Star

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A star is a ball in outer space that is made of very hot plasma (which is like a gas). Stars get energy through nuclear fusion so they can give off light, making them look like they are on fire. Most stars look like shining or twinkling dots from Earth, because they are far away, and Earth's atmosphere 9 the air ) makes it harder to see them well. Our Sun is a star that is much closer 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 too. Scientists have found other planets moving around other stars, but no planet that they have found are similar to Earth.

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 amount 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. That is the estimated age of the Universe.

Some stars are small, and some stars are big. The smallest neutron stars (which are actually dead stars) are no bigger than a city. Supergiant stars are the largest objects in the Universe. They have a diameter about 1,000 times bigger than the Sun. They are about 1.6 billion kilometers across. The star Polaris (the North Star), and the star Betelgeuse, in the Orion constellation are supergiant stars.

The light, heat, and other energy of stars comes from nuclear fusion -- a process of turning one chemical element into another one. Most stars are mostly made of hydrogen and helium. They turn the hydrogen into helium. (When a star is near the end of its life, it begins to change the helium into other 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. It is electromagnetic radiation. This is mostly visible light.

When seen in the night sky without a telescope, some stars appear brighter than other stars. This difference is called 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.

Beside light, stars also give off a solar wind and neutrinos. These are very small particles.

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.

[edit] Birth and life

Stars are thought to begin in nebulas. These are large areas of slightly higher density than normal space.

When very big stars die, they explode. This is called a supernova. When a supernova happens in a nebula, it becomes unstable. This makes parts of the nebula collapse. Stars form in these collapsed areas. The Orion Nebula is an example of a place where stars form.

Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high pressure reactions near the core. Such stars are said to be on the main sequence.

Small stars (called red dwarfs) burn their fuel very slowly and might last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs.

As most stars use up their supply of hydrogen, their outer layers expand and cool to form a red giant. (Astrophysicists think that in about 5 billion years, when the sun is a red giant, it will swallow up Mercury and Venus.) Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. (Larger stars will also fuse heavier elements, all the way to iron.)

An average-size star will then shed its outer layers as a "planetary nebula". The core that remains will be a tiny ball of degenerate matter, not heavy enough for further fusion to take place, supported only by its own pressure, called a white dwarf. It will fade into a black dwarf over very, very long stretches of time.

In larger stars, fusion continues until collapse ends up causing the star to explode in a supernova. This is the only cosmic process that happens on human timescales; historically, supernovae have been observed as "new stars" where none existed before. Most of the matter in a star is blown away in the explosion (forming nebulae such as the Crab Nebula) but what remains will collapse into a neutron star (a pulsar or X-ray burster) or, in the case of the largest stars, a black hole.

The blown-off outer layers includes heavy elements, which are often converted into new stars and/or planets. The flowing out of the gas from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.