The Centaurus Constellation

Posted on Friday, 2 February, 2018 by Zheluo Cai

The Centaurus Constellation.

Centaurus is a bright constellation in the northern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. In Greek mythology, Centaurus represents a centaur; a creature that is half human, half horse. It is best seen during May at right ascension 12h 57min and declination -44 degrees.

Centaurus contains several very bright stars. Its alpha and beta stars are used as “pointer stars” to help observers find the constellation Crux. Centaurus has 281 stars above magnitude 6.5, meaning that they are visible to the unaided eye, the most of any constellation. Alpha Centauri, the closest star system to the Sun. It is a triple star system that contains Proxima Centauri, the nearest star to the Sun. Traditionally called Rigil Kentaurus or Toliman, meaning “foot of the centaur,” the system has an overall magnitude of -0.28 and is 4.4 light-years from Earth. The primary and secondary are both yellow-hued stars; the primary is of magnitude -0.01 and the secondary is of magnitude 1.35. Proxima, the tertiary star, is a red dwarf of magnitude 11.0. Also a flare star, Proxima has minutes-long outbursts where it brightens by over a magnitude. In addition to Alpha Centauri (the third-brightest star in the sky), a second first magnitude star, Beta Centauri, also called Hadar and Agena, is a double star; the primary is a blue-hued giant star of magnitude 0.6, 525 light-years from Earth. The secondary is of magnitude 4.0 and has a very small separation.

ω Centauri (NGC 5139), despite being listed as the constellation’s “omega” star, is, in fact, a globular cluster, located at a distance of 17,000 light-years, with a diameter of 150 light-years. It is the largest and brightest globular cluster in the Milky Way at over one million solar luminosities; it has a magnitude of 3.7. Omega Centauri is classified as a Shapley class VIII cluster, which means that its centre is loosely concentrated. It is also the only globular cluster to be designated with a Bayer letter and containing several million stars, most of which are yellow dwarf stars, but also possesses red giants and blue-white stars with an average age of 12 billion years.

Centaurus is also home to open clusters. NGC 3766 is an open cluster 6,300 light-years from Earth. It contains approximately 100 stars, the brightest of which are 7th magnitude. NGC 5460 is another naked-eye open cluster, 2,300 light-years from Earth, that has an overall magnitude of 6 and contains approximately 40 stars. There is one bright planetary nebula in Centaurus, NGC 3918, also known as the Blue Planetary. It has an overall magnitude of 8.0 and a central star of magnitude 11.0; it is 2600 light-years from Earth. The Blue Planetary was discovered by John Herschel and named for its colour’s similarity to Uranus, though the nebula is apparently three times larger than the planet.

Centaurus is rich in galaxies as well. NGC 4622 is a face-on spiral galaxy located 200 million light-years from Earth (redshift 0.0146). Its spiral arms wind in both directions, which makes it nearly impossible for astronomers to determine the rotation of the galaxy. Astronomers theorize that a collision with a smaller companion galaxy near the core of the main galaxy could have led to the unusual spiral structure. NGC 5253, a peculiar, irregular galaxy, is located near the border with Hydra and M83, with which it likely had a close gravitational interaction 1-2 billion years ago. This may have sparked the galaxy’s high rate of star formation, which continues today and contributes to its high surface brightness. NGC 5253 includes a large nebula and at least 12 large star clusters. NGC 4945 is a spiral galaxy seen edge-on from Earth, 13 million light-years away. It is “shaped like a candle flame.”

One of the closest active galaxies to Earth is the Centaurus A galaxy, NGC 5128, at a distance of 11 million light-years (redshift 0.00183). It has a supermassive black hole at its core, which expels massive jets of matter that emit radio waves due to synchrotron radiation. Astronomers posit that its dust lanes, not common in elliptical galaxies, are due to a previous merger with another galaxy, probably a spiral galaxy. Its overall magnitude is 7.0. ESO 270-17, also called the Fourcade-Figueroa Object, is a low-surface brightness object believed to be the remnants of a galaxy; it does not have a core and is very difficult to observe with an amateur telescope. It measures seven arcminutes by one arcminute. It likely originated as a spiral galaxy and underwent a catastrophic gravitational interaction with Centaurus A around 500 million years ago, stopping its rotation and destroying its structure. NGC 4650A is a polar-ring galaxy located at a distance of 136 million light-years from Earth (redshift 0.01). It has a central core made of older stars that resemble an elliptical galaxy and an outer ring of young stars that orbit around the core. The plane of the outer ring is distorted, which suggests that NGC 4650A is the result of a galaxy collision about a billion years ago. One of the closest galaxy clusters to Earth is the Centaurus Cluster, located at a distance of 160 million light-years (redshift 0.0114). It has a cooler, denser central region of gas and a hotter, more diffuse outer region. The intracluster medium in the Centaurus Cluster has a high concentration of metals (elements heavier than helium) due to a large number of supernovae. This cluster also possesses a plume of gas whose origin is unknown.