Apus (the Bird of Paradise) is a small, faint constellation close to the south celestial pole. It was introduced in the late 16th century by Dutch cartographer Petrus Plancius, using observations made by navigators exploring the Southern Hemisphere. Although Apus contains no bright stars or Messier objects, it is home to two interesting globular clusters, NGC 6101 and IC 4499.
Where Is Apus and When Can You See It?
Apus is the 67th-largest constellation, covering approximately 206 square degrees of the southern sky. It lies close to Octans, which contains the south celestial pole, and borders Chamaeleon, Circinus, Musca, Ara, Pavo, and Triangulum Australe.
The constellation is best placed for evening observation around July, although it is circumpolar and visible throughout the year from much of the Southern Hemisphere. Apus is not visible from Britain or most northern locations.
To locate it, start with the bright stars of Triangulum Australe or the nearby constellation Ara, then look south toward the celestial pole. Alpha Apodis, a magnitude 3.8 star, is its brightest member, but the remaining stars are faint enough that a chart is useful for identifying the pattern.
History of the Apus Constellation
Apus was one of 12 southern constellations introduced by Petrus Plancius using observations made by Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman during voyages to the East Indies. It appeared in Johann Bayer’s 1603 star atlas, Uranometria, and later became one of the 88 constellations officially recognized by the International Astronomical Union.
The constellation represents a bird-of-paradise, a tropical bird from New Guinea and surrounding regions known for its elaborate plumage. The name Apus derives from a Greek expression meaning “without feet.” This reflects a misconception that developed when early bird-of-paradise specimens were prepared for trade with their feet removed. Europeans unfamiliar with the living birds sometimes believed they spent their entire lives in flight, never landing on the ground.
Unlike many ancient northern constellations, Apus has no established connection with Greek mythology.
Notable Stars in Apus
- Alpha Apodis – The constellation’s brightest star, an orange giant approximately 410 light-years away, shining at magnitude 3.8. It has expanded to many times the Sun’s diameter as it has evolved beyond the main sequence.
- Gamma Apodis – A yellow giant approximately 160 light-years away, with an apparent magnitude of 3.9. It is only slightly fainter than Alpha Apodis and is one of the constellation’s most recognizable stars.
- Beta Apodis – An orange giant approximately 160 light-years away, shining at magnitude 4.2. Like Alpha Apodis, it has exhausted much of the hydrogen fuel in its core and expanded into a giant star.
- Delta Apodis – A visual double star whose components appear close together in the sky. Delta-1 Apodis is a red giant, while Delta-2 Apodis is an orange giant. Their apparent proximity does not necessarily mean they form a gravitationally bound system.
- Eta Apodis – A chemically peculiar A-type star belonging to the metallic-line, or Am, class. Such stars show unusual abundances of certain elements in their atmospheres.
Several stars in Apus are also known to host exoplanets, including HD 134606 and HD 137388.
Notable Deep-Sky Objects in Apus
Apus contains no Messier objects, but two globular clusters stand out among its deep-sky targets.
NGC 6101 — An Ancient Globular Cluster
NGC 6101, also known as Caldwell 107, is a globular cluster approximately 50,000 light-years away. It contains a large population of old stars held together by gravity and is estimated to be more than 12 billion years old. Unlike some globular clusters with extremely dense centers, NGC 6101 has a relatively loose appearance. This makes its stellar distribution particularly interesting to astronomers studying how clusters evolve.
The cluster also contains blue stragglers, stars that appear hotter and younger than expected for such an ancient population. These may form through interactions between stars, including mass transfer in binary systems or stellar mergers.
With an apparent magnitude around 9, NGC 6101 is beyond naked-eye visibility. It appears as a faint, diffuse object in smaller telescopes, while larger instruments under dark skies can begin to resolve some of its stars.
IC 4499 — A Globular Cluster With an Interesting Age
IC 4499 is another ancient globular cluster, approximately 55,000 light-years away.
For many years, astronomers believed it might be several billion years younger than most Milky Way globular clusters. However, more detailed observations with the Hubble Space Telescope suggested an age of approximately 12 billion years, making it broadly comparable with other old clusters in our galaxy.
IC 4499 is relatively loosely concentrated and has an apparent magnitude around 9.8. It is a challenging target for smaller telescopes but becomes more accessible with larger apertures and dark southern skies.
Studies of IC 4499 help astronomers investigate the ages, chemical compositions, and evolutionary histories of the Milky Way’s globular clusters.
Other Deep-Sky Objects
Apus also contains several faint galaxies, including IC 4633, IC 4635, and NGC 6392. These are considerably more difficult targets than the constellation’s globular clusters and are mainly of interest to experienced observers using larger telescopes.