Meet Gacrux, the Top of the Cross

Gacrux Vitals

Official name (IAU-approved)	Gacrux
Other designations	Gamma Crucis, HIP 61084, HR 4763, HD 108903
Nicknames	Rubidea (in Brazil)
Apparent magnitude	1.64
Distance from Earth	88.6 light-years
Type	Red giant, M3.5III
Color	Red/orange
Mass	~1.3 M☉
Radius	~120 R☉
Constellation	Crux
Right ascension	12h 31m 10s
Declination	-57° 06' 48"
Multiple system?	No
Variable star?	Yes
Exoplanets status	None known
Probable fate	Planetary nebula/white dwarf

Physical Characteristics

In this installment of Meet the Stars, we take a trip to the southern skies and examine Gacrux, a red giant star in one of the brightest and beloved asterisms — the Southern Cross.

Of the four main stars that make up the Southern Cross, Gacrux is the only red member of the group, a fact that stands out clearly in long-exposure photographs. Its pleasant red color contrasts nicely with the blue-white colors of the other stars in the Cross. The effect is similar to the Betelgeuse’s standout color in Orion (though Gacrux is a much smaller red giant).

Many giant stars are quite distant, often in the range of hundreds or thousands of light-years away from Earth. Gacrux is somewhat unusual in that it’s only about 88 light-years away — the closest red giant to Earth. Gacrux has a diameter 120 times the Sun’s, but it only has about 30% more mass. Like other red giants, Gacrux is huge but fairly diffuse. When the star began using up its fuel reserves, the gas within the star cooled and expanded, a process that will continue as Gacrux advances in its stellar development.

Origin / Mythology

The Southern Cross is a brilliant object, but one that is mostly absent from Western mythology. Though once visible from the Northern Hemisphere, precession, the wobbling progression of Earth’s axis that takes place over thousands of years, slowly shifted the Southern Cross out of view; by around 400 AD it was out of sight and out of mind. (Before it slipped over the horizon, the Greeks considered the Southern Cross stars part of Centaurus.) Europeans rediscovered the Southern Cross by the 1500s. Its modern name is attributed to French cartogropher Augustine Royer, who published maps of the southern sky in 1679, though the constellation appeared in earlier maps too. The name is probably a Christian reference.

Of course, while the Southern Cross was lost from northern view, it remained prominent and important in southern regions, with a wide variety of stories associated with it. The fishing communities in northern Australia, for example, saw the Southern Cross as a stingray, while in the desert regions of central Australia, aboriginals saw in the constellation the track of an eagle. To the Incas, the Southern Cross may have been a “chakana,” (bridge or crossing). A carved stone at Machu Picchu in Peru might represent the Southern Cross.

Today, the Southern Cross appears prominently on the national flags of Australia, New Zealand, Brazil, Papua New Guinea, and Samoa.

The name Gacrux appears to be a contraction of Gamma Crux, from the Latin designation indicating it as the third-brightest star in the constellation. It’s a strictly utilitarian name, like Alpha Centauri.

How to See Gacrux

You must be south of 25°N latitude to see the entire Southern Cross, so it’s virtually impossible to view from the continental U.S. It’s possible to a catch glimpse of Gacrux from the extreme southernmost portions, like the Florida Keys, but catching Gacrux as it briefly pops over the horizon requires a perfectly clear view to the south and precise timing. (Gacrux is the constellation’s northernmost star and therefore the most likely one to see.)

A better plan for viewing the Southern Cross from the U.S. would be a trip to Hawai‘i. The island chain lies just below 25°N, so the constellation is visible in its entirety during winter and spring. Even still, the Southern Cross hangs quite low in the south and only rises for a couple of hours before diving back below the horizon.

For the best views of Gacrux, you’ll need to get into the Southern Hemisphere. (They don’t call it the “Southern Cross” for nothing!) Large portions of South America, Africa, and Australia will show the Cross perfectly, and if you go below 35°S, the Cross becomes circumpolar, staying up all night, just like Cassiopeia does in the north.

There isn’t a convenient south pole star — no Polaris counterpart exists in that portion of the sky — but the Cross can be used as a pointer. An imaginary line from Gacrux through Acrux (the constellation’s southernmost star) points roughly at the celestial south pole.

Seeing Gacrux and the Southern Cross is nothing unusual for citizens of the Southern Hemisphere, but for visitors who have never seen these objects, they can be the highlight of a trip. While you’re there, make sure to check out the Coalsack Nebula within Crux and the Magellanic Clouds (the Milky Way’s miniature companion galaxies) too.

---