Star
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A star is a luminous spheroid of plasma held together by self-gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night; their immense distances from Earth make them appear as fixed points of light. The most prominent stars have been categorised into constellations and asterisms, and many of the brightest stars have proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated 1022 to 1024 stars. Only about 4,000 of these stars are visible to the naked eye—all within the Milky Way galaxy.
Image of the Sun, a G-type main-sequence star, the closest to Earth
A star-forming region in the Large Magellanic Cloud
Stars in the night sky
Infrared image from NASA's Spitzer Space Telescope showing hundreds of thousands of stars in the Milky Way galaxy
Plasma (physics)
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Plasma is one of four fundamental states of matter characterized by the presence of a significant portion of charged particles in any combination of ions or electrons. It is the most abundant form of ordinary matter in the universe, mostly in stars, but also dominating the rarefied intracluster medium and intergalactic medium.
Plasma can be artificially generated, for example, by heating a neutral gas or subjecting it to a strong electromagnetic field.
Lightning as an example of plasma present at Earth's surface: Typically, lightning discharges 30 kiloamperes at up to 100 megavolts, and emits radio waves, light, X- and even gamma rays. Plasma temperatures can approach 30000 K and electron densities may exceed 1024 m−3.
Artificial plasma produced in air by a Jacob's Ladder
Hall-effect thruster
Plasma spraying