Everything that exists makes up what we call the universe. Broadly speaking, the universe is made up of matter, energy and space. The Earth and everything on it - rocks, air, water, plants, humans - form part of the matter of the universe. So do the other planets, the Moon, the Sun and the stars we see in the night sky. All these forms of matter float in, or rather travel through, space. But exactly how big is space, is the universe?
When we look up at the night sky, we are peering into a tiny part of the universe. It is occupied by stars seemingly suspended in the inky blackness of space. Through our telescopes, no matter how powerful they are, the stars always appear only as tiny points of light. Yet we know that the stars are in reality other Suns, great balls of searing-hot gas a million kilometres or more in diameter. So, as they show only as tiny points, the stars must lie far, far away.
And so they do. Astronomers tell us that even the nearest stars lie more than 40 million million km away. And the farthest star systems, or galaxies, we can see in powerful telescopes lie thousands of millions of times farther away still! Such distances are more than the human mind can grasp or even imagine. So, to the question: How big is the universe? we must reply: Bigger than we can ever imagine.
But we can get a rough idea of the scale of the universe, if we take an imaginary trip in a spaceship, starting from Earth and travelling out through the solar system to the stars and beyond.
But an ordinary rocket craft like the space shuttle is no good - it is far too slow. We must ride in a super spaceship that can travel at the highest speed possible. This is the speed of light. Light travels at a speed of about 300,000 km a second, or 1,080 million km an hour.
Science fiction stories and films feature starships propelled by photon rockets, which achieve the speed of light by beaming intense light rays from their engines. The 'Star Trek' adventure series features the best known of these, the starship Enterprise. NASA called the first prototype space shuttle orbiter Enterprise in its honour.
We start out from Earth and visit the Moon first. The 385,000-km lunar journey, at the speed of light, is over in a blink of an eye, taking a little over a second. We next aim for the Sun, nearly 150 million km away. We get there in 81 minutes.
From there we venture forth to the planet Pluto in the outskirts of the solar system. We reach the planet, currently about 4,400 million km away, in just four hours.
Even though we are on the edge of the solar system, the stars appear no closer. They are still only pinpoints of light. But we make course for a small dim red star in the constellation Centaurus (the Centaur). We call it Proxima Centauri. It is the nearest star to us.
The journey to Proxima Centauri takes not just hours, nor days, nor months, but years. In fact about four years and three months (4.28 years)! That is how long it has taken us, travelling at the speed of light, to cover the distance just to the nearest star.
And, of course, that is also the time it takes light from the star to reach the Earth. So we can say that from Earth the star is 4.28 years away, travelling at the speed of light. And we can also say that it lies at a distance 4.28 times the distance light travels in a year.
Astronomers use the distance light travels in a year as a convenient unit for measuring distances in space. They term it the light-year. It is equal to about 9.5 million million km. Using this unit, distances in space become easier to express. So Proxima Centauri is 4.28 light-years away, which is easier to grasp than saying it is forty million million kilometres away.
From Proxima Centauri, one of the dimmest stars in our night sky, we travel to one of the brightest, Deneb in the constellation Cygnus (the Swan). To reach this brilliant supergiant star takes over 1,600 years. Yet Deneb is still a near neighbour in the star system, or galaxy, that we inhabit.
Our galaxy, which is called the Milky Way, measure no less than 100,000 light-years across. So it would take us 100,000 years to travel from one side to the other.
When we consider travelling to other galaxies, the distances, and our times of travel, increase by leaps and bounds. It would take us 170,000 years to reach the nearest galaxy, the Large Magellanic Cloud. It would take over 2,000,000 years to travel to the Andromeda galaxy. And this is still a neighbour as far as the universe is concerned!
Only if we journeyed for some 15,000 million years from Earth would we reach the most distant heavenly bodies we can see in the night sky. And that is the same time that has past since the universe was created!.
