### Theory:

Since ancient times, human has the fascination about the starry night sky. Objects found in the night sky have been taken care of and documented by our ancestors.
The field of investigation of the universe is called astronomy.
Whatever Existing and known to us are characterized as universe.
Although the whole universe's spatial size is unclear, we shall able to measure the known part of the universe. Currently, it is evaluated as $$93$$ billion light-years in diameter.

Astronomical distances are expressed in terms of a unit length called as light-year.
A light-year is the distance travelled by light in a year.
Example:
$$1$$ light-year  trillion kilometres ($9.46×{10}^{12}$ km) or $5.88$ trillion miles ($5.88×{10}^{12}$ mi).
The universe consists of galaxies, planets, stars, meteorites, satellites, and other forms of matter and energy.

Hierarchy of the Universe:
1. Universe
2. Galaxy
3. Solar system
4. Planets
5. Satellites