Also called:
Gamma Crucis
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star

Gacrux, third brightest star in the constellation Crux (after Acrux and Mimosa) and the 25th brightest star in the sky. Gacrux has an apparent magnitude of 1.64 and is about 89 light-years from Earth. The constellation Crux is also known as the Southern Cross, and Gacrux is at the top of the cross. Having a spectral type of M3.5, Gacrux is a red giant star, a stage late in a star’s evolution during which a star’s outer layers expand; Gacrux’s mass is 1.3 times that of the Sun, and its radius is about 100 times that of the Sun. Gacrux is a barium star, its outer layers being enriched in that element. Barium stars typically arise from material that has been transferred from a white dwarf companion, but no such white dwarf has been observed near Gacrux.

(List of Brightest Stars as Seen from Earth)

Erik Gregersen

variable star, any star whose observed light varies notably in intensity. The changes in brightness may be periodic, semiregular, or completely irregular.

A brief treatment of variable stars follows. For full treatment, see star: Variable stars.

Variable stars may be classified into three broad types according to the origin and nature of their variability: (1) eclipsing, (2) pulsating, and (3) explosive.

Ursa major constellation illustration art.  (Big Dipper) stars, space, night sky)
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Stars: Explosions in Space

In an eclipsing variable, one member of a double, or binary, star system partially blocks the light of its companion as it passes in front of the latter, as observed from Earth. Each time this happens, the brightness of the entire system fluctuates. Such an eclipsing variable is perhaps best exemplified by the binary star Algol, whose name means “blinking demon.”

Unlike eclipsing binaries, the other two types of variable stars are intrinsically variable—that is to say, their own output of radiant energy fluctuates with time. The pulsating variables expand and contract cyclically, causing them to pulsate rhythmically in brightness and size. The Cepheids and RR Lyrae stars are typical examples of such variables. The explosive (or eruptive) variables include novas, supernovas, and similar stars that undergo sudden outbursts of radiant energy, which results in rapid brightening. This increase in brightness lasts only for a short period of time, followed by relatively slow dimming.

Besides these three major classes, there are also several miscellaneous variables: R Coronae Borealis stars, T Tauri stars, flare stars, pulsars (neutron stars), spectrum and magnetic variables, X-ray variable stars, and radio variable stars. Tens of thousands of variable stars are known.

This article was most recently revised and updated by Chelsey Parrott-Sheffer.