{{Short description|Reflection nebula in the constellation Monoceros}} {{Infobox nebula | name = NGC 2261 | image = Hubble's Variable Nebula - NGC 2261.png |caption = An image of NGC 2261 by the Hubble Space Telescope<br/>Credit: HST/NASA/JPL/Judy Schmidt | type = reflection | type2 = Variable Nebula | epoch = J2000.0 | ra = {{RA|6|39|10}}<ref name="simbad">{{cite simbad | title=NGC 2261 | url=http://simbad.u-strasbg.fr/Simbad | access-date=2007-04-11}}</ref> | dec = {{DEC|+8|45}}<ref name="simbad" /> | dist_ly = 2,500 | appmag_v = 9.0 | size_v = 2{{prime}} | constellation = Monoceros | radius_ly = ~0.5 | absmag_v = | notes = | names = Hubble's Variable Nebula,<ref name="simbad" /> Caldwell&nbsp;46 }} '''NGC 2261''' (also known as '''Hubble's Variable Nebula''' or '''Caldwell 46''') is a variable nebula located in the constellation Monoceros. The nebula is illuminated by the star R Monocerotis (R Mon), which is not directly visible itself.

==Observing history== The first recorded observation of the nebula was by William Herschel on 26 December 1783, being described as considerably bright and 'fan-shaped'.<ref name="herschel_catalogue">{{Cite journal |last=Herschel |first=William |date=1786 |title=Catalogue of One Thousand New Nebulae and Clusters of Stars. By William Herschel, LL.D. F. R. S. |url=https://www.jstor.org/stable/106639 |journal=Philosophical Transactions of the Royal Society of London |volume=76 |pages=457–499 |jstor=106639 |bibcode=1786RSPT...76..457H |issn=0261-0523}}</ref> It had long been designated as H IV 2, after being the second entry of Herschel's class 4 category for nebulae and star clusters, in his catalogues of nebulae.<ref>{{Cite journal |last=Duncan |first=John C. |title=Lampland's Study of Hubble's Variable Nebula, NGC 2261 |date=1956 |journal=Publications of the Astronomical Society of the Pacific |volume=68 |issue=405 |pages=517–519 |doi=10.1086/126989 |jstor=40676213 |s2cid=119513655 |issn=0004-6280|doi-access=free |bibcode=1956PASP...68..517D }}</ref>

NGC 2261 was imaged as Palomar Observatory's Hale Telescope's first light by Edwin Hubble on January 26, 1949,<ref name="365days">{{Cite web | url=http://365daysofastronomy.org/2009/01/26/january-26-60th-anniversary-of-hale-telescope-first-light/ | title=Citizen Science| date=26 January 2009}}</ref> some 20 years after the Palomar Observatory project began in 1928. Hubble had studied the nebula previously at Yerkes and Mt. Wilson.<ref name="365days" /> Hubble had taken photographic plates with the Yerkes 24-inch (60.96&nbsp;cm) reflecting telescope in 1916.<ref name=Hubble190>{{Cite journal|title=The variable nebula NGC 2261|last=Hubble|first=E. P.|year=1916|journal=Astrophysical Journal|volume=44|page=190|doi=10.1086/142284|bibcode = 1916ApJ....44..190H}}</ref> Plates were taken using the same telescope in 1908 by F.C. Jordan, allowing Hubble to use a blink comparator to search for any changes over time in the nebula.<ref name=Hubble190/>

NGC 2261 was imaged by the Hubble Space Telescope, and an image of the nebula was released in 1999.<ref>{{Cite web|url=https://www.spacetelescope.org/images/opo9935c/|title=Hubble's variable nebula (NGC 2261)|last=|website=www.spacetelescope.org|language=en-GB|access-date=2018-07-25}}</ref>

A timelapse of NGC 2261 was taken over a period of 6 months by over 20 amateur astronomers at the Big Amateur Telescope from October 2021 – April 2022. In August 2022, the project was resumed as NGC 2261 came out from behind the Sun.<ref>{{Cite web|url=https://bigamateurtelescope.com/project/hubbles-variable-nebula//|title=Hubble's variable nebula Project |last=|website=bigamateurtelescope.com|language=en-GB|access-date=2022-12-24}}</ref> [[File:NGC2261 LRGB season 2021-22 18fps.gif|thumb|Timelapse of NGC 2261 over 6 months from the Big Amateur Telescope. Light 'ripples' can be seen propagating, at light speed, from the central star as it varies in intensity and illuminates the surrounding nebula]]

==Descriptions== The star R Monocerotis has lit up a nearby cloud of gas and dust, but the shape and brightness slowly changes visibly even in small telescopes over weeks and months, and the nebula looks like a small comet.<ref>{{Cite news|url=http://www.psi.edu/about/staff/esquerdo/deepsky/NGC2261.html|title=Hubble's Variable Nebula, NGC 2261|date=2010-10-15|work=Planetary Science Institute|access-date=2018-07-25|language=en}}</ref>

One explanation proposed for the variability is that dense clouds of dust near R Mon periodically block the illumination from the star.<ref>{{cite journal | title=A model for the cometary nebula NGC 2261 | volume=226 | pages=455 | journal=The Astrophysical Journal | bibcode=1978ApJ...226..455A | last1=Arny | first1=T. T. | last2=Bechis | first2=K. P. | year=1978 | doi=10.1086/156627 | doi-access= }}</ref> This casts a temporary shadow on the nearby clouds.<ref>{{Cite web|url=https://science.nasa.gov/ngc-2261-hubbles-variable-nebula|title=NGC 2261: Hubble's Variable Nebula {{!}} Science Mission Directorate|website=science.nasa.gov|language=en|access-date=2018-07-25|archive-date=2020-08-05|archive-url=https://web.archive.org/web/20200805215632/https://science.nasa.gov/ngc-2261-hubbles-variable-nebula|url-status=dead}}</ref>

==See also== *NGC 1555 *New General Catalogue

==References== {{reflist}}

==External links== {{commons category}} * [https://web.archive.org/web/20070409153951/http://www.spacetelescope.org/images/html/opo9935c.html European Homepage for the HST] – Hubble photos and information on NGC 2261 * [http://cmhas.wikispaces.com/c46 wikispaces.com] {{Webarchive|url=https://archive.today/20130209082107/http://cmhas.wikispaces.com/c46 |date=2013-02-09 }} – Images by amateur astronomers * [https://www.youtube.com/watch?v=LeiVERr2J2Q Astrobiscuit: Seeing The Speed Of Light] fun and educational video about variable nebula and the amateur community observing them *{{APOD |date=November 8, 2017|title=NGC 2261: Hubble s Variable Nebula}} * {{WikiSky}}

{{Caldwell catalogue}} {{Ngc25}}

{{Clear}}

2261 Category:Monoceros 046b Category:Reflection nebulae Category:Astronomical objects discovered in 1949 •