Star

A star-forming region in the Large Magellanic Cloud. NASA/ESA image

Home to some of the largest stars known, the open stellar cluster Pismis 24 is in the nebula NGC 6357.
Several stars in the clusters have over 100 times the mass of the Sun. The strange shapes taken by the clouds are a result of the vast radiation emitted by these massive, hot stars.
This image combines observations with three different filters in visible light with the 1.5-metre Danish telescope at the ESO La Silla Observatory in Chile.

The Crab Nebula, remains of a supernova that was first seen around 1050 AD

A star is a massive ball of plasma (very hot gas) held together by gravity. It radiates because of the nuclear reactions inside it.

It radiates heat and light, and every other part of the electromagnetic spectrum, such as radio waves, microwaves, X-rays, gamma rays, UV. The proportions vary according to the mass and age of the star.

The energy of stars comes from nuclear fusion -- a process of turning one chemical element into another. 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.

Birth of a star

A star begins as a collapsing cloud of material composed primarily of hydrogen, with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes. The star's radiation pressure prevents it from collapsing further under its own gravity. Once the hydrogen fuel at the core is exhausted, those stars with at least 0.4 times the mass of the Sun expand to become a red giant, in some cases fusing heavier elements. The star then evolves into a degenerate form, putting out a portion of its matter into space. There it forms a new generation of stars with a higher proportion of heavy elements.

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.

Planets

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 too. Scientists have found other planets moving around other stars, but no planets that they have found are similar to Earth.

Numbers, distances

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 micron 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.

Supergiant stars are the largest objects in the Universe. They have a diameter about 1,500 times bigger than the Sun. If you changed the sun into one of these supergiant stars down where the sun is, its outer surface would reach between the orbits of Jupiter and Saturn and the earth would be inside the star. The star Betelgeuse, in the Orion constellation is a red supergiant star.

Stars vary widely in size

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.

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.

History of seeing stars

People have seen patterns in the stars from long ago. This, from 1690, is the constellation of Leo the lion, as imagined by Johannes Hevelius.

Stars have been important to civilizations around the world through history. They have been part of religious practices, and long ago, many people believed that stars could never die. Astronomers organized stars into constellations and used them to see the motions of the planets and guess the position of the Sun. The motion of the Sun with the stars was used to make calendars to keep track of agricultural practices. The Gregorian calendar, which is used almost everywhere in the world, is a solar calendar.

Birth and life

People think that stars are made in nebulas. These are big 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 lives 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 become dimmer and dimmer.

Because 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, the sun will be a red giant.) 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 include 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.

Related websites

Simple English Wiktionary has the word meaning for:

star

• Media related to Star at Wikimedia Commons
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• Kaler, James. "Portraits of Stars and their Constellations". University of Illinois. http://www.astro.uiuc.edu/~kaler/sow/sow.html. Retrieved 2010-08-20. 
• "Query star by identifier, coordinates or reference code". SIMBAD. Centre de Données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/sim-fid.pl. Retrieved 2010-08-20. 
• "How To Decipher Classification Codes". Astronomical Society of South Australia. http://www.assa.org.au/sig/variables/classifications.asp. Retrieved 2010-08-20. 
• "Live Star Chart". Dobsonian Telescope Community. http://www.mydob.co.uk/community_star.php. Retrieved 2010-08-20.  View the stars above your location
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