Gacrux, also known as Gamma Crux or Gamma Crucis, third brightest star in the southern constellation Crux, the Southern Cross. Gacrux marks the top of the Southern Cross at a position only 33 from the south celestial pole, a point in the sky about which the stars visible in the southern hemisphere appear to rotate. This appearance is actually caused by Earth`s rotation. Because it is so close to the south celestial pole, Gacrux can never be seen north of about latitude 30 north. From many locations in the southern hemisphere, however, Gacrux can be seen all night throughout the year, circling the south celestial pole. For this reason, it is called a south circumpolar star. The name Gacrux, which is a contraction of the star`s more formal designation Gamma Crucis, was probably applied by European navigators of the 17th and 18th centuries, who needed names for the prominent stars of the southern hemisphere. The Southern Cross was known to early navigators and poets as a prominent part of the much larger constellation Centaurus, and it was not formally designated as a separate constellation until the late 17th century.

Stars that are visible to the unaided eye

Stars that are visible to the unaided eye, such as Gacrux, belong to Earth`s home galaxy, the Milky Way, and tend to be very bright or relatively close. Gacrux is fairly close to our solar system and shines in the night sky as one of the 50 brightest stars as seen from Earth.

The estimated surface temperature of Gacrux

The estimated surface temperature of Gacrux is comparatively cool at 3000C (5400F), which is not quite half the temperature of the Sun and gives the star a dull red color. Gacrux`s diameter is an estimated 124 million km (77 million mi), which is about 90 times greater than the diameter of the Sun. From its composition, astronomers classify Gacrux as a red giant staran older star that has undergone several internal changes that make it much larger than its original size and hence much brighter.

Many star charts and references show Gacrux

Many star charts and references show Gacrux as a double star, with a fainter companion star just to its northeast. This stellar pairing is inaccurate, however, because the stars are not actually near each other in space. They only appear to lie near one another from the perspective of an observer on Earth. Astronomers refer to this type of pairing as an optical double star.

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