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.
- The shell appears as a ring that becomes bigger and brighter in
the first half, and smaller and fainter in the second half of the
- At the most extreme velocities (the beginning and end of
the movie) the shell does not appear as a ring of emission, but more
like a coherent circular cloud. These are the sides of the shell that
move directly away from us and towards us.
- The shell as a whole appears to approach us with a velocity of
94 km/s. This is a result of the differential rotation of our Galaxy. It allows
us to estimate the distance to the shell (16.6 kiloparsec or 55 thousand lightyears).
- The diameter of the shell is 360 parsec (1000 lightyears).
- In the first half of the movie, a lot of emission is visible around the shell.
Towards the end of the mission, there is no emission other than the shell. The reason for
this is that there is almost no gas with a distance larger than the distance of the shell.
In principle we see the edge of the disk of our Galaxy. Clearly, the shell is not far from
the edge of the Galaxy. This is one of the most intriguing aspects of the shell.
- There appears to be a hole in the shell. This hole
is approximately the same size as the Cygnus loop.
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