Submitted by admin on April 22, 2009 - 22:32
Stars in the Constellation
Symbolic representations of the Cross, such as on national flags, show either four or five stars; and often the question is raised as to just how many stars are in the constellation. The four main stars give the cross effect, although the cross-member is somewhat askew. The fifth next brightest star is something of an interloper. Examination by the naked eye on a very clear night will reveal about 34 stars within the modern boundaries of the constellation, but with telescopes the number must run into many thousands. It is correct to say, however, that there are five principal stars. Two bright stars on one side of the Cross are often referred to as the Pointers, since the imaginary line joining them appears to point towards the constellation. They are, however, members of the constellation Centaurus, the brighter one (Alpha Centauri or Rigil Kent) being the closest star to our sun. Light travelling at 186,000 miles per second takes four and one-third years to reach us from it.
One of the very rich and highly interesting portions of the Milky Way passes through the Southern Cross, and examination by the naked eye shows many condensations which break up into magnificent clouds of stars when viewed through binoculars or a small telescope. Close by Beta Crucis is a magnificent telescopic star cluster known as the Jewel Casket, since it contains stars of varied brightness and colours. Adjacent to Alpha and Beta Crucis is a large, dark, cloud-like area, from which stars appear to be absent as seen by the naked eye. Popularly known as the Coal Sack, it is a cloud of gas and dust obscuring the light from the more distant stars of the Milky Way which silhouettes its outline.
The principal stars of the Southern Cross (scientifically called Crux) are given in the following table:
| (Epoch 1950.0) | ||||||||
| Star | Right Ascension | Declination | Magnitude | Spectral Type | Distance (Light Years) | Radial Velocity (Km/sec) | ||
| hr | min | degrees | min | |||||
| Alpha | 12 | 23.8 | –62 | 49 | 1.05 | B1 | 217 | –12 |
| Beta | 12 | 44.8 | –59 | 25 | 1.50 | B1 | 326 | +20 |
| Gamma | 12 | 28.4 | –56 | 50 | 1.61 | Mb | .. | +21 |
| Delta | 12 | 11.5 | –58 | 28 | 3.08 | B3 | 192 | +26 |
| Epsilon | 12 | 18.7 | –60 | 08 | 3.57 | K2 | 171 | –5 |
Alpha Crucis (sometimes called Acrux) is a triple star, the two principal components having magnitudes of 1.58 and 2.09. Gamma Crucis is a double star, and noticeably of an orange colour to the naked eye. Positive radial velocity values indicate that a star is receding from us, and negative values indicate that it is approaching us. Such values have been determined spectroscopically.
by Ivan Leslie Thomsen, F.R.A.S.(LOND.), Director, Carter Observatory, Wellington.
