Cosmic Quest: Astronomy Basics

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.

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.

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.

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.

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.