Alpha Centauri

Alpha Centauri is a special group of stars in the southern sky, in the constellation of Centaurus. This star system has three stars: Rigil Kentaurus (also called Alpha Centauri A), Toliman (Alpha Centauri B), and Proxima Centauri. Proxima Centauri is the closest star to our Sun, about 4.25 light-years away.

Rigil Kentaurus and Toliman are similar to our Sun in some ways. They move around each other as a pair, called a binary star system. From Earth, they look like one bright star. They are the third-brightest star we can see at night, after Sirius and Canopus.

Proxima Centauri is faint but the closest star to us. Scientists have found planets around Proxima Centauri, including one about the size of Earth. We are still learning more about the planets in this system.

Etymology and nomenclature

Alpha Centauri (Latinised to Alpha Centauri) is the name given to this star system by a man named J. Bayer in 1603. It is part of a group of stars called the Centaurus constellation, named after a creature from old stories that is half human and half horse. In these stories, a hero named Heracles accidentally hurt this creature and placed it in the sky.

Alpha Centauri marks the right front hoof of this creature. It is also known by the name Rigil Kentaurus, which comes from an old Arabic phrase meaning "the Foot of the Centaur." Another name, Toliman, comes from an Arabic word meaning "the (two male) Ostriches."

The smallest star in the group, Alpha Centauri C, was found in 1915. It is called Proxima Centauri, meaning "the nearest [star] of Centaurus." This name was officially approved by a group of scientists called the International Astronomical Union.

In 2016, this group decided to give each star in the system its own special name. They chose Rigil Kentaurus for the biggest star, Alpha Centauri A, and Proxima Centauri for the smallest one, Alpha Centauri C. In 2018, they approved the name Toliman for the middle star, Alpha Centauri B.

Other names for Alpha Centauri have been used too. One old name was Bungula, and in Chinese it is called 南門二 (Nán Mén Èr), meaning the Second Star of the Southern Gate. Different cultures have their own names and stories for these bright stars.

Observation

The Very Large Telescope open to the night sky, with the Milky Way running diagonally across the sky above it and many southern stars and constellations labelled and connected by lines, including Alpha Centauri and the not visible Proxima Centauri.

Alpha Centauri looks like one bright star when you look up at the southern sky with your eyes alone. It is very bright and one of the closest stars we can see without any help. This group of stars is called The Pointers. It helps us find the Southern Cross in the night sky.

Proxima Centauri is another star in this group, but it is very faint. You need telescopes to see it. Sometimes it can become much brighter for just a few minutes. People have watched these stars for a very long time. Ancient astronomers wrote about them, and scientists have kept learning more about how they move and how far away they are.

Location and motion

Alpha Centauri is inside a space area called the Local Bubble. The closest known system is a pair of brown dwarfs named Luhman 16, about 3.6 light-years away.

Relative positions of Sun, Alpha Centauri AB and Proxima Centauri. Grey dot is projection of Proxima Centauri, located at the same distance as Alpha Centauri AB.

The stars in Alpha Centauri move slowly across the sky over time. This movement is called proper motion. Ancient astronomers thought stars were fixed in place. In 1718, Edmond Halley discovered that some stars had moved from where they were seen before. Later, Thomas Henderson measured the distance to Alpha Centauri and found it moves quickly across the sky.

Over time, Alpha Centauri will change its place in the sky and become brighter. In about 6,200 years, it will appear very close to another bright star, Beta Centauri. Much later, around 26,700 years from now, Alpha Centauri will come closer to Earth than it is today.

Alpha Centauri AB historical distance estimates
Source	Year	Subject	Parallax (mas)	Distance
Source	Year	Subject	Parallax (mas)	parsecs	light-years	petametres
H. Henderson	1839	AB	1160±110	0.86+0.09 −0.07	2.81 ± 0.53	26.6+2.8 −2.3
T. Henderson	1842	AB	912.8±64	1.10 ± 0.15	3.57 ± 0.5	33.8+2.5 −2.2
Maclear	1851	AB	918.7±34	1.09±0.04	3.55+0.14 −0.13	32.4 ± 2.5
Moesta	1868	AB	880±68	1.14+0.10 −0.08	3.71+0.31 −0.27	35.1+2.9 −2.5
Gill & Elkin	1885	AB	750±10	1.333±0.018	4.35±0.06	41.1+0.6 −0.5
Roberts	1895	AB	710±50	1.32 ± 0.2	4.29 ± 0.65	43.5+3.3 −2.9
Woolley et al.	1970	AB	743±7	1.346±0.013	4.39±0.04	41.5±0.4
Gliese & Jahreiß	1991	AB	749.0±4.7	1.335±0.008	4.355±0.027	41.20±0.26
van Altena et al.	1995	AB	749.9±5.4	1.334±0.010	4.349+0.032 −0.031	41.15+0.30 −0.29
Perryman et al.	1997	AB	742.12±1.40	1.3475±0.0025	4.395±0.008	41.58±0.08
Söderhjelm	1999	AB	747.1±1.2	1.3385+0.0022 −0.0021	4.366±0.007	41.30±0.07
van Leeuwen	2007	A	754.81±4.11	1.325±0.007	4.321+0.024 −0.023	40.88±0.22
van Leeuwen	2007	B	796.92±25.90	1.25±0.04	4.09+0.14 −0.13	37.5 ± 2.5
RECONS TOP100	2012	AB	747.23±1.17	1.3383±0.0021	4.365±0.007	41.29±0.06

Stellar system

Alpha Centauri is a system with three stars. The two main stars, called Alpha Centauri A and Alpha Centauri B, orbit each other. Together, they form a binary pair. The third star, Proxima Centauri, orbits this pair from a much greater distance.

Orbital plot of Proxima Centauri around the bright apparent star Alpha Centauri AB, with position change marked (in thousands of years).

The stars A and B take about 80 years to orbit each other. They move closer and farther apart during this time. Proxima Centauri is much farther away from the pair, taking around 500,000 years to complete one orbit. All three stars are held together by gravity and move as one group through space.

Apparent and true orbits of Alpha Centauri. The A component is held stationary, and the relative orbital motion of the B component is shown. The apparent orbit (thin ellipse) is the shape of the orbit as seen by an observer on Earth. The true orbit is the shape of the orbit viewed perpendicular to the plane of the orbital motion. According to the radial velocity versus time, the radial separation of A and B along the line of sight had reached a maximum in 2007, with B being further from Earth than A. The orbit is divided here into 80 points: each step refers to a timestep of approx. 0.99888 years or 364.84 days.

Planetary system

The Alpha Centauri system has two known planets, both around Proxima Centauri. Scientists are still looking to see if there are more planets around the other stars in this system.

Planets of Alpha Centauri A

In 2021, astronomers found a possible planet around Alpha Centauri A. They think it might be about the size of a planet between Neptune and half of Saturn, but they are not sure yet. Special telescopes are planned to look more closely at this star to find out if there are any planets there.

Observations with the James Webb Space Telescope in August 2024 found a possible object that might be a planet. If it is a planet, it could be about 90 to 150 times the mass of Earth and have a temperature of about -48°C.

Planets of Alpha Centauri B

In 2012, scientists thought they found a planet around Alpha Centauri B, but later they found out it was not a real planet. More searches were done to see if there might be other planets, but none were confirmed.

The discovery image of Alpha Centauri's candidate Neptunian planet, marked here as "C1"

Planets of Proxima Centauri

Main articles: Proxima Centauri b, c, and d

Proxima Centauri b is a planet found in 2016. It is about the size of Earth and orbits very close to its star.

Proxima Centauri c might be a bigger planet, but scientists are still not sure if it really exists.

In 2022, another small planet called Proxima Centauri d was found orbiting very close to the star.

Hypothetical planets

Scientists think there might be more planets around Alpha Centauri A or B, but they have not been found yet. These stars are similar to our Sun, so scientists are very interested in searching for planets there.

Circumstellar discs

Some observations suggest there might be a thin ring of dust around Alpha Centauri A and B, but it is very small compared to the dust we see around our Sun.

View from this system

From Alpha Centauri AB, the sky would look similar to Earth's, but without its brightest star, Alpha Centauri AB itself. The Sun would look like a bright white star, about as shiny as Betelgeuse looks from Earth. It would be in the constellation Cassiopeia, standing out from other stars there.

Nearby stars would appear in new places. Sirius, the brightest star we see, would still shine brightly but move closer to Betelgeuse in the sky. Procyon would also shift its position.

A planet orbiting either Alpha Centauri A or B would see the other star as a very bright companion. Even though it would be dimmer than the planet’s main star, it would still be much brighter than the full Moon we see from Earth. Proxima Centauri would look like just another small star in the sky.

Future exploration

In culture

Alpha Centauri has been important to many cultures. In the Southern Hemisphere, Polynesians used it for star navigation and called it Kamailehope. In the Ngarrindjeri culture of Australia, Alpha Centauri and Beta Centauri represent two sharks chasing a stingray, the Southern Cross. In Incan culture, it and Beta Centauri form the eyes of a llama-shaped dark constellation in the Milky Way. Ancient Egypt also revered it, and in China, it is part of the South Gate asterism.

The Sagan Planet Walk in Ithaca, New York includes a walkable scale model of our solar system. An obelisk showing the position of Alpha Centauri can also be found at the ʻImiloa Astronomy Center in Hawaii.