the amplitude is now increasing again, a reversal not seen in any other Cepheid.<ref name=lee/>

[[File:Integrated Flux Nebula Surrounding Polaris - Kush Chandaria.jpg|thumb|left|Polaris and its surrounding [[Integrated Flux Nebula|integrated flux nebula]]]]
The period, roughly 4 days, has also changed over time. It has steadily increased by around 4.5 seconds per year except for a hiatus in 1963–1965. This was originally thought to be due to secular redward (a long term change in [[redshift]] that causes light to stretch into longer wavelengths, causing it to appear red) evolution across the Cepheid [[instability strip]], but it may be due to interference between the primary and the first-[[overtone]] pulsation modes.<ref name=evans>{{Cite journal | last1 = Evans | first1 = N. R. | last2 = Sasselov | first2 = D. D. | last3 = Short | first3 = C. I. | doi = 10.1086/338583 | title = Polaris: Amplitude, Period Change, and Companions | journal = The Astrophysical Journal | volume = 567 | issue = 2 | pages = 1121 | year = 2002 |bibcode = 2002ApJ...567.1121E | doi-access = free }}</ref><ref name=turner>{{Cite journal | last1 = Turner | first1 = D. G. | last2 = Savoy | first2 = J. | last3 = Derrah | first3 = J. | last4 = Abdel-Sabour Abdel-Latif | first4 = M. | last5 = Berdnikov | first5 = L. N. | title = The Period Changes of Polaris | doi = 10.1086/427838 | journal = Publications of the Astronomical Society of the Pacific | volume = 117 | issue = 828 | pages = 207 | year = 2005 |bibcode = 2005PASP..117..207T | doi-access = free }}</ref><ref name=neilson>{{Cite journal | last1 = Neilson | first1 = H. R. | last2 = Engle | first2 = S. G. | last3 = Guinan | first3 = E. | last4 = Langer | first4 = N. | last5 = Wasatonic | first5 = R. P. | last6 = Williams | first6 = D. B. | doi = 10.1088/2041-8205/745/2/L32 | title = The Period Change of the Cepheid Polaris Suggests Enhanced Mass Loss | journal = The Astrophysical Journal | volume = 745 | issue = 2 | pages = L32 | year = 2012 |arxiv = 1201.0761 |bibcode = 2012ApJ...745L..32N | s2cid = 118625176 }}</ref> Authors disagree on whether Polaris is a fundamental or first-overtone pulsator and on whether it is crossing the instability strip for the first time or not.<ref name=fadeyev/><ref name=neilson/><ref name=engle>{{cite journal|doi=10.3847/2515-5172/aad2d0|title=Toward Ending the Polaris Parallax Debate: A Precise Distance to Our Nearest Cepheid from Gaia DR2|journal=Research Notes of the AAS|volume=2|issue=3|pages=126|year=2018|last1=Engle|first1=Scott G|last2=Guinan|first2=Edward F|last3=Harmanec|first3=Petr|bibcode=2018RNAAS...2..126E|s2cid=126329676 |doi-access=free }}</ref>

The temperature of Polaris varies by only a small amount during its pulsations, but the amount of this variation is variable and unpredictable. The erratic changes of temperature and the amplitude of temperature changes during each cycle, from less than 50&nbsp;[[kelvin|K]] to at least 170&nbsp;K, may be related to the orbit with Polaris Ab.<ref name=usenko2005/>

Research reported in ''[[Science (journal)|Science]]'' suggests that Polaris is 2.5 times brighter today than when [[Ptolemy]] observed it, changing from third to second magnitude.<ref>{{cite journal|doi=10.1126/science.304.5678.1740b|pmid=15205508|year=2004|last1=Irion|first1=R|title=American Astronomical Society meeting. As inconstant as the Northern Star|journal=Science|volume=304|issue=5678|pages=1740–1|s2cid=129246155}}</ref> Astronomer [[Edward Guinan]] considers this to be a remarkable change and is on record as saying that "if they are real, these changes are 100 times larger than [those] predicted by current theories of [[Stellar Evolution|stellar evolution]]".

In 2024, researchers led by Nancy Evans at the [[Harvard–Smithsonian Center for Astrophysics|Harvard & Smithsonian]], have studied with more accuracy the Polaris' smaller companion orbit using the [[CHARA Array]]. During this observation campaign they have succeeded in shooting Polaris features on its surface; large bright places and dark ones have appeared in close-up images, changing over time. Further, Polaris diameter size has been re-measured to {{solar radius|46}}, using the ''Gaia'' distance of {{val|446|1}} light-years, and its mass was determined at {{solar mass|5.13}}.<ref name=evans2024>{{Cite journal |last1=Evans |first1=Nancy Remage |last2=Schaefer |first2=Gail H. |last3=Gallenne |first3=Alexandre |last4=Torres |first4=Guillermo |last5=Horch |first5=Elliott P. |last6=Anderson |first6=Richard I. |last7=Monnier |first7=John D. |last8=Roettenbacher |first8=Rachael M. |last9=Baron |first9=Fabien |last10=Anugu |first10=Narsireddy |last11=Davidson |first11=James W. |last12=Kervella |first12=Pierre |last13=Bras |first13=Garance |last14=Proffitt |first14=Charles |last15=Mérand |first15=Antoine |date=2024-08-01 |title=The Orbit and Dynamical Mass of Polaris: Observations with the CHARA Array |bibcode=2024ApJ...971..190E |journal=The Astrophysical Journal |volume=971 |issue=2 |pages=190 |doi=10.3847/1538-4357/ad5e7a |doi-access=free |arxiv=2407.09641 |issn=0004-637X}}</ref>