Castor (star)

Castor (α Gem, α Geminorum, Alpha Geminorum) is the second brightest star in the constellation Gemini and one of the brightest stars in the night sky. Although it has the Bayer designation "alpha", it is actually fainter than Beta Geminorum (Pollux).

The Castor System

Castor was recorded as a visual binary in 1718 by James Pound. It may have been resolved in 1678 by Cassini. The separation of the two stars has increased from 2" in 1907 to 7" in 1997. The two stars form a visual double, with magnitudes of 1.9 and 3.0.

A third star is 73" distant from the main components. It was discovered to vary in brightness with a regular period and was thought to be an eclipsing binary, but the variations are now considered to be due to areas of different brightness on the surface of one or both stars. It was given the variable star designation YY Geminorum.

All three of the visual components are actually spectroscopic binaries and Castor is a complex multiple star system made up of six individual stars. Castor A and B both have orbits of a few days with a much fainter companion. The Castor C components orbit in less than a day. Castor C is believed to be in orbit around the bright pair, but with an extremely long period of several thousand years.

Castor

Castor most commonly refers to:

Castor or CASTOR may also refer to:

Science and technology

Biology

Star

A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun. Other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations.

For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Once the hydrogen in the core of a star is nearly exhausted, almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime and, for some stars, by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, luminosity, and spectrum respectively. The total mass of a star is the principal determinant of its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.