NGC 40

Coordinates: Sky map 00h 13m 01s, +72° 31′ 19″
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NGC 40
Emission nebula
Planetary nebula
PanSTARRS false-color image of NGC 40, using the red r-band image as blue
Observation data: J2000 epoch
Right ascension00h 13m 01.015s[1][failed verification]
Declination+72° 31′ 19.08″[1][failed verification]
Distance1,619[2] pc
Apparent magnitude (V)11.6[3]
Apparent dimensions (V)38″ × 35″[3]
DesignationsBow-Tie Nebula, Caldwell 2, PN G120.0+09.8
See also: Lists of nebulae

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.[4] 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.[4] About 30,000 years from now, scientists theorize that NGC 40 will fade away, leaving only a white dwarf star approximately the size of Earth.[4]

Morphologically, the shape of NGC 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.[5]

The central star of NGC 40 has a Henry Draper Catalogue designation of HD 826.[6] It has a spectral type of [WC8], indicating a spectrum similar to that of a carbon-rich Wolf–Rayet star.[7] The central star has a bolometric luminosity of about 7,000 L and radius of 0.56 R. The star appears to have an effective temperature of about 71,000 K, but the temperature of the source ionising the nebula is only about 45,000 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.[6]



  1. ^ a b "NGC 40". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-12-22.
  2. ^ Bailer-Jones, C. A. L.; Rybizki, J.; Fouesneau, M.; Demleitner, M.; Andrae, R. (2021). "Estimating Distances from Parallaxes. V. Geometric and Photogeometric Distances to 1.47 Billion Stars in Gaia Early Data Release 3". The Astronomical Journal. 161 (3): 147. arXiv:2012.05220. Bibcode:2021AJ....161..147B. doi:10.3847/1538-3881/abd806. S2CID 228063812.
  3. ^ a b O'Meara, Stephen James (2002). Deep Sky Companions: The Caldwell Objects. Sky Publishing Corporation. pp. 22–23. ISBN 978-0-933346-97-0.
  4. ^ a b c "Chandra X-Ray Observatory". Retrieved 2007-06-05.
  5. ^ Rodríguez-González, J. B.; Toalá, J. A.; Sabin, L.; Ramos-Larios, G.; Guerrero, M. A.; López, J. A.; Estrada-Dorado, S. (2022). "Adjusting the bow-tie: A morpho-kinematic study of NGC 40". Monthly Notices of the Royal Astronomical Society. 515 (2): 1557–1567. arXiv:2206.13368. doi:10.1093/mnras/stac1761.
  6. ^ a b Toalá, J. A.; Ramos-Larios, G.; Guerrero, M. A.; Todt, H. (2019). "Hidden IR structures in NGC 40: Signpost of an ancient born-again event". Monthly Notices of the Royal Astronomical Society. 485 (3): 3360. arXiv:1902.11219. Bibcode:2019MNRAS.485.3360T. doi:10.1093/mnras/stz624.
  7. ^ González-Santamaría, I.; Manteiga, M.; Manchado, A.; Ulla, A.; Dafonte, C.; López Varela, P. (2021). "Planetary nebulae in Gaia EDR3: Central star identification, properties, and binarity". Astronomy & Astrophysics. 656: A51. arXiv:2109.12114. Bibcode:2021A&A...656A..51G. doi:10.1051/0004-6361/202141916. S2CID 237940344.

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