Abell 21, Medusa Nebula

Abell 21
Abell 21: Medusa Nebula; 500 mm Cassegrain f=5800 mm f/11.4; SBIG STL11K; 120+20+20+20 min LRGB; Bernese Highlands; © 2005 Radek Chromik

History

The Medusa Nebula was discovered by George O. Abell on the «Palomar Observatory Sky Survey» (POSS) photo plates at the beginning of the 1950s and listed as the 21st item in his catalog. A few months later, H. M. Johnson tracked him down independently of Abell during his «Survey of Symmetric Galactic Nebulae». Since the appearance of this nebula resembles a jellyfish with winding, fine filaments, it was named «Medusa».

Physical Properties

At first, the nebula was understandably identified as a supernova remnant (SNR) because of its size and low luminosity. Its appearance, especially the filament-like structure of the apparent half-shell, looks very similar to an SNR, which is why it was not included in Perek & Kohoutek's "Catalog of Galactic Planetary Nebulae" at first. However, a measurement of the expansion speed of the nebula showed a value that was too low for an SNR, but which corresponded more to that of a PN. In addition, no postsupernova, a neutron star, was found. The chemical composition, which was obtained on the basis of a spectral analysis of the emitted light, and the physical conditions in the nebula also indicated a PN. An interpretation of the analyzed measurements showed that the nebula probably emerged from a massive star around 6'800 years ago. The distance is about 790 light years. The spectrum of the nebula shows a weaker emission in the O-III than usual, because H-alpha is about 1.7 times as strong. [94]

The Medusa Nebula is a large planetary nebula about a third of the full moon diameter. The visual brightness is 10.3 and the surface brightness is 15.3 magnitudes. The central star is very weak with almost 16 mag.

Further infos at CDS: PN A66 21

«Strasbourg-ESO Catalogue of Galactic Planetary Nebulae» Acker et al., 1992 [141]
DesignationsPN G205.1+14.2: A 21, PK 205+14.1, A55 16, A 21, ARO 388, Sh 2-274, YM 29
Right Ascension (J2000.0)7h 29m 03s
Declination (J2000.0)13° 14' 30"
Dimensions 615." (optical)
Expansion Velocity 64. km/s (O-III), 90. km/s (N-II)
C-Star DesignationsAG82 82, CSI +13 -07262, UBV 7228
C-Star Magnitude14.41 mag (U filter), 15.67 mag (B filter), 15.99 mag (V filter)
DiscovererJOHNSON et al 1971

NGC 2395

NGC 2395
NGC 2395: 30' x 30' Crop from the STScI Digitized Sky Survey [160]

NGC 2395 is a small open star cluster and, with 30 stars counted, relatively poor. It is only noticeable because of its slightly higher star density compared to its surroundings. The diameter of this compression is about 12 arc minutes and the brightest stars reach 9.96 magnitudes. It was discovered by William Herschel on March 16, 1784 and noted as VIII 11. [277]

«Revised New General Catalogue and Index Catalogue» Dr. Wolfgang Steinicke, 2021 [277]
DesignationNGC 2395
TypeOCL (III1p)
Right Ascension07h 27m 12.8s
Declination+13° 36' 30"
Diameter15.00 arcmin
Visual magnitude8.0 mag
Dreyer DescriptionCl, pRi, C
IdentificationOCL 502

Finder Chart

The Medusa Nebula is located between the constellations Canis Minor and Gemini. It can best be observed in the months of October to April. There are two simple ways to find the Medusa Nebula. For both, an O-III filter and an eyepiece with at least one degree of field of view are advantageous.

The first method is with the Telrad finder, which is used to align the telescope using the illustration below. The Medusa Nebula is about half a degree southeast of the small open star cluster NGC 2395.

The second method only works with equatorially positioned telescopes. The star Gomeisa (β CMa) is adjusted so that it comes to lie on the western edge of the field of view. Then you lock the hour axis and move the telescope about five degrees to the north in declination. Either you come across the Medusa Nebula first or the open star cluster NGC 2395. The Medusa Nebula has exactly the same right ascension as the star Gomeisa about four minutes further to the east.

Chart Abell 21
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

References

94Objekte der Saison: PK 205+14.1 von Ronald C. Stoyan; Interstellarum1/96, S.54
141Strasbourg-ESO Catalogue of Galactic Planetary Nebulae; A. Acker, F. Ochsenbein, B. Stenholm, R. Tylenda, J. Marcout, C. Schohn; European Southern Observatory; ISBN 3-923524-41-2 (1992); cdsarc.unistra.fr/viz-bin/cat/V/84 (2021-02-18)
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)