140 Chapter 3 The Universe Was Born With a Big Bang
What Is the Large-Scale Structure of the Cosmos?
The philosopher Kant and many others reasoned that there must be a hierarchical, large-
scale structure in the universe, just like the hierarchical group structures in the world we
live in—that is, just as houses make up neighborhoods, neighborhoods make up cities, many
cities make up states, and states make up a country, so too must the heavenly bodies be
grouped into patterns. However, this really became clear when the existence of galaxies out-
side of our own galaxy was verified. Later observations and research also made it apparent
that many galaxies collect together to create a group, and these in turn form aggregations
at even higher levels of the cosmic hierarchy.
Planetary System
A planetary system is a system like our solar system, in which planets, asteroids, satellites,
comets, and other matter form a single system orbiting around a star.
A galaxy is a heavenly body that is formed by the gravitational attraction of several tens of
billions to several hundreds of billions of stars and interstellar matter (including dark matter).
Since galaxies exist in space like islands in the sea, they used to be referred to as island uni-
verses. The galaxy to which our solar system belongs is called the Milky Way galaxy.
The phrase “island universe” fell out of use once astronomers realized that there were
many “island universes” (that is, galaxies) besides the Milky Way.
Group of Galaxies or Cluster of Galaxies
Groups and clusters are multiple galaxies that are gravitationally unified. When the number
of galaxies is fewer than 50, we call the aggregation a group of galaxies; when the number
is greater (up to several thousand), we call it a cluster. Our Milky Way galaxy belongs to the
Local Group, which is made up of 30 to 40 galaxies, including the Andromeda galaxy and
the Large and Small Magellanic Clouds. The closest cluster of galaxies to the Local Group
is the Virgo Cluster, which is approximately 60 million light-years away. It appears in the
constellation Virgo, and it has a diameter of approximately 12 million light-years.
Milky Way
of galaxies
of galaxies
Large-scale structure of the cosmos

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