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Stars
Stars appear as shining points in the night sky. They "twinkle" because of the effect of the Earth's atmosphere. The Sun is an exception: it is the only star sufficiently close to Earth to appear as a round object (a disc) and to provide daylight.
Humans use to find patterns and throughout history have grouped stars that appear close to one another into constellations (or smaller groups called asterisms). Most constellations have been created in ancient times and are related with mythology.
The stars in a constellation rarely have a real physical relationship to each other: they just happen to appear close together in the sky as viewed from Earth. In other words, in real three-dimensional space most stars may be far away from one another, but can appear to be grouped on the celestial sphere of the night sky. However, real clusters also exist (such as the famous Pleiades in Taurus).
The ancient Greek word for "star" is "astron", that is origin of several modern words such as "astronomy". Common language does not always reflect the astronomical usage: the term "star" ordinarily does not include the Sun, and sometimes includes meteors ("shooting stars" or "falling stars") or even the visible planets, that are in our Solar System.
Modern astrophysics has discovered that stars are massive gaseous bodies with high surface temperature, that causes them to emit light and other radiation.
The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres = 4.2 ly (light years) = 1.29 pc (parsecs) away. Thus light from Proxima Centauri takes 4.2 years to reach Earth.
Astronomers estimate that there are at least 70 sextillion stars in the known universe, that is 70 000 000 000 000 000 000 000, or 230 billion times as much as the 300 billion in the Milky Way, the our own galaxy.
Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13 or 14 billion years old, which is the estimated age of the universe. Stars range in size from the tiny neutron stars (which are actually dead stars) no bigger than a city, to supergiants like the North Star (Polaris) and Betelgeuse, in the Orion constellation, which have a diameter about 1,000 times larger than the Sun. However, these have a much lower density than the Sun. One of the most massive stars is Eta Carinae, with 100-150 times as much mass as the Sun.
Scientifically, stars are self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their energy through the process of nuclear fusion.
The energy produced by stars radiates into space as electromagnetic radiation (mostly visible light), and as streams of neutrinos. The apparent brightness of a star is measured by its magnitude.
Some stars are gravitationally bound to other stars, forming binary stars. Larger groups called star clusters also exist. Modern reserach have shown that stars and clusters are not spread uniformly across the universe, but are typically grouped into galaxies. A typical galaxy contains hundreds of billions of stars.
All the stars that we can see by naked eyes belong to our own galaxy, the Milky Way. However, 3 galaxies are barely visible by naked eyes: M31 (the Great Galaxy in Andromeda) and the 2 Magellanic clouds. Only M31 can be seen from the Northern hemisphere.
Star classification
Stars are classified by their temperature and spectral type: for more info, read the page about Stellar classification. Common spectral classes are O,B,A,F,G,K,M, and can easily remembered using the mnemonic "Oh, Be A Fine Girl, Kiss Me" (variant: change "girl" to "guy"), invented by Annie Cannon (1863-1941).
Each letter has 10 subclassifications. Our Sun is a G2, which is very near the middle in terms of quantities observed.
More info: Constellations - Star naming - Star classification.
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