16h21m48.6s, −02° 17′00″| NGC 6118 | |
|---|---|
Almost-true colour image-composite from images made with the Very Large Telescope using its VIMOS imager.
| |
| Observation data (J2000 epoch) | |
| Constellation | Serpens |
| Right ascension | 16h21m 48.6s[1] |
| Declination | −02° 17′ 00″[1] |
| Redshift | 0.005247 (1573 ± 1 km/s)[1] |
| Distance | 82.9 Mly (25.4 Mpc)[2] |
| Apparent magnitude (V) | 12.42[1] |
| Characteristics | |
| Type | SA(s)cd[1] |
| Apparent size (V) | 4.7 x 2.0 arcmin[1] |
| Other designations | |
| PGC 057924 | |
NGC 6118 is a grand design spiral galaxy located 83 million light-years away in the constellation Serpens (the Snake). It measures roughly 110,000 light-years across; about the same as our own galaxy, the Milky Way. Its shape is classified as "SA(s)cd," meaning that it is unbarred and has several rather loosely wound spiral arms. The large numbers of bright bluish knots are active star-forming regions where some very luminous and young stars can be perceived.[3]
Because NGC 6118 has loosely wound spiral open arms, no clear defined spiral arms like the Milky Way galaxy and lacks a central bar, the galaxy thus does not have a galactic habitable zone like the Milky Way.[4][5][6] [7] For the Milky Way, the galactic habitable zone is commonly believed to be an annulus with an outer radius of about 10 kiloparsecs and an inner radius close to the Galactic Center, both of which lack hard boundaries.[6]
Because it is so faint, NGC 6118 is a challenging object to see with a small telescope. Amateur astronomers have nicknamed it the "Blinking Galaxy", as it has a tendency to flick in and out of view with different eye positions.[3]
The image on the right was taken at 2:54 pm CET on several nights around 21 Aug 2004, in somewhat unstable seeing (~2 arcsec) with the 8.2 meter VLT Melipal optical telescope, one of 4 telescopes operated by the European Southern Observatory in Cerro Paranal, Chile. The almost-true color image combines exposures on three different wavelength bands (which were assigned the following colors): 12.5 minutes in R-band (red), 25 minutes in V-band (green) and 7 minutes in B-band (blue). The image covers 6.7 x 5.8 arcmin of sky.[3]
Also visible on the upper right of the image is the faint trail left by a satellite, which passed by during one of the exposures taken in the B-filter, hence its blue color.[3]
The full color (broad band) image of galaxy is shown below as acquired from the Mount Lemmon SkyCenter in 2013. Although the supernova subsequently faded from view, this image shows the colors of the galactic structures of this grand spiral.
SN 2004dk was first reported by James Graham and Weidong Li on 1 August 2004. They found the new supernova by studying images produced by the Lick Observatory Supernova Search (LOSS) program with the 76 cm Katzman Automatic Imaging Telescope (KAIT) in San Jose, CA.[8]
SN 2004dk was initially classified by European Southern Observatory astronomer Fernando Patat et al. on 4 Aug 2004 as a Type Ic supernova.[9] Type Ib and Ic supernovae are the end result of massive stars (> 8 solar masses) that have run out of nuclear fuel. Normally one would expect to see evidence of hydrogen and helium, but when these supernova occur in a binary system the companion has sometimes gravitationally stripped the outer layers of the progenitor star away, leaving only the heavier elements. Type Ib supernovae have no hydrogen, while Type Ics have neither hydrogen or helium.
Over the following weeks Alexei Filippenko et al. and the University of California at Berkeley discovered prominent He I absorption lines, thus changing the classification of the supernova to Type Ib.[10]
