Fifteen billion years ago our universe did not exist. From the tiny planet we came home, to our Local Galaxy and beyond, here's the story of how everything came to be. 

Our universe came into being in a cosmic explosion, the big bang, expanding from nothing to 2 billion billion km (1.25 billion billion miles) wide in a single second, and it is still expanding today. At present, scientists can only speculate about why the big bang occurred, but we are beginning to understand what happened in the first few moments.

There are billions of galaxies in the universe, each containing billions of stars. Galaxies typically cluster together. Our Galaxy is one of about 30 in a cluster known as the Local Group. One of the nearest galaxies to ours is Andromeda, 2.2 million light years away. 


Early evolution of the universe

The big bang

Within a second after the big bang, the building blocks of all matter were created, but it took another 2 billion years before the first stars and galaxies started to form.

After 1 millionth of a trillionth of a trillionth of a second (10-43 seconds)

The temperature of the infant universe is 100,000 billion billion billion°C. The universe expands rapidly and fills with radiation, mostly in the form of light and heat. Gravity appears as a distinct force.

After 10 000 trillionths of a trillionth of a second (10-32 seconds)

Expansion slows down. Quarks, the smallest known particles, appear and start to combine to create larger subatomic particles.

After 10millionths of a second (10-5 seconds)

Subatomic particles combine to form protons and neutrons, the two components of the nuclei of atoms.

After 100 seconds

The temperature drops to 1billion°C. Space is now filled with protons, neutrons, and electrons; the three particles that make up atoms. Over the next 32,000 years, protons and neutrons react with background radiation to combine and form nuclei of hydrogen and helium—the two simplest chemical elements.

After 1 billion years

The universe becomes transparent and its temperature drops to about 4000°C, low enough for complete atoms to form. These are pulled together by gravity, creating clumps of matter.

After 2 billion years

The first stars and galaxies begin to condense from clouds of gaseous hydrogen and helium. 


How do we know what happened billions of years ago? 

Simply put, we can see it. Looking across the vast distances to stars, space and time become impossible to separate. We can only see objects when the light from them reaches us. The farther away an object is, the longer it takes. 

For instance, it takes eight years for the light from the brightest star, Sirius, to travel to Earth, so we are actually looking at it as it was eight years ago. With more distant objects we are looking even farther back in time. It takes the light from the Virgo Cluster 50 million years to reach us, so we are looking at it as it was long before human beings even existed.

Stellar objects emit other types of radiation, such as radio waves, as well as light. These can be detected by specialized telescopes, and the data they provide helps to build up a fuller picture of the universe.

At the moment, the farthest galaxies that we can perceive are 13 billion light years away, only 2 billion years after the big bang. In theory, if we could see far enough we should be able to see right to the beginning of the universe. 



Stars are not distributed evenly throughout the universe; they clump together in galaxies. In turn, galaxies group together in clusters and superclusters. Although stars appear closely packed in galaxies, they are separated by vast distances. If our Sun were the size of a grain of sand, its nearest star neighbor would be 6 km (4 miles) away. 

Galaxies are classified by shape. There are three main types:

Spiral Galaxy

Spiral Galaxy

About 30 percent of galaxies are believed to be spiral. There are two kinds. Normal spirals are pinwheel-shaped with a central bulge and spiral arms. Barred spirals (left) have an elongated central region and protruding arms.



Elliptical Galaxy

Most galaxies are thought to be this shape, a stretched sphere. They range from the virtually spherical to almost flattened. M87 in Virgo is an example. 


Irregular Galaxy

Irregular Galaxy

Many galaxies have an ill-defined structure with no definite outline. The Magellanic Cloud in our Local Cluster is an example of an irregular galaxy. 


The Milky Way

The Milky Way

Our Galaxy consists of at least 200 billion stars and their planets, grouped into a flattened disk with spiral arms and a bulge at its center.

Looking up from Earth along the plane of this disk, the Galaxy appears as a luminous band of stars and glowing gas—the Milky Way—spanning the sky.

The whole Galaxy is sometimes referred to as the Milky Way Galaxy, but, strictly speaking, the term refers to the luminous band of stars visible from Earth.


Galaxy features


Black holes

The name given to immeasurably dense collapsed stars with such a strong gravitational pull that nothing, not even light, can escape from them. The size of black holes is dependent upon the mass of the collapsed star. Because they are invisible, no black hole has been detected directly. Their existence can only be inferred from the effect they have on other objects.



These are cores of very active distant galaxies, possibly with black holes at their centers. They are point sources of radio waves. Because they are so distant, light from them has taken a long time to reach us. When we look at a quasar, we are looking at a galaxy in a very early stage of its evolution.


Colliding galaxies

If galaxies move close enough for their gravitational fields to affect each other, the structure of one or both galaxies can alter radically. They may collide and even merge. The closest colliding galaxies to us are NGC 4038 and 4039, known as the Antennae. They are just 80 light years apart and streams of material from them are already converging. Eventually the two systems will merge.


Dark matter

Also known as missing mass, this is matter that cannot be seen directly because it emits little or no radiation. Its presence can be inferred from the effect it has on other bodies. Its gravitational force explains the rotation speeds of galaxies and the fact that they tend to group together into clusters.

It has been estimated that as much as 90 percent of the matter in the Universe is dark matter, in the form of particles left over from the big bang. 


Professor Lucy Green on the wonder of our sun:


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