Basic Objects in the Universe

This module starts with the question, “What is Astronomy?”Simply stated, Astronomy is the study of the Universe. Astronomers classify objects into various groups based on characteristics and locations in our Universe. Some of these classifications, such as stars, are easy to understand scientifically. Other objects are more difficult to classify, and in some instances, to entirely understand. This module identifies basic objects in our Solar System and those beyond, the overall scale of the Universe, and patterns in the night sky.


Stars are glowing balls of gas that undergo nuclear fusion; the Sun is a star.

Image of stars in the night sky.
Public Domain | Image courtesy of NASA / ESA.


Planets are moderately large objects orbiting a star. We see planets because they reflect the light of their central star, or in some cases, stars. Planets are generally rocky or gaseous in nature and spherical-shaped.

A new group of objects has been recently defined: the Dwarf Planets or Plutoids. These are objects that orbit the Sun, but have not cleared their orbits. Pluto is an example of a Dwarf Planet.

Image of Planets in our Solar System, including Mercury, Venus, Earth, Mars, Saturn, Jupiter, Uranus, Neptune, and the moon.

Public Domain | Image courtesy of NASA.


satellite orbits a planet; these objects are also called moons. For example, the Earth’s satellite is the Moon – a proper name.

Image of the Moon, Earth’s Satellite, behind Earth in the distance.
Public Domain | Image courtesy


An asteroid is a relatively small, rocky/metallic object usually orbiting a star.

Image of the asteroid, Eros.

Public Domain | Image courtesy of NASA.


comet is a relatively small, icy object usually orbiting a star. Asteroids, comets, and miscellaneous small/irregular objects and “dust” are often categorized as Minor Bodies.

Image of Comet Hyakutake by NASA photographer Bill Ingalls.
Public Domain | Image courtesy of NASA.

Solar System

The Solar System is the Sun and all the objects that orbit the Sun, including the planets and their moons.

Image of our Solar system, including Mercury, Venus, Earth, Mars, Saturn, Jupiter, Uranus, Neptune, Pluto, and the Sun.
Public Domain | Image courtesy of

Stellar System

Stellar System is a star and other objects such as planets and/or other stars and other materials that orbit it.


galaxy is a large island of stars, a few hundred million to over a trillion stars.

Image shows spiral galaxy ESO 137-001, framed against a bright background as it moves through the heart of galaxy cluster Abell 3627.
CC BY 3.0 | Image courtesy of ESA/Hubble. NASA, ESA Acknowledgements: Ming Sun (UAH), and Serge Meunier

Galactic Cluster

Galactic Cluster is a collection of galaxies gravitationally bound.


Supercluster is a region where galaxies and galactic clusters are tightly packed.


The Universe is all matter and energy, and is also called the Cosmos.

Image of a universe with vast amounts of stars and planets.
Public Domain | Image courtesy of