The Crux is a constellation located in the southern sky in a bright portion of the Milky Way. It is among the most easily distinguished constellations, as all of its four main stars have an apparent visual magnitude above +2.8, even though it is the smallest of all 88 modern constellations. Its name is Latin for cross, and it is dominated by a cross-shaped or kite-like asterism that is commonly known as the Southern Cross. The Crux is bordered by the constellations Centaurus (which surrounds it on three sides) on the east, north and west, and Musca to the south. Covering 68 square degrees and 0.165% of the night sky, it is the smallest of the 88 constellations. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of four segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 11h 56.13m and 12h 57.45m, while the declination coordinates are between −55.68° and −64.70°. The whole constellation is visible to observers south of latitude 25°N. In tropical regions, Crux can be seen in the sky from April to June. The Crux is exactly opposite to Cassiopeia on the celestial sphere, and therefore it cannot appear in the sky with the latter at the same time. For locations south of 34°S, Crux is circumpolar and thus always visible in the night sky. The Crux is sometimes confused with the nearby False Cross by stargazers. The Crux is somewhat kite-shaped (a Latin cross), and it has a fifth star (ε Crucis). The False Cross is diamond-shaped (a Greek cross), somewhat dimmer on average, does not have a fifth star and lacks the two prominent nearby “Pointer Stars.”
Within the constellation’s borders, there are 49 stars brighter than or equal to apparent magnitude 6.5. The four main stars that form the asterism are Alpha, Beta, Gamma, and Delta Crucis. Also known as Acrux, Alpha Crucis is a triple star 321 light-years from Earth. Blue-tinged and magnitude 0.8 to the unaided eye, it has two close components of magnitude 1.3 and 1.8, as well as a wide component of magnitude 5. Beta Crucis, called Mimosa, is a blue-hued giant of magnitude 1.3, 353 light-years from Earth. It is a Beta Cephei-type Cepheid variable with a variation of fewer than 0.1 magnitudes. Gamma Crucis, called Gacrux, is an optical double star. The primary is a red-hued giant star of magnitude 1.6, 88 light-years from Earth. The secondary is of magnitude 6.5, 264 light-years from Earth. Delta Crucis is a blue-white hued star of magnitude 2.8, 364 light-years from Earth. It is the dimmest of the Southern Cross stars. Like Beta, it is a Beta Cepheid.
Crux boasts four Cepheid variables that reach naked eye visibility. BG Crucis ranges from magnitude 5.34 to 5.58 over 3.3428 days, T Crucis ranges from 6.32 to 6.83 over 6.73331 days, S Crucis ranges from 6.22 to 6.92 over 4.68997 days, and R Crucis ranges from 6.4 to 7.23 over 5.82575 days. BH Crucis, also known as Welch’s Red Variable, is a Mira variable that ranges from magnitude 6.6 to 9.8 over 530 days. Discovered in October 1969, it has become redder and brighter (mean magnitude changing from 8.047 to 7.762) and its period lengthened by 25% in the first thirty years since its discovery.
The star HD 106906 has been found to have a planet—HD 106906 b—that has a larger orbit than any other exoplanet discovered to date.
The Coalsack Nebula is the most prominent dark nebula in the skies, easily visible to the naked eye as a prominent dark patch in the southern Milky Way. It is large, five degrees by seven degrees, and is 600 light-years from Earth. Not all of the nebula is in the borders of Crux; some of it is technically in Musca and Centaurus. The open cluster NGC 4755, better known as the Jewel Box or Crucis Cluster, has an overall magnitude of 4.2—to the naked eye it appears to be a fuzzy star—and is about 7600 light-years from Earth. The cluster was given its name by John Herschel. About seven million years old, an age that makes it one of the youngest open clusters in the Milky Way, it appears to have the shape of a letter A. The Jewel Box Clusters is a Shapley class g and Trumpler class I 3 r cluster; it is a very rich, centrally-concentrated cluster detached from the surrounding star field. It has more than 100 stars that range significantly in brightness. The brightest stars are mostly blue supergiants, though the cluster contains a few bright red supergiants. Kappa Crucis is a true member of the cluster that bears its name and is one of the brighter stars at magnitude 5.9. Credit: Wikipedia.