There are many scales of building blocks of which the Universe is made. Quarks are fundamental components out of which normal matter is constructed. Most of the gravitating Universe is composed of Dark Matter, but we simply don't know what this stuff is. On the other extreme, the material Universe is composed of clusters of glowing galaxies bound together by the gravity of a huge distributions of dark matter. These dark-matter dominated galaxy clusters form a basic unit which astrophysicists use to understand the cosmic web created when tiny temperature fluctuations present at the Big Bang grew into the distribution of galaxies, stars and planets we see all around us. These enormous galaxy clusters can be built up by accreting smaller groups of galaxies. The Chandra X-ray Observatory has helped resolve this process in exquisite detail. The image above shows, in the lower right, an X-ray image (in blue) of a galaxy cluster called Abell 2142, superimposed on an optical image from the Sloan Digital Sky Survey. In the upper left, Chandra shows x-ray emitting hot gas from a smaller group of galaxies which is plunging towards Abell 2142. As this group of galaxies falls towards Abell 2142, X-ray emitting hot gas is swept behind by the motion of the group. As Abell 2142 absorbs this smaller group, its gravitational pull increases, helping it to pull even more material into itself, growing ever larger, and forming one more nexus in the cosmic web.
CXC: Making Head or Tail of a Galactic Landscape
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