What are stars?

What a wondrous sight it is to see the night sky ablaze with thousands of twinkling, yellow lights contrasted against the black sky. Have you ever wondered what those stars are made of and just why they twinkle so brightly?

The stars that we see on a clear night may resemble tiny, scattered fairy lights from here on Earth, but in reality they are very different. Stars are, in fact, huge balls of hot gas very much like our sun. The only difference between our sun and other stars is their distance from the Earth. The sun is the closest star to the Earth. That is why it appears so big to us and why we are able feel its heat and share its light.

How big are stars?

Other stars that we see in the sky can vary greatly in size. Some are much bigger than the sun and others much smaller. Apart from the sun, others that we see in the sky all look roughly the same size when we look from Earth. This is because they are so far away from us.

The sizes of stars actually differ greatly. Extremely big stars are known as giantstars and much smaller stars are called dwarf stars. To help understand the vast size difference of stars, just imagine that our sun was the size of a balloon. Compared to a balloon, giant stars could be as big as the size of a small suburb and dwarf stars could be as small as a five cent piece.

It is difficult to imagine how far away these stars must be from us for them to appear so small in our sky. As an example, the closest star to us beyond the sun is over four light years away (one light year is approximately 9.5 billion kilometres).

How hot are stars?

The temperatures of stars vary greatly also, depending on the star's size and stage of life. Astronomers can tell the temperature of stars by the colour and brightness of the light they emit (send out). Stars with a bright blue/white light are the hottest stars of all. Their temperatures can reach approx 30 000 degrees Celsius. Stars like our sun that are yellow in colour are estimated to be approximately 5500 degrees Celsius. Stars that are slightly dimmer in appearance and have reddish glow have the lowest temperatures of around 3000 degrees Celsius.

How are stars formed?

The universe contains many more stars than planets. The space that exists in between the stars and planets is sprinkled with dust and gas. Clouds of this dust and gas are called nebulae. Over time, gravity causes billions of particles to be forced together into enormous balls. The pressure created inside these balls heats up the particles until nuclear reactions are caused. Stars are then born as a result of these nuclear reactions.

Why do stars appear to twinkle?

When you look closely at stars in the sky you may notice that they do not look like solid lights, but rather they appear to twinkle rapidly. The truth is that stars do actually shine steadily and do not get brighter and dimmer as we would expect. The reason they appear to twinkle is because air currents in the atmosphere make the starlight that we see bend slightly. Therefore, some light reaches our eyes while other light is bent away from our eyes, creating a flickering effect. See animation 1

What is the life cycle of a star?

Individual stars do not last forever. Many stars that we see in the sky may not even exist any more, they may have died millions of years ago. We can still see them, however, because they are so far away. In other words, it takes so long for the light to reach the Earth that we can still see some stars long after they have died.
Each star has a life cycle. Stars are born, they mature, they grow old and then they die. This is similar to our own life cycle as humans.

As explained earlier, stars are formed from nebulae. When the centre of a nebula becomes very hot as a result of the pressure built up inside, it begins to crush hydrogen gas into helium. This chemical reaction releases energy and, as a result, the star begins to shine. Most stars then live for millions of years. During this time they radiate heat and light.

Eventually there comes a time when the hydrogen supplies begin to run out. This causes the star to swell up and become a red giant star. The outer layers of red giants explode to form a colourful cloud of gas which surrounds the star.

After millions of years the star will die. There are two possible ways a star can die. Some red giants shrink into a white dwarf (the last, much cooler stage in the life of a medium-sized star) and then continue shrinking to become a black dwarf (a very small, cold, dead star).

Other red giants actually swell up to become super giants and then explode assupernovae.

A supernova is the spectacular final explosion of a super giant at the end of its life. They are the biggest explosions in the whole universe and are as bright as the light of billions of suns put together. Occasionally, supernovae can be seen with the naked eye. A supernova will appear briefly as a bright new star in the sky and then will disappear again about a week or so later. Even though it is rare to see a supernova, there is one exploding somewhere in the universe at any moment in time. See animation 2

What is a black hole?

After a star explodes as a supernova, the remaining matter shrinks very quickly until all that is left is a small area of space with enormous gravitational force. The gravity is so strong that this tiny area sucks in all matter that is nearby. This can also include nearby stars. This is called a black hole. It is called a black hole because the gravitational pull is so strong and powerful that not even light can resist it. See image 1

What are constellations?

Constellations are patterns of stars grouped together in the sky. They are useful in helping us pinpoint individual stars in the sky. Think of them as being like a map of the sky. Stars within a constellation are not related to each other in any physical way. They are only grouped for reference purposes. They are still light years apart even though they might appear close to each other from Earth.

If there were no recognised constellations, trying to find particular stars in the sky would be almost impossible. Constellations allow astronomers to monitor the stars more closely.

Most constellations were identified by stargazers of Ancient Babylon and Egypt. The stars within each constellation are named after letters from the Greek alphabet. The brightest star in each constellation is called 'alpha', the next brightest is called 'beta' and so on.

In total, the sky is divided into 88 constellations. You can never see all the constellations at once. What you are able to see will depend on things like where the Earth is on its orbit around the sun, where you live and what time of night you are looking.

For many years people have imagined that particular constellations, when joined together in a dot-to-dot manner, resemble creatures, gods or other objects. Among the most well known are the twelve constellations of the zodiac.

What are the constellations of the zodiac?

You may have heard of zodiac signs in relation to the months. Astrologers call these 'star signs' and they believe that human lives are affected by the stars. (Astrologers are not scientists but people who believe that our lives are affected by the movement of planets and stars.)

The zodiac is the band of constellations that the sun appears to travel in front of during the year while the Earth is orbiting the sun. Like other constellations, the twelve zodiac constellations form imaginary symbols related to the patterns of the stars. The Ancient Greeks named each one according to the picture they could see in each one. The twelve constellations of the zodiac are: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.

The word 'zodiac' comes from the Greek word zoidiakos which means 'circle of animal signs'. Seven of the twelve zodiac constellations form animal shapes (such as Leo the Lion), four form people (such as Aquarius, the water bearer) and just one forms an object that is not a living creature (Libra, the scales). See image 2