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The number of stars in the celestial sphere is beyond imagination. The most prominent stars have since centuries been grouped into constellations.
Even stars do not live forever.
They move through different phases, from birth to death, which can be seen by their physical status.
Over 2000 stars are visible on clear nights. Greek scholars around 500 BC still believed them to be burning objects suspended in the sky.
All stars are hot balls of gas that radiate energy. However, they can have very different characteristics. Their sizes alone range from twenty kilometers in diameter to 2.5 thousand times the diameter of the sun. There are also great differences in luminosity(measurement for the total energy emitted by a star)as well as in density.
The temperature of a star can be determined by its apparent color. red stars, with a surface temperature of less than 4000 Celsius are considered "cold".
Yellow stars have temperatures between 5500 and 8000 Celsius. white stars range from 8000 to 10,000 Celsius in temperatures and blue stars are hottest, with temperatures of up to 20,000Celsius.
A special phenomenon is the collapse of a massive star, when it becomes a supernova
Especially bright stars seem to form conspicuous figures in the sky, which are known as constellations.
Every star undergoes different phases during its evolution. These phases depend on the star's mass and chemical composition.
All stars are formed from a cloud of gas and dust, as was our sun. The size of the cloud, which determines the further development of the star, can vary.
When a cloud of gas begins to contract under the influence of its own gravity(gravitational force), it becomes increasingly hotter until it reaches a stage of compression where it begins to emit a dark red glow. At this stage it is called a proto star. The temperature and the luminosity(measurement for the total energy emitted by a star) depend on the mass of the star. The T-Tauri stars are thought to belong to this category.
Nuclear reactions, which release enormous amounts of energy, begin when a temperature of about 10 million degrees Celsius is reached.
Hydrogen fuses to helium in the centre of the star.
It has now become a main-sequence_star(star that is located within the clearly defined diagonal section of the Hertzsprung-Russell diagram. more than 90% of known stars are assigned to this group. These stars emit energy generated by the fusion of hydrogen to helium), with a long life-span.
A star the size of our sun remains in the main-sequence phase for several billions of years. For a star with significantly more mass, this stage of its life lasts "only" a few million years. Then its reserve of hydrogen is used up. Now its interior begins to change. If it is a high-mass star, it becomes a red supergiant and later a supernova, then a tiny neutron_star (core of a star of extremely high density, wich is left over after a supernova explosion. Supernovas are thought to be the explosions of high-mass stars) or a black hole.
In a star of less mass, the helium core inside it continues to contract and further nuclear reactions take place in the stellar envelope surrounding it. The outer layers cool slowly and expand until the star becomes a red giant. The next stages of its development are white dwarf and later black dwarf.
There are two types of brightness associated with a star. The "apparent" brightness to us on Earth, and the "absolute", which is a standard distance of 32.6 light-years(unit for distance used in astronomy)
Neutron stars and pulsars
Very old stars that have nearly reached the end of their evolution are particularly interesting objects. They referred to as neutron stars or pulsars and consist mainly of neutron gas.
Neutron stars arise at the end of the development a star. They are formed from the remains of the core after a supernova pulsars are nothing neutron stars that emit radiation impulses at regular intervals.
They were discovered by radio telescopes.
In 1967 an astrophysicist named joce
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