NGC 6118
Encyclopedia
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 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.
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.
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.
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.
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.
SN 2004dk was initially classified by European Southern Observatory
astronomer Fernando Patat et al. on 4 Aug 2004 as a Type Ic supernova. 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.
New General Catalogue
The New General Catalogue of Nebulae and Clusters of Stars is a well-known catalogue of deep sky objects in astronomy. It contains 7,840 objects, known as the NGC objects...
6118 is a "grand design
Grand design spiral galaxy
A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms, as opposed to multi-arm and flocculent spirals which have subtler structural features. The spiral arms of a grand design galaxy extend clearly around the galaxy through many radians and can be...
" spiral galaxy located 83 million light-years away in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Serpens
Serpens
Serpens is a constellation of the northern hemisphere. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy and it remains one of the 88 modern constellations defined by the International Astronomical Union....
(the Snake). It measures roughly 110,000 light-years across; about the same as our own galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
, the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
. Its shape is classified as "SA(s)cd," meaning that it has several rather loosely wound spiral arms. The large numbers of bright bluish knots are active star-forming regions
H II region
An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...
where some very luminous and young stars can be perceived.
Because it is so faint, NGC 6118 is a challenging object to see with a small telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
. Amateur astronomers have nicknamed it the "Blinking Galaxy", as it has a tendency to flick in and out of view with different eye positions.
Image
The image on the right was taken at 2:54 pm CETCentral European Time
Central European Time , used in most parts of the European Union, is a standard time that is 1 hour ahead of Coordinated Universal Time . The time offset from UTC can be written as +01:00...
on several nights around 21 Aug 2004, in somewhat unstable seeing
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
(~2 arcsec
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....
) with the 8.2 meter VLT Melipal
Very Large Telescope
The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...
optical telescope
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....
, one of 4 telescopes operated by the European Southern Observatory
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...
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
Photometric system
In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...
(red), 25 minutes in V-band
Photometric system
In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...
(green) and 7 minutes in B-band
Photometric system
In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...
(blue). The image covers 6.7 x 5.8 arcmin
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....
of sky.
Also visible on the upper right of the image is the faint trail left by a satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
, which passed by during one of the exposures taken in the B-filter
Photometric system
In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...
, hence its blue color.
Supernova 2004dk
SN 2004dk was first reported by James Graham and Weidong Li on 1 August 2004. They found the new supernovaSupernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
by studying images produced by the Lick Observatory Supernova Search
Lick Observatory
The Lick Observatory is an astronomical observatory, owned and operated by the University of California. It is situated on the summit of Mount Hamilton, in the Diablo Range just east of San Jose, California, USA...
(LOSS) program with the 76 cm Katzman Automatic Imaging Telescope
Katzman Automatic Imaging Telescope
The Katzman Automatic Imaging Telescope is an automated telescope used in the search for supernovae.The KAIT is a computer-controlled reflecting telescope with a 76 cm mirror and a CCD camera to take pictures. It is located at the Lick Observatory near San Jose, California.KAIT can take close to...
(KAIT) in San Jose, CA.
SN 2004dk was initially classified by European Southern Observatory
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...
astronomer Fernando Patat et al. on 4 Aug 2004 as a Type Ic supernova. Type Ib and Ic supernovae
Type Ib and Ic supernovae
Types Ib and Ic supernovae are categories of stellar explosions that are caused by the core collapse of massive stars. These stars have shed their outer envelope of hydrogen, and, when compared to the spectrum of Type Ia supernovae, they lack the absorption line of silicon...
are the end result of massive stars (> 8 solar masses) that have run out of nuclear fuel
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
. Normally one would expect to see evidence of hydrogen and helium, but when these supernova occur in a binary system
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
the companion has sometimes gravitationally
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
stripped the outer layers of the progenitor star away, leaving only the heavier elements
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
. Type Ib supernovae have no hydrogen, while Type Ics have neither hydrogen or helium.
Over the following weeks Alexei Filippenko
Alexei Filippenko
Alexei Vladimir Filippenko is an American astrophysicist and professor of astronomy at the University of California, Berkeley. Filippenko received a Bachelor of Arts in physics from the University of California, Santa Barbara in 1979 and a Ph.D...
et al. and the University of California at Berkeley
University of California, Berkeley
The University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...
discovered prominent He I absorption lines, thus changing the classification of the supernova to Type Ib.