Alpha Centauri – 10 Facts about Our Neighbor

Alpha Centauri – 10 Facts about Our Neighbor
Where is Alpha Centauri? [Image via ESO/BBC]

One of the sky's brightest stars is often seen in fiction, but in reality unseen by much of the Earth's population. Alpha Centauri, in the constellation Centaurus (the Centaur), is not only a significant feature of the southern hemisphere sky, but also the Solar System's closest stellar neighbor.

1. Alpha Centauri is the third brightest star in the night sky.
Although prominent in the southern hemisphere, it's not visible in the northern hemisphere above 29° north.

2. Ptolemy's 2nd-century star catalogue listed Alpha Centauri.
In Ptolemy's time, Alpha Centauri was visible from Alexandria, Egypt at latitude 31° N. It can't be seen that far north now because of a little wobble in the Earth's axis. Over thousands of years it makes the north pole point to a different pole star, zodiac constellations appear in different months, and at a given latitude some stars may disappear or others become visible. This process is called precession.

3. To the naked eye Alpha Centauri appears as one bright star, but it's turned out to be a triple star system.
In the 17th century, telescopes revealed this bright star as two stars. The first recorded sighting of the double was made by Father Jean Richaud in 1689. Then 226 years later Scottish astronomer Robert Innes discovered a red dwarf which is gravitationally bound to the other two.

4. Alpha Centauri is the first star whose distance was determined by parallax.
The credit for measuring the first reliable stellar parallax is given to Friedrich Bessel for his 1839 measurement of 61 Cygni. Although Thomas Henderson had measured the parallax of Alpha Centauri in 1838, he didn't feel confident about publishing it. Parallax is like the triangulation that surveyors use on Earth. If a star seems to move against the background stars as the Earth moves in its orbit, the apparent movement can be used to calculate its distance.

5. The two brighter stars are sunlike stars and they form the binary system Alpha Centauri AB.
Alpha Centauri A is slightly bigger than the Sun and Alpha Centauri B is slightly smaller than the Sun. They orbit the center of the binary system every 80 years, coming as close together as 11.2 AU. At their most distant, they're 35.6 AU apart, which is about the distance between the Sun and Pluto. Alpha Centauri AB is currently about 13,000 AU away from Proxima. (The AU - astronomical unit - is the Earth-Sun distance.)

6. The stars of Alpha Centauri have designations as parts of a triple system, but each one also has a name officially sanctioned by the International Astronomical Union (IAU).
Alpha Centauri A has the old name Rigil Kentaurus which means “the foot of the centaur”. Alpha Centauri B is listed as Toliman, a vaguely phonetic rendering of its Arabic name. And as the closest known star to our Sun, the red dwarf Alpha Centauri C is named Proxima (Latin for “nearest”).

7. Having a pair of sunlike stars nearby has made astronomers hopeful of finding an earthlike planet there.
Long-term monitoring of Alpha Centauri by NASA’s Chandra X-ray Observatory indicates that the stars of the binary don't emit large amounts of X-ray radiation. This would be good news for a terrestrial planet in the habitable zone of either of the stars. (The habitable zone is the region around a star where water could exist in liquid form.)

8. Although no planets have yet been found in Alpha Centauri AB, two have been confirmed orbiting Proxima.
Proxima b was discovered in 2016 in Proxima's habitable zone. It seems to be a super-earth, a rocky planet larger than Earth. Proxima c was confirmed in June 2020. It's also a super-earth, but several times bigger than Proxima b. It orbits at about the same distance as Mars does from the Sun and takes over five Earth years to do so.

9. Proxima is not a benevolent star.
A red dwarf is small and comparatively cool, so Proxima's habitable zone is quite close to the star. Proxima b orbits in about 11 Earth days. It's probably tidally locked, the same side always facing Proxima as the Moon does to the Earth. Proxima b receives much more high energy radiation than Earth gets from the Sun. If that weren't enough, Proxima is a flare star. On average, it sends the planet a dose of X-rays about 500 times greater than what Earth gets from the Sun. But when it flares, the amount is up to 50,000 times greater. The atmospheres of both exoplanets may well have been stripped away by the stellar winds like the solar wind did to the Martian atmosphere.

10. Alpha Centauri is one of the Southern Pointers.
The Southern Cross in the constellation Crux is a navigation aid. Sometimes people are misled by another cross-shaped asterism known as the False Cross. The trick is to remember that the bright stars Alpha Centauri and Beta Centauri are “the pointers”. They point to Gacrux, the star at the top of the Southern Cross.

You Should Also Read:
Centaurus the Centaur
Crux - the Southern Cross
ABC of Astronomy – D Is for Double Star

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