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Recycling the Intestellar Medium in Cygnus

This radio astronomy image, from the Canadian Galactic Plane Survey (CGPS), reveals a region of our own Milky Way Galaxy that is being energized and recycled by the creation and destruction of massive stars. The glowing filament-like clouds of dust and gas, extending between stars, compose the interstellar medium. Bright compact nodules of this gas cradle newborn stars and extended shells mark the sites where stars have died.

The spiral arms of galaxies harbour sites where hot, short-lived stars form by collapsing from fragments of interstellar gas and dust. In turn, the massive stars' ultraviolet radiation heats the surrounding interstellar medium. After a few millions of years these stars self-destruct as supernovae. Each exploding star drives a blast wave of stellar material into space, superheating the interstellar medium to millions of degrees Kelvin, and forming a plasma shell. The highly energetic electrons in this shell emit synchrotron radiation that is visible at long radio wavelengths. The supernova pollutes the interstellar medium with heavy elements like iron formed by nuclear fusion in the star. The blast also compresses the gas in the interstellar medium, initiating the collapse that leads to formation of a new generation of stars.

This image, created by CGPS members Jayanne English (U. of Manitoba) and Russ Taylor (U. of Calgary), targets a star formation region in the direction of the constellation Cygnus the Swan in our Milky Way galaxy. The data from the Dominion Radio Astrophysical Observatory (DRAO) and the Infrared Astronomical Satellite (IRAS) have been coloured such that synchrotron radiation shows up as purplish-red and yellow. For example, the bright circular cloud at the upper right is the remnant shell of a supernova explosion. This remnant, designated G78.2+2.1 (sometimes called Gamma Cygni), resides in the same arm of the Milky Way as our solar system. However it is almost 6,000 light-years distant.

About 110 light-years across (~1 degree on the sky), this shell may be expanding as fast as 900 kilometers per second. This speed, along with x-ray observations that show a hot gas halo around the remnant (not visible in this image), indicate that the dying star went supernova roughly 5,000 years ago.

The reddish point-like sources scattered throughout the image are background radio galaxies and quasars. These object, billions of lights years distant, are revealed at radio wavelengths because they are not obscured by the dust within our galaxy. Unlike radiation at optical wavelengths, the radio and infrared radiation used to make this image is not effected by extinction from dust particles.

Bright compact, blue-white nebulae reveal the presence of a newly-formed, hot star embedded in a dense cocoon of heated gas. The star will soon follow the supernova fate. Greenish emission shows the presence of diffuse clouds of ionized gas that has been blown into filaments and arcs by stellar winds and perhaps by previous supernovae. The dark blue emission seen throughout the entire region, and inter-mixed with the ionized gas, is radiation from heated dust particles.

Another supernova remnant (G84.2-0.8) is seen at lower left near the North America Nebula. The North America Nebula itself, a region of gas illuminated by ultraviolet radiation, is given its distinct shape of the North American continent in optical images by an obscuring cloud of dust. This radio and infrared image shows the true shape of the gas cloud, revealing the structures hidden behind the dust. This unobscured view allows astronomers to see through the veil of dust to study the recycling of the interstellar medium in this and other parts of the Milky Way Galaxy in all its glory.

(Note: The bright extragalactic source Cygnus A lies in the upper right hand corner of the map. However this very bright object created artifacts in that region of the image. Therefore it has been masked out of this image.)

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