Education: the universe
WHEN you look at the sky on a dark, cloudless night you can see countless stars and the moon, while in the daytime you can see the sun rising and setting. We take a look at how the universe began and learn more about celestial bodies.
THE BIG BANG
Most astronomers believe the universe started about 14 billion years ago with a massive explosion – the Big Bang. They speculate that before the Big Bang the universe was just a tiny ball, a thousand times smaller than a needle’s point. But it was extremely hot and dense and suddenly exploded with immense force.
The resulting Big Bang, scientists believe, was what created time, space and matter and gave birth to the universe. In a split second the universe expanded, from being smaller than a single atom to becoming bigger than a galaxy – and according to science the universe has continued, and continues, to expand to this day at an incredible pace.
Scientists base their theories on evidence such as the night glows (or cosmic background microwave radiation) from the Big Bang that can be observed with scientific instruments.
American scientist Edwin Hubble (18891953) documented the movement of the galaxies in the 1920s. He determined that the speed of a galaxy is equal to its distance from Earth. This comparative rate is known as Hubble’s Law.
OUR PLACE IN SPACE
Centuries ago people thought Earth was at the centre of the universe and everything revolved around it – the sun, moon and stars. Today we know Earth and the other planets in our solar system move around the sun (ABOVE RIGHT).
We also know that our solar system is part of a vast spiral-shaped galaxy known as the Milky Way.
There are scientists who think our solar system originated when large clouds of dust and gas imploded, causing a downward circular motion – like water washing down a drain. The sun and planets then emerged out of the mist (nebula).
THE MILKY WAY
Our solar system is located in one of the spirals of the Milky Way (ABOVE LEFT), a galaxy made up of billions of stars and their planets, as well as dust particles and gas. It looks like a giant whirlpool that turns at its centre once every 200 million years.
This galaxy is so vast that it takes light about 100 000 years to travel from one end to the other. There are billions of other galaxies in the universe.
OUR SUN
The sun is massive and comprises
about 98% of the matter (any substance that takes up space) in our solar system. Because of its enormous mass, the sun’s gravity is the strongest in our solar system and draws everything else towards it. But the planets also have their own gravity, which causes them to remain in set orbital paths around the sun.
The sun is the closest star to Earth and emits life-giving sunlight and heat. But sunlight also contains potentially harmful ultraviolet rays that can cause sunburn and skin cancer.
The sun is actually a huge ball of superhot gas some 1,4 million km in diameter (the size of 109 planets the size of Earth placed next to one another). Its mass is equal to 330 000 Earths, and about 1,3 million Earths could fit inside it!
Looking at it from Earth the sun may not seem that big, but remember it’s about 150 million km from us. At this distance it takes sunlight (at a speed of about 300 million m/sec) about eight minutes to reach us!
THE PLANETS
In addition to the sun, our solar system has eight main planets. The four rocky ones closest to the sun – Mercury, Venus, Earth and Mars – are relatively small. Beyond Mars there’s an asteroid belt – a collection of rocky objects and space debris left over after the planets were formed.
Beyond this rocky belt we find the four gaseous giants – Jupiter, Saturn, Uranus and Neptune. Although much bigger than Earth, these planets are relatively light in weight because they’re made up mainly of hydrogen and helium gas. Pluto, an icy world on the outskirts of our solar system, is regarded as a dwarf planet because it’s considerably smaller than our moon.
THE MOON
Earth’s natural satellite, the moon, is made of rock. The moon’s orbit around Earth takes 27 days, but the moon also takes 27 days to make a full turn on its own axis. Because Earth also turns (on its own axis as well as around the sun) it seems to us as if the moon’s orbit around Earth takes 29 days. The moon’s gravity is responsible for ocean tides on Earth.
Scientists believe the moon originated 4,5 billion years ago when Earth was created. They speculate that a massive rock collided with our newly formed planet, slicing off a part of it which then became the moon.
The moon doesn’t have an atmosphere (layer of gas and liquid) around it to protect it against comets and meteors, or from the sun’s radiation. It therefore gets extremely hot and cold.
Earth’s atmosphere causes most incoming meteors to burn out before they can hit the ground, but on the moon meteorite strikes can cause huge craters. The moon’s surface is covered in a dust layer about 5cm thick. The dust might come from space or be the result of the impact of meteorites.