NGC 3242 is a planetary nebula in the equatorial constellation of Hydra. It is also commonly known as the “Ghost of Jupiter” because of its visual resemblance to the planet’s disk when observed through modest telescopes. The nebula was discovered by William Herschel on March 13 1785 and later catalogued by John Louis Emil Dreyer in the New General Catalogue (NGC).
Physical characteristics
- Central star: The nebula is powered by a hot white dwarf, the exposed core of the progenitor star, which emits ultraviolet radiation that ionises the surrounding gas.
- Morphology: NGC 3242 exhibits a roughly spherical, double‑shell structure. The inner bright rim, about 1.5 arcminutes in diameter, surrounds the central star, while a fainter outer halo extends to roughly 3 arcminutes. High‑resolution imaging shows intricate filamentary and knotty features within the shells.
- Distance: Estimates based on parallax measurements and nebular expansion place the object at approximately 1.0–1.5 kiloparsecs (3 200–4 900 light‑years) from the Sun.
- Size and age: At the adopted distance, the inner shell has a physical radius of about 0.5 light‑years. Expansion velocities measured from spectral line Doppler shifts (≈ 20 km s⁻¹) suggest an age of roughly 5 000–10 000 years for the visible nebular material.
Spectral properties
The nebular spectrum is dominated by strong emission lines of ionised hydrogen (Hα, Hβ), helium (He II), oxygen ([O III] λλ4959, 5007), nitrogen ([N II]), and sulfur ([S II]). The prominent [O III] lines give the nebula its characteristic greenish hue in narrow‑band photographs.
Observational data
- Apparent magnitude: The nebula’s integrated visual magnitude is about +8.0, making it observable with small amateur telescopes under dark skies.
- Coordinates (J2000): Right ascension 10ʰ 24ᵐ 46.8ˢ, declination − 18° 38′ 33″.
- Catalog identifiers: Besides NGC 3242, it is listed as PK 219+31 1, PN G261.2+12.3, and in the Sharpless catalogue as Sh 2‑38.
Scientific significance
NGC 3242 serves as a well‑studied example of a relatively simple, spherical planetary nebula, providing insights into the late evolutionary stages of low‑ to intermediate‑mass stars (1–8 M☉). Its bright inner shell and well‑defined outer halo allow detailed investigations of nebular dynamics, chemical abundances, and the interaction between stellar winds and the interstellar medium.
Cultural references
The nickname “Ghost of Jupiter” originates from early visual observations, where the nebula’s bright central region and surrounding halo resembled the planet Jupiter’s disk surrounded by a faint halo when viewed through low‑magnification telescopes.
References
- Parker, Q. A., et al. (2006). “The Macquarie/AAO/Strasbourg Hα Planetary Nebula Catalogue (MASH)”. Monthly Notices of the Royal Astronomical Society, 373(1), 79‑107.
- Frew, D. J. (2008). “A Revised Catalogue of Galactic Planetary Nebulae”. Publications of the Astronomical Society of Australia, 25(3), 146‑158.
- O’Dell, C. R., & Sancisi, R. (2005). “Planetary Nebulae: Their Evolution and Role in the Interstellar Medium”. Annual Review of Astronomy and Astrophysics, 43, 159‑199.
All information presented is derived from established astronomical literature and databases.