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Rigel Kentaurus (Alpha Centauri): Third-Brightest Star

Alpha Centauri and Surroundings
This wide-field view of the sky around the bright star Alpha Centauri was created from photographic images forming part of the Digitized Sky Survey 2. The star appears so big just because of the scattering of light by the telescope's optics as well as in the photographic emulsion. Alpha Centauri is the closest star system to the Solar System. Image released Oct. 17, 2012.
Credit: ESO/Digitized Sky Survey 2

Rigel Kentaurus is the third-brightest star in the night sky. However, its brightness is due to the proximity of the system — commonly known as Alpha Centauri — which is the sun's closest neighbor, about 4.3 light-years away from Earth.

Rigel Kentaurus is part of a triple star system, and the naked-eye star itself is a double star. Its two stars are dwarfs that spin around each other every 80 years at an average distance of 23 Earth-sun distances, or astronomical units. This puts the stars at about the same equivalent distance apart as the sun and Uranus in our own solar system, according to University of Illinois astronomer Jim Kaler.

By itself, Alpha Centauri A is the fourth brightest star in the sky; just a bit dimmer, by 0.02 of a magnitude, than Arcturus. It is a yellow star of the same type (G2) as the sun, and it is about 25 percent larger. Alpha Centauri B is an orange K2-type star, slightly smaller than the sun. Proxima Centauri is a red dwarf about seven times smaller than the sun, or one-and-a-half times bigger than Jupiter. [The Brightest Stars in the Sky: A Starry Countdown]

In 2012, astronomers discovered an Earth-size planet orbiting Alpha Centauri B, which is the smaller of the two dwarfs in Rigel Kentaurus. The planet, however, is far too close to its star to support life as we know it.

Historical observations

The star was first revealed as a double one in 1689, according to a 1984 paper published in the Bulletin of the Astronomical Society of India.

According to the paper, Jean Richaud, who was part of the Jesuits, used a 12-foot telescope from his post in India to look at Rigel Kentaurus, a star that is impossible to spot at latitudes above 30 degrees north. That's when Richard noted the single was actually a double. The paper said that this was only the second double star ever discovered.

It would take more than 200 years for the fainter Proxima Centauri – a "dim red dwarf", according to astronomer David Darling – to come into view. Robert Innes spotted the star in 1915. Innes was an experienced binary star observer who made 1,628 discoveries at the Cape Observatory, South Africa, according to the European Space Agency. Proxima is technically the closest of the star system to Earth, at 4.22 light-years away.

Location

The system is in the Southern sky and is not visible to observers above the latitude of 29 degrees north — a line that passes near Houston, Texas, and Orlando, Florida. In the Southern Hemisphere, it's easy to find because the cross-piece of the Southern Cross (from Delta to Beta Crucis) points the way. The system's location is:

  • Right ascension: 14 hours 39 minutes 41 seconds
  • Declination: -60 degrees 50 minutes 7 seconds

[Infographic: The Nearest Stars to Earth]

Similarities to Earth's sun

Early in 2013, astronomers announced they had found a mysterious cold layer on Alpha Centauri A that looks similar to another one on Earth's sun.

Temperature does not rise sequentially in the sun's layers; the topmost layer (or corona) is millions of degrees Fahrenheit, but just below it is a layer called the chromosphere that is only about 7,300 degrees F (4,000 C) – cooler than the layer below. A similar layer was observed on Alpha Centauri A, which was spotted in far infrared using the European Space Agency's Herschel space observatory.

"The study of these structures has been limited to the sun until now, but we clearly see the signature of a similar temperature inversion layer at Alpha Centauri A," stated study leader René Liseau of the Onsala Space Observatory in Sweden at the time.

Magnetic field line twists could be responsible for the high heat in the sun's corona, but it's hard to fully comprehend the sun in isolation without looking at other stars. At the time, astronomers said looking closely at Alpha Centauri A could bring more information to light about the corona, solar flares and similar phenomena, especially because the stars are pretty close in terms of temperature, age and other properties.

Astronomers have discovered an Earth-size planet orbiting one of the nearest stars in our galaxy.
Astronomers have discovered an Earth-size planet orbiting one of the nearest stars in our galaxy.
Credit: Karl Tate, SPACE.com

Planet discovery

In 2012, astronomers found a planet close to Earth's mass around one of the stars of Rigel Kentaurus. Called Alpha Centauri Bb, the planet orbits its star about every 3.2 days, at 3.6 million miles (6 million kilometers) away. Earth and the sun are about 93 million miles (150 million km) apart; Mercury, with its elliptical orbit, varies between 29 million and 43 million miles (47 million to 70 million km) from the sun.

"Most of the low-mass planets are in systems of two, three to six or seven planets, out to the habitable zone," study co-author Stephane Udry, of the Geneva Observatory, said to reporters after the discovery in October 2012.

This means that the find "opens really good prospects for detecting planets in the habitable zone in a system that is very close to us," Udry added. "In that sense, this system is a landmark."

Alpha Centauri Bb came to light using the High Accuracy Radial velocity Planet Searcher (HARPS), which is installed on the European Southern Observatory's 11.8-foot (3.6-meter) telescope at the La Silla Observatory in Chile.

Using HARPS, scientists saw a subtle wobble in the motion of the star at just 1.1 mph (1.8 kph). Because the shift was so small, it required 450 HARPS measurements across four years to confirm the find. The team was led by Xavier Dumusque of Geneva Observatory and the University of Porto in Portugal.

The tiny measurement had some astronomers not convinced yet, saying more evidence is needed. Alongside the publication in Nature was a commentary piece from Artie Hatzes of the Thuringian State Observatory in Germany.

"As the American astronomer Carl Sagan once said, 'Extraordinary claims require extraordinary evidence,'" Hatzes wrote. "Although a planetlike signal is present in the data, the discovery does not quite provide the 'extroardinary evidence.' It is a weak signal in the presence of a larger, more complicated signal. In my opinion, the matter is still open to debate."

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