Editing Betelgeuse (section)

=== Distance measurements ===
[[File:USA.NM.VeryLargeArray.02.jpg|thumb|right|[[National Radio Astronomy Observatory|NRAO]]'s [[Very Large Array]] used to derive Betelgeuse's 2008 distance estimate]]

[[Parallax]] is the apparent change of the position of an object, measured in seconds of arc, caused by the change of position of the observer of that object. [[Parallax in astronomy|Parallax is used in astronomy]] to estimate distances to the nearest stars. As the Earth orbits the Sun, every star is seen to shift by a fraction of an arc second, which measure, combined with the baseline provided by the Earth's orbit gives the distance to that star. Since the first successful [[parallax]] measurement by [[Friedrich Bessel]] in 1838, astronomers have been puzzled by Betelgeuse's apparent distance. Knowledge of the star's distance improves the accuracy of other stellar parameters, such as [[Stellar luminosity|luminosity]] that, when combined with an angular diameter, can be used to calculate the physical radius and [[effective temperature]]; luminosity and [[Natural abundance|isotopic abundances]] can also be used to estimate the [[Stellar evolution|stellar age]] and [[Intermediate mass star|mass]].<ref name="HARPER">
{{cite journal
 | last1=Harper |first1=Graham M.
 | last2=Brown  |first2=Alexander
 | last3=Guinan |first3=Edward F.
 | title=A new VLA-Hipparcos distance to Betelgeuse and its implications
 | journal=The Astronomical Journal
 | date=April 2008
 | volume=135 | issue=4 | pages=1430–40
 | bibcode=2008AJ....135.1430H
 | doi=10.1088/0004-6256/135/4/1430 | doi-access=free
 }}</ref>

When the first interferometric studies were performed on the star's diameter in 1920, the assumed parallax was {{val|0.0180|ul="}}. This equated to a distance of {{val|56|ul=pc}} or roughly {{val|180|ul=ly}}, producing not only an inaccurate radius for the star but every other stellar characteristic. Since then, there has been ongoing work to measure the distance of Betelgeuse, with proposed distances as high as {{val|400|u=pc}} or about {{val|1300|u=ly|fmt=commas}}.<ref name="HARPER" />

Before the publication of the [[Hipparcos Catalogue]] (1997), there were two slightly conflicting parallax measurements for Betelgeuse. The first, in 1991, gave a parallax of {{val|9.8|4.7|ul=mas}}, yielding a distance of roughly {{val|102|u=pc}} or {{val|330|u=ly}}.<ref name=YALEPLX>
{{cite journal
 | last1=van&nbsp;Altena |first1=W.F.
 | last2=Lee  |first2=J.T.
 | last3=Hoffleit |first3=D.
 | title=Yale Trigonometric Parallaxes Preliminary
 | journal=Yale University Observatory (1991)
 | date=October 1995
 | bibcode=1995yCat.1174....0V
 | volume=1174
 | page=0
}}
</ref> The second was the [[Hipparcos#Hipparcos Input Catalogue|Hipparcos Input Catalogue]] (1993) with a trigonometric parallax of {{val|5|4|u=mas}}, a distance of {{val|200|u=pc}} or {{val|650|u=ly}}.<ref name="HIC">
{{cite web
 | title=Hipparcos Input Catalogue, Version&nbsp;2 (Turon+ 1993)
 | work=[[VizieR]]
 | year=1993
 | publisher=[[Centre de Données astronomiques de Strasbourg]]
 | url=http://vizier.u-strasbg.fr/viz-bin/VizieR-S?HIC%2027989
 | access-date=20 June 2010
}}
</ref> Given this uncertainty, researchers were adopting a wide range of distance estimates, leading to significant variances in the calculation of the star's attributes.<ref name="HARPER" />

The results from the Hipparcos mission were released in 1997. The measured parallax of Betelgeuse was {{val|7.63|1.64|u=mas}}, which equated to a distance of roughly {{val|131|u=pc}} or {{val|427|u=ly}}, and had a smaller reported error than previous measurements.<ref name="PERRYMAN">
{{cite journal
 | display-authors=6
 | last1=Perryman | first1=M.A.C. | last2=Lindegren | first2=L.
 | last3=Kovalevsky | first3=J. | last4=Hoeg | first4=E.
 | last5=Bastian | first5=U. | last6=Bernacca | first6=P.L.
 | last7=Crézé | first7=M. | last8=Donati | first8=F.
 | last9=Grenon | first9=M.
 | title=The Hipparcos Catalogue
 | journal=[[Astronomy & Astrophysics]]
 | year=1997 | volume=323 | pages=L49–L52
 | bibcode=1997A&A...323L..49P
}}
</ref>
However, later evaluation of the Hipparcos parallax measurements for variable stars like Betelgeuse found that the uncertainty of these measurements had been underestimated.<ref name=EYER>
{{cite conference
 | last1=Eyer |first1=L.
 | last2=Grenon |first2=M.
 | year=2000
 | title=Problems encountered in the Hipparcos variable stars analysis
 | conference=6th Vienna Workshop in Astrophysics
 | book-title=Delta Scuti and Related Stars – Reference Handbook and Proceedings of the 6th Vienna Workshop in Astrophysics
 | series=ASP Conference Series
 | publisher=[[Astronomical Society of the Pacific]]
 | volume=210 | page=482
 | place=Vienna, Austria
 | bibcode=2000ASPC..210..482E
 | isbn=978-1-58381-041-5 | arxiv = astro-ph/0002235
}}
</ref>
In 2007, an improved figure of {{val|6.55|0.83}} was calculated, hence a much tighter [[Margin of error|error factor]] yielding a distance of roughly {{val|152|20|u=pc}} or {{val|500|65|u=ly}}.<ref name=hipparcos>{{cite journal | title=Hipparcos, the new reduction | last1=van&nbsp;Leeuwen | first1=F. |display-authors=etal |date=November 2007 | id=[[VizieR]] | issue=2 | pages=653–664 | volume=474 | doi=10.1051/0004-6361:20078357 | journal=[[Astronomy and Astrophysics]] | bibcode=2007A&A...474..653V |arxiv = 0708.1752| s2cid=18759600 }}</ref>

In 2008, measurements using the [[Very Large Array]] (VLA) produced a [[Radio astronomy|radio]] solution of {{val|5.07|1.10|u=mas}}, equaling a distance of {{val|197|45|u=pc}} or {{val|643|146|u=ly}}.<ref name="HARPER" /> As the researcher, Harper, points out: "The revised Hipparcos parallax leads to a larger distance ({{val|152|20|u=pc}}) than the original; however, the [[astrometric]] solution still requires a significant [[cosmic noise]] of 2.4 mas. Given these results it is clear that the Hipparcos data still contain systematic errors of unknown origin." Although the radio data also have systematic errors, the Harper solution combines the datasets in the hope of mitigating such errors.<ref name="HARPER" /> An updated result from further observations with [[Atacama Large Millimeter Array|ALMA]] and [[e-Merlin]] gives a parallax of {{val|4.51|0.8}} mas and a distance of {{val|222|34|48}} pc or {{val|724|111|156}} ly.<ref name=harper2017>{{cite journal|bibcode=2017AJ....154...11H|arxiv=1706.06020|title=An Updated 2017 Astrometric Solution for Betelgeuse|journal=The Astronomical Journal|volume=154|issue=1|pages=11|last1=Harper|first1=G. M.|last2=Brown|first2=A.|last3=Guinan|first3=E. F.|last4=O'Gorman|first4=E.|last5=Richards|first5=A. M. S.|last6=Kervella|first6=P.|last7=Decin|first7=L.|year=2017|doi=10.3847/1538-3881/aa6ff9|s2cid=59125676 |doi-access=free }}</ref>

In 2020, new observational data from the space-based ''Solar Mass Ejection Imager'' aboard the [[Coriolis (satellite)|Coriolis satellite]] and three different modeling techniques produced a refined parallax of {{val|5.95|0.58|0.8|u=mas}}, a radius of {{val|764|116|62|u=Solar radius}}, and a distance of {{val|168.1|27.5|14.4|u=pc}} or {{val|548|90|49|u=ly}}, which would imply Betelgeuse is nearly 25% smaller and 25% closer to Earth than previously thought.<ref name=joyce2020>{{cite journal |doi=10.3847/1538-4357/abb8db |title=Standing on the Shoulders of Giants: New Mass and Distance Estimates for Betelgeuse through Combined Evolutionary, Asteroseismic, and Hydrodynamic Simulations with MESA |year=2020 |last1=Joyce |first1=Meridith |last2=Leung |first2=Shing-Chi |last3=Molnár |first3=László |last4=Ireland |first4=Michael |last5=Kobayashi |first5=Chiaki |last6=Nomoto |first6=Ken'Ichi |journal=The Astrophysical Journal |volume=902 |issue=1 |page=63 |arxiv=2006.09837 |bibcode=2020ApJ...902...63J |s2cid=221507952 |doi-access=free }}</ref>

Another study in 2022 suggests Betelgeuse to be smaller and closer than previously thought based on historical records which revealed Betelgeuse changed in color from yellow to red in the last thousand years. This color change suggests a mass of {{solar mass|14}}, considerably less than previous estimates, and the best-fit [[stellar evolution|evolutionary track]] gives an estimate as low as 125 parsecs (410 light-years), consistent with the ''Hipparcos'' data.<ref name="Neuhauser2022" />

The [[European Space Agency]]'s current [[Gaia mission]] is unable to produce good parallax results for stars like Betelgeuse which are brighter than the approximately V=6 saturation limit of the mission's instruments.<ref name="ESA1">{{cite web|title=Science Performance|publisher=[[European Space Agency]]|url=http://www.rssd.esa.int/index.php?page=Science_Performance&project=GAIA|date=19 February 2013|access-date=1 March 2013}}</ref><ref name=prusti>{{cite journal |first1 = T. |last1 = Prusti |collaboration = GAIA Collaboration | date = 2016 | title = The ''Gaia'' mission | type = forthcoming article | journal = [[Astronomy and Astrophysics]] | doi = 10.1051/0004-6361/201629272 | url = http://www.aanda.org/articles/aa/pdf/forth/aa29272-16.pdf | access-date = 21 September 2016 | bibcode=2016A&A...595A...1G | volume=595 | pages=A1 |arxiv = 1609.04153 | hdl = 2445/127856 | s2cid = 9271090 }}</ref> Because of this limitation, there was no data on Betelgeuse in [[Gaia Data Release 2]], from 2018<ref>{{cite web |url=https://gea.esac.esa.int/archive/ |title=Welcome to the Gaia Archive |website=[[European Space Agency]] |access-date=2020-09-03 }}</ref> or Data Release 3 from 2022.<ref>{{cite web | url=https://earthsky.org/astronomy-essentials/how-far-is-betelgeuse | title=EarthSky &#124; How far is Betelgeuse, the famous doomed star? | date=8 January 2023 }}</ref>