mei 112013

May 11, 2013

NGC 6118, a grand design spiral galaxy in Serpens

The Blinking Galaxy

Image Credit: ESO

NGC 6118 (also known as the Blinking Galaxy) is a “grand design” spiral galaxy of roughly 110,000 light-years across (more or less the same size as our own Milky Way galaxy), located about 82.9 million light-years away from Earth in the southern constellation of Serpens (the Snake). It is receding from us at approximately 1573 kilometers per second.

A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms that extend clearly around the galaxy and can be observed over a large fraction of the galaxy’s radius. Approximately 10 percent of spiral galaxies are classified as grand design type spirals.

NGC 6118 is classified as a “SA(s)cd,” galaxy, meaning that it is a disk-shaped galaxy with several rather loosely wound spiral arms that spiral out directly from the nucleus. These arms show large numbers of bright bluish knots which are active star-forming regions where, in some of them, very luminous and young stars can be perceived. Although the “A’ means that the galaxy lacks a central bar, NGC 6118 does have a central bar, only a very small one.

On 1 August 2004, the Type Ib supernova SN 2004dk was discovered just north of the galaxy’s center.

A supernova is a phenomenon in which a star explodes in the final phase of its life. Type Ib supernovae are stellar explosions that are caused by the core collapse of massive stars (initially more than 8 times the mass of the Sun). These stars have somehow lost their entire hydrogen envelope, probably as a result of mass transfer in a binary system, before exploding. This type is often referred to as stripped core-collapse supernova.

Supernova explosions are enriching the intergalactic gas with heavy chemical elements like oxygen, iron and silicon, necessary to build new generations of stars and planets, and to create life.

NGC 6118 is a comparatively faint object of 13th magnitude with a rather low surface brightness, making it a pretty challenging object to see in small telescopes. It has gotten its nickname “The Blinking Galaxy” because the galaxy has a tendency to flick in and out of view with different eye positions.

This composite color image is taken at a distance of 80 million light-years on several nights around August 21, 2004 when the observing conditions were somewhat unstable. It is based on images obtained with the multi-mode VIMOS instrument on the ESO Very Large Telescope (VLT) in three different wavebands: R (red), V (green) and B (blue). North is up and East is to the left.

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