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It radiates heat and light, and every other part of the electromagnetic spectrum, such as radio waves, micro-waves, X-rays, gamma-rays and ultra-violet radiation. The proportions vary according to the mass and age of the star.
The energy of stars comes from nuclear fusion. This is a process that turns a light chemical element into another heavier element. Stars are mostly made of hydrogen and helium. They turn the hydrogen into helium by fusion. When a star is near the end of its life, it begins to change the helium into other heavier chemical elements, like carbon and oxygen. Fusion produces a lot of energy. The energy makes the star very hot. The energy produced by stars radiates away from them. The energy leaves as electromagnetic radiation.
Birth of a star
A star begins as a collapsing cloud of material made mostly of hydrogen, with helium and tiny amounts of heavier elements. Once the stellar core is dense enough, some of the hydrogen is changed into helium through nuclear fusion. The energy moves away from the core by a combination of radiation and convection. The star's radiation stops it from collapsing further under its own gravity. Once the hydrogen fuel at the core has been used up, those stars with at least 0.4 times the mass of the Sun expand to become a red giant. In some cases they fuse heavier elements. When the star is very old it might expand until its outer layers are pushed away. If the star is heavier, it might explode and spread most of its mass into space. The matter it spreads into space may make a new generation of stars. If the star is even larger it may collapse and form a black hole.
Earth and Sun
The star nearest to Earth is the Sun. It is not accurate to say "most of the energy on Earth comes from it". It is more accurate to say the energy of sunlight supports almost all life on Earth by photosynthesis, and drives Earth's climate and weather. The Earth has its own source of heat in its interior: see Age of the Earth. The internal heat comes from the original gravitational formation of the Earth, and from the radioactive materials inside it.
We can see other stars in the night sky when the Sun goes down. They are made mostly of hydrogen and a little bit of helium plus other elements. The sun is such a star.
Most stars look like shiny dots from Earth, because they are far away. Our Sun is the closest star to us. The earth moves around (orbits) the Sun in an oval shape. The Sun and all things that orbit it are called the Solar System. Many other stars sometimes have planets orbiting them: they are called exoplanets.
The nearest star to our Solar System, and the second nearest star to Earth after the Sun, is Proxima Centauri. It is 39.9 trillion kilometres away. This is 4.2 light years away, meaning that light from Proxima Centauri takes 4.2 years to reach Earth.
Astronomers think there are a very large number of stars in the Universe. They estimate (guess) that there are at least 70 sextillion stars. That is 70,000,000,000,000,000,000,000; which is about 230 billion times the number of stars in the Milky Way (our galaxy).
Most stars are very old. They are usually thought to be between 1 and 10 billion years old. The oldest stars are thought to be around 13.7 billion years old. Scientists think that is close to the age of the Universe.
Stars vary greatly in size. The smallest neutron stars (which are actually dead stars) are no bigger than a city. The neutron star is incredibly dense. If you were to take a layer a micrometre thick and apply it onto a tank, it would be a very tough armor. The tank would be so heavy, it would sink into the center of the Earth.
Hypergiant stars are the largest stars in the Universe. They have a diameter over 1,500 times bigger than the Sun. If you changed the sun into one of these huge stars down where the sun is, its outer surface would reach beyond the orbit of Jupiter, and the earth would be well inside the star. The star Betelgeuse, in the Orion constellation is a red supergiant star.
When seen in the night sky without a telescope, some stars appear brighter than other stars. This difference is measured in terms of apparent magnitude. There are two reasons for stars to differ in apparent magnitude. If one star is much closer than another otherwise similar star, it will appear much brighter, in just the same way that a candle that is near looks brighter than a big fire that is far away. If one star is much larger or much hotter than another star at about the same distance, it will appear much brighter, in just the same way that if two fires are the same distance away, the bigger or hotter one will look brighter. A star's true luminosity is its absolute magnitude.
Stars are a source of a gravity field. This is what keeps planets close to them. It is also not unusual for two stars to orbit each other. This happens when they are close together. This is also because of gravity, in the same way as the Earth orbits the Sun. These binary stars (binary meaning "two") are thought to be very common. There are even groups of three or more stars orbiting each other. Proxima Centauri is the smallest star in a group of three.
Stars are not spread evenly across all of space. They are grouped into galaxies. A typical galaxy contains hundreds of billions of stars.
History of seeing stars
Stars have been important to people all over the world for all of history. Stars have been part of religious practices. Long ago, people believed that stars could never die. Astronomers organized stars into groups called constellations. They used the constellations to help them see the motion of the planets and to guess the position of the Sun. The motion of the Sun and the stars was used to make calendars. The calendars were used by farmers to decide when to plant crops and when to harvest them.
The life of stars
Stars are made in nebulas. These are big areas that has more gas than normal space has. The gas in a nebula is attracted to all the other gas by gravity. This makes the gas in the nebula very thick. Stars form in these thick areas. The Orion nebula is an example of a place where gas is coming together to form stars.
Stars spend most of their lives burning (fusing) hydrogen to make energy. When hydrogen is fused it makes helium. To fuse hydrogen it must be very, very hot and the pressure must be very, very high. Fusion happens at the center of stars. The center part of the star is called "the core".
The smallest stars (called red dwarfs) fuse their hydrogen very slowly and live for 100 billion years. Red dwarfs live longer than any other type of star. At the end of their lives, they become dimmer and dimmer. Red dwarfs do not explode.
When very heavy stars die, they explode. This is called a supernova. When a supernova happens in a nebula, the explosion pushes the gas in the nebula together. This makes the gas in the nebula very thick. Gravity and exploding stars both help to make new stars in nebulas.
Most stars use up the hydrogen at their core. When they do, their core shrinks and becomes very, very hot. It becomes so hot it pushes away the outer layer of the star. As the outer layer expands it makes a red giant star. Astro-physicists think that in about 5 billion years, the sun will be a red giant. Our sun will be so large it will eat the Earth. After our sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Elements as heavy as iron are made in heavy stars.
Average stars (like our sun) will push away their outer gases. The gas it pushes away makes a cloud called a planetary nebula. The core part of the star will remain. It will be a ball as big as the Earth and called a white dwarf. It will fade into a black dwarf over a very, very long time.
In heavier stars, heavier and heavier elements are made by fusion. Finally the star makes a supernova explosion. Most things happen in the universe so slowly we do not notice. But supernova explosions happen in only 100 seconds. When a supernovae explodes it's flash is as bright as a 100 billion stars. The dying star is so bright it can be seen during the day. Supernova means "new star" because people used to think it was the beginning of a new star. Today we know that a supernova is the death of an old star. The gas of the star is pushed away by the explosion. It forms a giant cloud of gas called a planetary nebulae. The Crab Nebula is a good example. All that remains is a neutron star. If the star was very heavy, the star will make a black hole in the universe. Gravity in a black hole is very, very strong. It is so strong, not even light can escape from a black hole.
The heaviest elements are made in the explosion of a supernova. After billions of years of floating in space, the gas and dust come together to make new stars and new planets. The gas and dust in space comes mostly from supernovae. Our sun, the Earth, and all living things are made from star dust.
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