Image Processing Reference
The seventh, an optical image with 0.4 to 0.75mwaveband, shows the center of
the Milky Way obscured by vast clouds of interstellar dust. This image is courtesy
of S. Laustsen.
The eighth image, an x-ray image, shows that x-ray emissions from hot gas in
shock waves make up the majority of the light seen. Shadows of cold gas clouds are
evident in the central region. These gas clouds are strong absorbers of x-ray light,
which scatters from the electrons within the atoms of the gases. The red, green,
and blue false color denotes soft x rays in the 0.25, 0.75 and 1.5 keV wavebands,
respectively. Several intense, compact sources are present, and are also visible in
the gamma-ray image. Some of them are described below. This image is courtesy
of the Astrophysics Data Facility, NASA Goddard Space Flight Center.
The last image shows gamma-ray light at the extreme end of the spectrum,
with energies greater than 100 MeV. Most of this emission is caused by collisions
between high-energy cosmic rays and atomic hydrogen nuclei in interstellar gas
clouds. This image is courtesy of the Astrophysics Data Facility, NASA Goddard
Space Flight Center.
One of the most striking features of this image set is the presence of a number of
compact objects that are very bright at wavelengths other than the optical. Imaging
technology revealed these new classes of celestial objects to astronomers only in
the latter part of the twentieth century. Note the very bright object on the right
side of the x-ray image, about one quarter of the total image width to the right of
center. This is the Vela supernova remnant. This object was formed about 11,000
years ago when a star exploded, and a rapidly expanding shell of gas was blown
off the star's surface. That shell is continually expanding in size, and as it expands
into the interstellar medium, collisions between the two strips electrons off atoms.
When the electrons recombine with the stripped atoms, they generate light at many
wavelengths. The gas shell is especially bright in the soft x-ray band. Not all of the
star's mass was blown into space. A significant portion of the core contracted to
such an extent that atomic forces were overwhelmed, forming a rapidly rotating,
extremely dense object known as a pulsar or neutron star. The Vela pulsar rotates
11 times per second, sweeping a powerful beam of radio waves (like the beam
from a lighthouse) around in space. The Vela pulsar is thus clearly visible in both
the 73-cm-wavelength and the 12-cm-wavelength images, and in the gamma-ray
image as an intense, compact source. It is much less prominent in the optical image.
Figure 6.12 shows five views of Vela in these five wavebands.
Two other pulsars are prominent on the far right side of the 73-cm microwave,
x-ray and gamma-ray images. They are the Crab and Gemiga pulsars, both of which
lie at the center of supernova remnants associated with the stars that formed them.
On the left side of the center of the x-ray image, about the same distance to the left
as Vela is to the right, is a very bright, compact object. This is Cygnus X-1, located
in the constellation Cygnus and thought to be a black hole orbiting a companion