Ara (“the altar”) is a small, faint constellation that is the southernmost of the 48 listed by the Greek astronomer Ptolemy in his 2nd century astronomical treatise called Almagest. The origin of this southern hemisphere constellation dates back to at least the time of the Babylonians, with Ara at the time symbolizing the altar honoring the Tower of Babel. The constellation’s brightest star is Beta Arae, an orange giant situated 603 light years distant of magnitude 2.84.
Ara is the 63rd largest constellation, and takes up 237 square degrees, or 0.6% of the southern celestial sphere. It can be seen by observers located between +25° and -90° of latitude, although best seen in July. Ara can be found south of Scorpio’s “tail”, lying between the constellations of Lupus and Cygnus, with other neighboring constellations including Apus, Corona Australis, Norma, Pavo, Telescopium, and Triangulum Australe.
Hercules Constellation Family
Ara is a member of the Hercules family of constellations, along with Aquila, Centaurus, Corona Australis, Corvus, Crater, Crux, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Sagitta, Scutum, Serpens, Sextans, Triangulum Australe, and Vulpecula.
In ancient Greece, Ara was associated with the altar that the centaur Chiron (“Centaurus”) was depicted sacrificing Lupus (“the Wolf”) upon in order to unite the gods of Olympus against the Titans. Centaurus appears to be facing eastwards towards Ara, while the smoke imagined rising southwards from the sacrificial altar follows the line of the Milky Way.
– Beta Arae, the brightest star in Ara, is an orange giant (K3 Ib-IIa) found 603 light years from our solar system that shines with an apparent magnitude of +2.81. This 50 million year old star is around 100 times bigger than our sun, with 7 times its mass, and 4,600 its brightness. Beta Arae also has a slow rotational velocity of 5 km/s, meaning it takes 2.33 years to complete a full rotation.
– Alpha Arae (Choo), the constellation’s second brightest star, is a blue dwarf (B2Vne) situated 240 light years from the Sun that shines with an average apparent magnitude of 2.90, although being an “eruptive” star its luminosity can dip as low as 2.76. It is around 4 times bigger than our sun, 10 times more massive, and 5,800 times brighter. Alpha Arae has a fast rotational velocity of about 375 km/s, and as a result is surrounded by a dense equatorial shell of ejected material.
– Zeta Arae, the third brightest star in Ara, is an orange giant located 490 light years distant with an apparent visual magnitude of 3.12.
Other stars of interest in Ara includes the blue-white supergiants Gamma Arae, Delta Arae, and Theta Arae; the orange giant stars Eta Arae and Epsilon-1 Arae; and the yellow dwarf Mu Arae, which is 50 light-years away, and has a planetary system with four known planets.
Notable Deep-Sky Objects
While there are no Messier objects in Ara, its proximity to the Milky Way galaxy means that it does contain a number of notable star clusters. It also contains several planetary nebulae, including the youngest yet discovered.
– NGC 6193 is an open cluster containing 27+ stars, many of which are binary systems, giving it a combined visual magnitude of 5.2. It is around 3 million years old, 4,300 light years distant, with its two hottest stars, HD 150135 and HD 150136, responsible for illuminating the vast emission nebula NGC 6188 in which they are embedded. These two massive blue O-type stars are so close together that their colliding stellar winds may be the cause of the exceptional amounts of X-rays being emitted from this system.
– NGC 6379 is a globular star cluster that at just 7,200 light years distant is one of the closest to our own solar system. It contains more than 400,000 stars, including several blue stragglers, which are older stars that through collisions with other stars have received extra mass and therefore stay on the main sequence for longer than usually expected.
– NGC 6362 is a globular cluster found 24,800 light years away that has an apparent magnitude of 8.3. This ancient cluster is an estimated 13.57 billion years old, and is composed mostly of aged red giants, although it also contains blue stragglers.
Other notable star clusters in Ara includes the globular clusters NGC 6352 and ESO 280-SC06; and the open clusters Westerlund 1, NGC 6250, IC 4651, NGC 6200, NGC 6204, and NGC 6208.
– The Stingray Nebula (Hen 3-1357) is situated 18,000 light years away, with its shape resembling the classic outline of an oceanic manta ray. It is around 130 times bigger than our solar system, which is still one-tenth the dimension of most planetary nebulae, and has a visual magnitude of 10.75. The Stingray Nebula also has the distinction of being the youngest planetary nebula discovered, as within the last 40 years it has evolved from being a protoplanetary nebula to releasing ionized gases that now surround it as rings. At its centre is a hot white dwarf that has a companion located 0.3 arcseconds away, while the nebula’s colors seen in the image show the gases being emitted by the nebula, which in this case are red (nitrogen), green (oxygen), and blue (hydrogen).
Other planetary nebulae in Ara includes the Water Lily Nebula and NGC 6326; while in the constellation can also be found the spiral galaxies NGC 6300, NGC 6215 and NGC 6221.