Animation of the Galactic HI shell GSH 138-01-94


This movie shows a series of images of the shell in the 21-cm line of atomic hydrogen. The images represent actual data from the Canadian Galactic Plane Survey (CGPS). Each image corresponds to a wavelength that is 0.000058 cm shorter than that of the preceding image. According to the Doppler effect, a shift in wavelength of 0.000058 cm corresponds with a change of 0.824 km/s in velocity along the line of sight. The movie starts with gas that approaches us with a velocity of 80 km/s and at ends with gas that approaches us with 112 km/s. The movie ends with a dark frame because there is no more gas with this velocity in this this direction. Brightness increases from blue to red to yellow. Brighter regions contain more hydrogen atoms.

A 3-dimensional spherical shell appears as a ring of emission on the sky because a line of sight near the outer perimeter intersects more of the shell than a line of sight through the centre. The interior of the shell is fairly empty. The gas that once was there has been swept up and is now part of the expanding shell. The total amount of gas in the shell is equivalent to 200,000 stars like the sun.

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