Galaxies are the largest single structures in nature, and certainly among the most beautiful. These vast, starry islands have slowly been sculpted over billions of years by the force of gravity. It’s believed that there are about 100 billion galaxies in the universe, and that most of them have a black hole at their centre.

Galaxies are the largest single structures in nature, and certainly among the most beautiful. These vast, starry islands have slowly been sculpted over billions of years by the force of gravity. It’s believed that there are about 100 billion galaxies in the universe, and that most of them have a black hole at their centre.

© Canadian Heritage Information Network, 2003

The galaxy of the Swirl

The galaxy of the Swirl (M51) and its galaxy partner NGC 5195.

National Research Councel of Canada

© Herzberg institute of astrophysiques


Most astronomers believe that the universe began about 15 billion years ago in a fiery explosion called the Big Bang .

In one instant, all the space, time, matter and energy in the universe came into being as a tiny speck, then started to expand rapidly. As the universe got bigger and cooler, the force of gravity started to take hold.

At the moment of the Big Bang

A fraction of a second after the Big Bang, the infant universe went through a sudden growth spurt, called inflation. In far less than a billionth of a second, the universe grew faster than at any other time in its history. Inflation only lasted an instant, and then the universe slowed down to the rate of expansion we see today.

After one second

As the universe continued to expand and cool, protons, neutrons and electrons condensed out of the background energy, especially in areas where the energy was slightly "denser."

After 100 seconds

Protons and neutrons combined to form helium nuclei. Hydrogen nuclei, which are simply protons, already existed. In less than two minutes, all the sub-atomic elements that exist today had been for Read More
Most astronomers believe that the universe began about 15 billion years ago in a fiery explosion called the Big Bang .

In one instant, all the space, time, matter and energy in the universe came into being as a tiny speck, then started to expand rapidly. As the universe got bigger and cooler, the force of gravity started to take hold.

At the moment of the Big Bang

A fraction of a second after the Big Bang, the infant universe went through a sudden growth spurt, called inflation. In far less than a billionth of a second, the universe grew faster than at any other time in its history. Inflation only lasted an instant, and then the universe slowed down to the rate of expansion we see today.

After one second

As the universe continued to expand and cool, protons, neutrons and electrons condensed out of the background energy, especially in areas where the energy was slightly "denser."

After 100 seconds

Protons and neutrons combined to form helium nuclei. Hydrogen nuclei, which are simply protons, already existed. In less than two minutes, all the sub-atomic elements that exist today had been formed.

After 300 000 years

By this time, the universe had cooled enough for electrons to combine with atomic nuclei, thereby forming atoms. These atoms slowly gravitated into vast strand-shaped clouds, from which the galaxies would soon emerge.

After one billion years

Clouds of hydrogen and helium began "clumping" under the force of gravity. As the clouds grew denser, early galaxies-called protogalaxies-began to take shape. These protogalaxies grew bigger and bigger. Some of them started spinning and flattening out into disk-like shapes. Others stayed more or less spherical as they grew.

Eventually, these primal galaxies gathered enough mass for stars to ignite within them, and for the first time, the universe took on the appearance we see today.

© Canadian Heritage Information Network, 2003

Cosmic Evolution

Temperature fluctuations in the oldest light in the universe.

NASA / WMAP

© NASA / WMAP


Cosmic Evolution

Matter condensing under the force of gravity.

NASA / WMAP

© NASA / WMAP


Cosmic Evolution

The first stars form, just 200 million years after the Big Bang.

NASA / WMAP

© NASA / WMAP


Cosmic Evolution

The first galaxy chains start to form, 1 billion years after the big bang.

NASA / WMAP

© NASA / WMAP


Cosmic Evolution

The modern era, nearly 14 billion years after the Big bang.

NASA / WMAP

© NASA / WMAP


Hubble Deep Field

This remarkable image, called the Hubble Deep Field, represents a "core sample" of the cosmos. It reveals about 1500 galaxies, seen in an area of sky no larger than a grain of sand held at arm's length! Many of these were among the first to emerge after the Big Bang.

STScI / NASA

© STScI / NASA


Like snowflakes, no two galaxies are exactly alike. However, all galaxies have certain features in common, which means we can group them into categories. Though there are many ways to do this, most people still use the system developed by Edwin Hubble in 1925. Hubble grouped galaxies according to two distinct shapes: elliptical and spiral.

Like snowflakes, no two galaxies are exactly alike. However, all galaxies have certain features in common, which means we can group them into categories. Though there are many ways to do this, most people still use the system developed by Edwin Hubble in 1925. Hubble grouped galaxies according to two distinct shapes: elliptical and spiral.

© Canadian Heritage Information Network, 2003

Spiral galaxy

Spiral galaxy NGC 268.

Canada-France-Hawaii Telescope

© CFHT


Galaxies come in many shapes and sizes. No two are exactly alike, but they do tend to follow certain general forms.

Spiral galaxies

Spiral galaxies are flat with bright and dark arms winding around a central core. Some have bright bars across their centres or a bulge in the middle. The spiral shape comes from a wave of star formation that proceeds along the disk-like a wave of sports fans standing up in the bleachers. Our Milky Way galaxy is spiral shaped.

Elliptical galaxies

Elliptical galaxies are fat balls of old, cool stars. Most formed when spiral galaxies collided and lost their star-forming gas as they merged. Elliptical galaxies take either an ellipsoidal (like a U.S. football) or spherical (like a beach ball) shape.

Irregular galaxies

Some galaxies are neither elliptical nor spiral-these galaxies are called irregular. Irregular galaxies are small and take a wide range of forms. They are often distorted by the violence of star birth or stretched into odd shapes by the gravitational pull of neighbouring galaxies.
Galaxies come in many shapes and sizes. No two are exactly alike, but they do tend to follow certain general forms.

Spiral galaxies

Spiral galaxies are flat with bright and dark arms winding around a central core. Some have bright bars across their centres or a bulge in the middle. The spiral shape comes from a wave of star formation that proceeds along the disk-like a wave of sports fans standing up in the bleachers. Our Milky Way galaxy is spiral shaped.

Elliptical galaxies

Elliptical galaxies are fat balls of old, cool stars. Most formed when spiral galaxies collided and lost their star-forming gas as they merged. Elliptical galaxies take either an ellipsoidal (like a U.S. football) or spherical (like a beach ball) shape.

Irregular galaxies

Some galaxies are neither elliptical nor spiral-these galaxies are called irregular. Irregular galaxies are small and take a wide range of forms. They are often distorted by the violence of star birth or stretched into odd shapes by the gravitational pull of neighbouring galaxies.

© Canadian Heritage Information Network, 2003

The Hubble "tuning fork"

The Hubble "tuning fork" arranges galaxies according to shape. However, the galaxies do not evolve from one shape to the next. Their shapes are the product of the initial conditions under which they formed.

STScI

© STScI


When galaxies collide, they pass through one another like ghosts moving through a wall.

Their stars are spaced so far apart that they rarely touch. However, the pull of gravity distorts the galaxies, spraying their stars across space.
When galaxies collide, they pass through one another like ghosts moving through a wall.

Their stars are spaced so far apart that they rarely touch. However, the pull of gravity distorts the galaxies, spraying their stars across space.

© Canadian Heritage Information Network, 2003

"Polar Ring" galaxy

Polar Ring galaxies temporarily form when two galaxies collide.

Space Telescope Science Institute

© US Gov public Domain


Learning Objectives

The learner will:

  • Define galaxy
  • Conceptualize the Big Bang, and describe the events from an atomic perspective that resulted in the formation of galaxies
  • Describe, in pictures and words, galaxy features

Teachers' Centre Home Page | Find Learning Resources & Lesson Plans