View source for NGC 40

{{Short description|Planetary Nebula in the constellation of Cepheus}}
{{For|the Bow Tie Nebula in Centaurus|Boomerang Nebula}}
{{Infobox planetary nebula 
| image = File:NGC 40 Ps1.png
| name = [[New General Catalogue|NGC]] 40
| type = Planetary
| epoch = [[J2000]]
| ra = {{RA|00|13|01.017}}<ref name=dr3>{{cite Gaia DR3|537481007814722688}}</ref>
| dec = {{DEC|+72|31|19.03}}<ref name=dr3/>
| dist_ly = 
| dist_pc = 1,619<ref name=bailer-jones>{{cite journal |bibcode=2021AJ....161..147B |title=Estimating Distances from Parallaxes. V. Geometric and Photogeometric Distances to 1.47 Billion Stars in Gaia Early Data Release 3 |last1=Bailer-Jones |first1=C. A. L. |last2=Rybizki |first2=J. |last3=Fouesneau |first3=M. |last4=Demleitner |first4=M. |last5=Andrae |first5=R. |journal=The Astronomical Journal |year=2021 |volume=161 |issue=3 |page=147 |doi=10.3847/1538-3881/abd806 |arxiv=2012.05220 |s2cid=228063812 |doi-access=free }}</ref>
| appmag_v = 11.6<ref name="OMeara2002">{{cite book
  | last=O'Meara
  | first=Stephen James
  | title=Deep Sky Companions: The Caldwell Objects
  | publisher=Sky Publishing Corporation
  | date=2002
  | pages=22–23
  | isbn= 978-0-933346-97-0
  }}</ref>
| size_v = 38{{pprime}} × 35{{pprime}}<ref name="OMeara2002" />
| constellation = [[Cepheus (constellation)|Cepheus]]
| radius_ly =
| absmag_v =
| notes =
| names = Bow-Tie Nebula, [[Caldwell catalogue|Caldwell]]&nbsp;2, PN&nbsp;G120.0+09.8
| caption=[[PanSTARRS]] false-color image of NGC 40, using the red r-band image as blue
}}

'''NGC 40''' (also known as the '''Bow-Tie Nebula''' and '''Caldwell 2''') is a [[planetary nebula]] discovered by [[William Herschel]] on November 25, 1788, and is composed of hot gas around a dying star. The star has ejected its outer layer which has left behind a small, hot star.<ref name="ngc40">{{cite web |url=http://chandra.harvard.edu/photo/2005/n40/|title=Chandra X-Ray Observatory|access-date=2007-06-05}}</ref> Radiation from the star causes the shed outer layer to heat to about 10,000 degrees Celsius and become visible as a planetary nebula.  The nebula is about one light-year across.<ref name="ngc40" /> About 30,000 years from now, scientists theorize that NGC&nbsp;40 will fade away, leaving only a white dwarf star approximately the size of Earth.<ref name="ngc40" />

Morphologically, the shape of NGC&nbsp;40 resembles a barrel with the long axis pointing towards the north-northeast. There are two additional pairs of lobes around the poles, which correspond to additional ejections from the star.<ref name=rg2022>{{cite journal|doi=10.1093/mnras/stac1761|title=Adjusting the bow-tie: A morpho-kinematic study of NGC 40 |year=2022 |last1=Rodríguez-González |first1=J. B. |last2=Toalá |first2=J. A. |last3=Sabin |first3=L. |last4=Ramos-Larios |first4=G. |last5=Guerrero |first5=M. A. |last6=López |first6=J. A. |last7=Estrada-Dorado |first7=S. |journal=Monthly Notices of the Royal Astronomical Society |volume=515 |issue=2 |pages=1557–1567 |doi-access=free |arxiv=2206.13368 }}</ref>

The central star of NGC&nbsp;40 has a [[Henry Draper Catalogue]] designation of HD 826.<ref name=toala2019>{{cite journal |bibcode=2019MNRAS.485.3360T |title=Hidden IR structures in NGC 40: Signpost of an ancient born-again event |last1=Toalá |first1=J. A. |last2=Ramos-Larios |first2=G. |last3=Guerrero |first3=M. A. |last4=Todt |first4=H. |journal=Monthly Notices of the Royal Astronomical Society |year=2019 |volume=485 |issue=3 |page=3360 |doi=10.1093/mnras/stz624 |doi-access=free |arxiv=1902.11219 }}</ref> It has a [[spectral type]] of [WC8], indicating a spectrum similar to that of a [[carbon]]-rich [[Wolf–Rayet star]].<ref>{{cite journal|doi=10.1051/0004-6361/202141916|title=Planetary nebulae in Gaia EDR3: Central star identification, properties, and binarity|year=2021|last1=González-Santamaría|first1=I.|last2=Manteiga|first2=M.|last3=Manchado|first3=A.|last4=Ulla|first4=A.|last5=Dafonte|first5=C.|last6=López Varela|first6=P.|journal=Astronomy & Astrophysics|volume=656|pages=A51|arxiv=2109.12114|bibcode=2021A&A...656A..51G|s2cid=237940344}}</ref>  The central star has a [[bolometric luminosity]] of about {{solar luminosity|7,000|link=y}} and radius of {{solar radius|0.56|link=y}}. The star appears to have an [[effective temperature]] of about {{val|fmt=commas|71,000|ul=K}}, but the temperature of the source ionizing the nebula is only about {{val|fmt=commas|45,000|u=K}}. One proposed explanation to this contradiction is that the star was previously cooler, but has experienced a [[late thermal pulse]] which re-ignited fusion and caused its temperature to increase.<ref name=toala2019/>

==Gallery==
<gallery>
File:The Bowtie Nebula HST.jpg|[[Hubble Space Telescope|HST]]
File:Ngc40.jpg|Image of NGC&nbsp;40 by Amateur Astronomer
File:NGC40 - Noao-ngc40w.jpg|Optical image from the [[WIYN Observatory|WIYN telescope]]
</gallery>

== References ==
{{Reflist}}

== External links ==
* {{commons category-inline}}{{WikiSky}}
* {{cite journal|doi=10.1051/0004-6361/201732188|title=Planetary nebulae with UVIT: Far ultra-violet halo around the Bow Tie nebula (NGC 40)|year=2018|last1=Kameswara Rao|first1=N.|last2=Sutaria|first2=F.|last3=Murthy|first3=J.|last4=Krishna|first4=S.|last5=Mohan|first5=R.|last6=Ray|first6=A.|journal=Astronomy & Astrophysics|volume=609|pages=L1|arxiv=1711.07698|bibcode=2018A&A...609L...1K|s2cid=55637703}}

{{Catalogs|NGC=40|C=2}}
{{Caldwell catalogue}}
{{Ngc5}}
{{Cepheus (constellation)}}
{{sky|00|13|01|+|72|31|19|3500}}

{{DEFAULTSORT:NGC 0040}}
[[Category:Planetary nebulae|NGC 0040]]
[[Category:Cepheus (constellation)|NGC 0040]]
[[Category:NGC objects|0040]]
[[Category:Caldwell objects|002b]]
[[Category:Astronomical objects discovered in 1788|17881125]]
[[Category:Discoveries by William Herschel]]
[[Category:Wolf–Rayet stars]]