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RCW 71 and the red carbon star RX Crux

Description of object:

There is little information about RCW 71 - also cataloged as Gum 46 or GN 12.47.3 - because it is a little-known DeepSky object. It is a small, with an apparent diameter of only 3 arcminutes, H-II region in the Southern Cross constellation (located just south of NGC 4755, Herschel's jewel box). RCW 71, with coordinates (J 2000) RA: 12 h 50m 22s and -61 degrees 35 minutes and 00 seconds, lies directly in the Coal Sack, the large dark cloud between the constellations Southern Cross (Crux), Centaur (Centaurus) surrounding it, and the Fly (Musca).

The star ionizing the H-II region at the center of RCW 71 is HD 311 999 with spectral class O9.5V. It is the brightest star of a small cluster [DBS 2003] 80 whose stars emit light mainly in the infrared spectrum. However, HD 311 999 is clearly visible in our image as the brightest star.

The distance of RCW 71 is given between 2000 and 2800 pc (1 pc = 3.26 light-years), which corresponds to 6500 to 9000 light-years, depending on scientific investigations. Since the distance of the dark cloud Coal Sack is only about 500 to 600 light years, RCW 71 stands much farther away and we are thus looking through a transparent region of the large dark cloud of Coal Sack.

The RCW catalog (Rodgers, Cambell, and Whiteoak) is an extension of the Gum catalog of H-II regions of the southern starry sky. More information about the catalogs here.
Further information and links to RCW 71 can be found here. More information about the star cluster in the center of RCW 71 [DBS 2003] 80 here.

« The annotated image shows the size comparison with the moon

Furthermore our image shows one of the rare red carbon stars, RX Crux with the B-V color index of 4.5 magnitudes and spectral class K. Carbon stars are very old giant stars, similar to red giant stars like Antares in the constellation Scorpio or Betelgeuse in Orion. Carbon stars contain in contrast to the "normal" red giant stars more carbon than oxygen.

The carbon forms different carbon molecules in the outer, cool layers of the star, which absorb blue and yellow light and this causes the star to appear in deep red color to the observer.
Carbon stars are divided into 5 classes and like many giant red stars are variable. RX Crux is irregularly variable with a period of one magnitude. More information about carbon stars can be found here.

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All Images und all Content are © by Franz Hofmann + Wolfgang Paech