When you look outside at night you can see many beautiful stars. If you are in a dark country area you might see as many as 3000 of them. In the city you can't see nearly as many, but they are still pretty. There are several different kinds of stars in the sky. Some are very big. A couple have been found that are 100 to 200 times bigger than the sun. At the end of their lives these large stars can stretch themselves out past the orbit of the planet Uranus. Some very old stars are smaller than the Earth. Scientists study stars, and place them in groups based on how they are alike, and how they are different.
Read about these star groups below:
Red Dwarf
Red Dwarf stars can range in size from a hundred times smaller than the sun, to only a couple of times smaller. Because of their small size these stars burn their fuel very slowly, which allows them to live a very long time. Some red dwarf stars will live trillions of years before they run out of fuel.
Why are red dwarf stars red?
Because red dwarf stars only burn a little bit of fuel at a time, they are not very hot compared to other stars. Think of a fire. The coolest part of the fire at the top of the flame glows red, the hotter part in the middle glows yellow, and the hottest part near the fuel glows blue. Stars work the same way. Their temperature determine what color they will be. Thus we can determine how hot a star is just by looking at its color.
Red dwarf stars are by far the most common type of star in outer space. However very few stars that you see in the sky are red dwarfs. This is because they are so small and make very little light. Imagine standing on a mountain. Pretend that there are one million kids five miles away holding flash lights. And 20 miles away there is a light house for ships. You will most likely not see any of the flash lights, while you will very easily see the light house. If the flash lights all glowed as brightly as the light house it would blind you. Like wise if all the red dwarf stars glowed as bright as the bigger stars night time would be very bright.
Yellow Star
Like the Sun, these medium sized stars are yellow because they have a medium temperature. Their higher temperature causes them to burn their fuel faster. This means they will not live as long, only about 10 billion years or so. Near the end of their lives, these medium sized stars swell up becoming very large. When this happens to the Sun it will grow to engulf even the Earth. Eventually they shrink again, leaving behind most of their gas. This gas forms a beautiful cloud around the star called a Planetary Nebula.
When will the Sun expand into a giant, and then shrink leaving behind a planetary nebula?
Don't worry, The sun is only about 5 billion years old. It still has another 5 billion years before it will expand, and then turn into a planetary nebula.
The Sun is so hot that when it dies, it will take a long time to cool off. The sun will die in about 5 billion years, but it will still glow for many billions of years after that. As it cools, it will be what is called a white dwarf star. Eventually, after billions maybe even trillions of years, it will stop glowing, at that point it will be what we call a black dwarf star. There are still no black dwarf stars in the Universe.
Planetary Nebula:
Blue Giant
Because blue stars are large, and compact, they burn their fuel quickly, which gives them a very high temperature. These stars often run out of fuel in only 10,000 - 100,000 years.
A blue giant is very bright. Like a light house, they shine across a great distance. Even though blue giant stars are rare, they make up many of the stars we see at night.
Blue giant stars die in a spectacular way. They grow larger just like the Sun sized stars, but then instead of shrinking and forming a planetary nebula they explode in what is called a super nova. Super nova explosions can be brighter than an entire galaxy, and can be seen from very far away.
Because blue giant stars only live a short time, scientists use them to find places in outer space where new stars are forming.
Giant Star
Remember when we talked about sun sized stars? We said that at the end of their lives these stars expand, taking up much more space then before. This is exactly what a Giant Star is.
As a sun sized star gets old, it starts to run out of its hydrogen fuel. When the burning of hydrogen in the star's core begins to slow down, the core gets more compact. This means all the stuff in the middle of the star gets really close together. As the center gets smaller and smaller it starts to heat up again. When it gets hot enough it will start to burn a new fuel called helium.
Once ignited, helium burns much hotter then hydrogen. The additional heat pushes the outside of the star much further than it used to be, making the star much larger. Imagine a hot air balloon. As the air gets hotter it stretches the outside of the balloon further and further. As the star gets hotter its outside stretches out further and further. When our own sun begins to stretch into a giant star it will engulf Mercury, Venus, Earth, and Mars.
Many of the stars you see at night are giant stars. This is because like a light house, giant stars glow very brightly. When the Sun becomes a giant star it will be seen from much further than it is right now.
Super Giant Star
A super giant star is the exact same thing as a giant star, only bigger. Remember that as a star gets older it begins to run out of fuel. As the star runs out of fuel, it will start to burn out.
Just like the sun sized stars blue giant stars also begin to burn helium. As they do these stars get much hotter. This extra heat makes the outside of an old blue giant star stretch out further. Remember how hot air balloons stretch out as the air in them gets hotter.
The only difference between Giant Stars and Super Giant Stars is their size. Super Giant Stars are much bigger. If the Sun were replaced by a super giant star it would extend almost all the way out to Uranus.
Dead super giant stars/blue giant stars often turn into black holes. A black hole is a very compact object. Learn more about black holes How does this happen? As the star dies it explodes in a huge explosion called a Super Nova. The super nova blasts away most of the star. Anything left begins to fall in to the middle of the star. It gets more and more compact, and smaller and smaller. If there is enough of the star left after the explosion, the star will be heavy enough to squash it down to the size of an atom, or even smaller.
