Physical Description

 
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The Sun is a typical star, with a visible surface called a photosphere, an overlying atmosphere of hot gases, and above them a more diffuse corona and an outflowing stream of particles called the solar (stellar) wind. Cooler areas of the photosphere, which on the Sun are called sunspots, are probably present on other typical stars; their existence on some large nearby stars has been inferred by a technique called speckle interferometry . The internal structure of the Sun and other stars cannot be directly observed, but studies indicate convection currents and density and temperature that increase until the core is reached, where thermonuclear reactions take place. Stars consist mainly of hydrogen and helium, with varying amounts of heavier elements.

Pistol Star

Astronomers published this infra-red, digitally enhanced image in October 1997. It is a star located in the centre of the Milky Way that could be 10 millions times more brilliant than the Sun. Some scientists claim that the "Pistol Star", as it is known, is the largest star ever discovered, but others suggest that the image represents a conglomeration of stars.



The largest stars known are supergiants with diameters that are more than 400 times that of the Sun, whereas the small stars known as white dwarfs have diameters that may be only 0.01 times that of the Sun. Giant stars are usually diffuse, however, and may be only 40 times more massive than the Sun, whereas white dwarfs are extremely dense and may have masses about 0.1 times that of the Sun despite their small size. Supermassive stars 1,000 times as massive as the Sun may exist, and, at the lower range, hot balls of gases may exist that are too small to initiate nuclear reactions. An object that may be such a brown dwarf was first observed in 1987, and others have been detected since then.

Star brightness is described in terms of magnitude. The brightest stars may be as much as 1,000,000 times brighter than the Sun; white dwarfs are about 1,000 times less bright.

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