Facts About the Alpha Centauri Star System
Let’s start off with some quick fire facts about Alpha Centauri
- Bearing the “alpha designation,” Alpha Centauri is the brightest star in the southern constellation of Centaurus. It shines with an apparent magnitude of -0.27.
- The Alpha Centauri star system is the third brightest star in our night sky. The only brighter stars are Sirius and Canopus.
- The stars of this star system are Rigil Kentaurus (Alpha Centauri A), Toliman (Alpha Centauri B), and Proxima Centauri (Alpha Centauri C). Rigil Kentaurus and Toliman make up a binary system that looks like a single star to the unaided eye.
- Rigil Kentaurus is a G-type star like the Sun while Toliman is a K-type star. The smaller and fainter Proxima Centauri C is a red dwarf with the class M spectral type.
- The stars of Alpha Centauri AB follow an elliptical orbit that brings them as far as 35.6 astronomical units (AU) or as close as 11.2 AU from each other. Their orbital period takes 79 years.
- Proxima Centauri is the closest single star to the solar system, located only about 4.25 light-years away. It is separated by 13,000 AU from the Alpha Centauri AB system.
- Proxima Centauri is home to an Earth-size exoplanet called Proxima b. This planet lies in the habitable zone of the red dwarf star, making it a strong candidate for life outside the solar system. Other exoplanets around this star are Proxima c and Proxima d.
- Alpha Centauri A has a candidate exoplanet that is believed to be the size of Neptune. Meanwhile, Alpha Centauri B has no known exoplanet.
- From the sky of the Alpha Centauri AB system, the Sun would appear as a bright yellow star with an apparent magnitude of 0.47. Also, our solar system would be located in the Cassiopeia constellation near the famous Heart Nebula.
The Alpha Centauri Star (α Centauri)
Alpha Centauri (α Cen) is not a single star but a star system made up of three stars:
- (Rigil Kentaurus) Alpha Centauri A
- (Toliman) Alpha Centauri B
- (Proxima Centauri) Alpha Centauri C
The star system has a combined apparent magnitude of -0.27, which means it is one of the brightest stars that we can easily see with the naked eye.
Aside from being the brightest point in the Centaurus constellation, it is also the third brightest point in the night sky. Sirius and Canopus are the other two brighter stars. It is followed by Arcturus, Vega, Capella, and Rigel which are in the 4th to 7th spot.
Rigil Kentaurus (Alpha Centauri A)
- Color: Yellow (G-type)
- Spectral type: G2V
- Apparent magnitude: 0.01
- Absolute magnitude: 4.38
- Mass: 1.1 solar masses
- Radius: 1.2 solar radii
- Luminosity: 1.5 Suns
- Constellation: Centaurus
- Distance: 4.37 light-years/ 1.34 parsecs from the solar system
Alpha Centauri A is a G-type main-sequence star like the Sun. Aside from its spectral type, G2V, it also shares many other similarities with the Sun.
This star has about the same mass as our Sun and is just about 20% larger. It shines with 1.5 solar luminosities. Individually, Alpha Centauri A or Rigil Kentaurus is a bright star with 0.01 visual magnitude. It completes a rotation every 22 days.
Alpha Centauri A forms a close binary system with Alpha Centauri B. They appear as a single point to the naked eye. However, if we look closely, Alpha Centauri AB can be resolved as a double star even with small telescopes.
Through radial velocity, it was confirmed that these two are gravitationally bound to each other. As the primary star, Alpha Centauri A is larger and more massive than its companion.
Alpha Centauri AB follows a highly eccentric orbit. This elliptical path takes the two stars as far as 36 AU from each other or as close as 11 AU. In the solar system, this is around the Sun-Pluto distance and Sun-Saturn distance, respectively. The pair takes 79 years to complete an orbit. These Sun-like stars are located 4.37 light-years from the Sun.
Toliman (Alpha Centauri B)
- Color: Orange (K-type)
- Spectral type: K1V
- Apparent magnitude: 1.33
- Absolute magnitude: 5.71
- Mass: 0.9 solar masses
- Radius: 0.9 solar radii
- Luminosity: 0.5 Suns
- Constellation: Centaurus
- Distance: 4.37 light-years/ 1.34 parsecs from the solar system
Alpha Centauri B is formally named Toliman. It is the companion star in the Alpha Centauri AB binary system.
Alpha Centauri B has the spectral class K1V. The “V” luminosity class indicates that it is a main-sequence star like the Sun. As a K-type star, it shines with an orange hue, as opposed to the larger Alpha Centauri A which has a yellow color.
Once resolved, the star Toliman has an apparent magnitude of 1.33. Though it is outshone by Alpha Centauri A, it is still easily visible to the unaided eye. This star only has about 90% of the Sun’s mass and size. It is less luminous than our Sun. This orange star indicated signs of being magnetically active and it emits more X-rays than the primary star.
Proxima Centauri (Alpha Centauri C)
- Color: Red (M-type)
- Spectral type: M5.5Ve
- Apparent magnitude: 10.43–11.11
- Absolute magnitude: 15.60
- Mass: 0.1 solar masses
- Radius: 0.2 solar radii
- Luminosity: 0.0017 Suns
- Constellation: Centaurus
- Distance: 4.2465 light-years/ 1.3020 parsecs from the solar system
Alpha Centauri C or Proxima Centauri is the third star in the Alpha Centauri triple star system. It is separated around 13,000 AU away from the binary stars Alpha Centauri AB. Because of that, it lies closer to the solar system—at only about 4.25 light-years away.
Proxima Centauri is a red dwarf star with the spectral type M5.5Ve. To review, red dwarfs are very small and cool stars in the main sequence. They are by the far the most numerous stars in the Milky Way galaxy.
Unlike the other two Alpha Centauri stars, Proxima Centauri is not visible to the unaided eye. Its visual magnitude which ranges from 10.43 to 11.11 indicates that it is way beyond the naked-eye limit which is around the 6th magnitude.
The range in its apparent magnitude indicates that it is a variable star. Specifically, it is a flare star which means that its brightness can change unpredictably in just a few minutes. A superflare in 2016 made Proxima Centauri about 68 times brighter. This M-type star is also known by its variable star designation V645 Centauri.
Proxima Centauri only has about 1/7 of the Sun’s size or about 1.5 times the diameter of the gas giant Jupiter. It only has roughly 10% of the Sun’s mass. The surface temperature of this star is 3,042 K and it shines with less than 1% of the Sun’s luminosity.
Proxima Centauri is a slow spinning star that takes around 83 days to rotate on its axis. It is located approximately 12,950 AU from Alpha Centauri AB. It takes a very long time to orbit the main pair which takes about 550,000 years.
While our Sun is a 4.6 billion years old star, our closest stellar neighbor is a little older at 4.85 years old. However, since red dwarfs are efficient at burning their hydrogen fuel, they have much longer lives than any other star type.
The Sun, which is an average mass star, will become a red giant after 10 billion years of its life. Proxima Centauri, however, will remain as a main-sequence star for approximately four trillion years. In about 3.5 billion years from now, it is predicted that this red star will diverge from the main pair in the system.
As seen in our night sky, Proxima Centauri is located 2.2° southwest of the binary Alpha Centauri AB. Being the closest star to the Sun, it is one of the most studied stars out there. Its close distance makes it one of the best places to search for life outside the solar system.
Where Does the Name Come From?
Alpha Centauri is called so because, in the Bayer designation, the Greek letter “alpha” is usually given to the brightest star in the constellation. Each star in this triple system is also known by other names.
Alpha Centauri A is also called Rigil Kentaurus which is sometimes shortened to Rigil Kent. Alpha Centauri B is Toliman. This name was derived from the Latinized version of the Arabic name al-Ẓulmān which means “the ostriches.” Alpha Centauri C is famously called Proxima Centaurus. This is from the Latin which translates to “the nearest star of Centaurus.”
Alpha Centauri and Beta Centauri, the second-brightest star in the constellation, are called “Southern Pointers.” This is because these stars can be used as guides that point to the Southern Cross in the Crux constellation.
Alpha Centauri Planets
Aside from its distance to us, Alpha Centauri is even more interesting because of the discovery of exoplanets in the system. There have been candidate planets around Alpha Centauri A and B, but none have been confirmed yet around these Sun-like stars.
The size of stars affects how close or far planets should be to have just the right temperature to allow liquid water to exist. This is called the habitable zone or the Goldilocks Zone. For Alpha Centauri A this should be between 1.2 AU and 2.1 AU. Since Alpha Centauri B is smaller, this star’s habitable zone is closer, between 0.7 AU and 1.2 AU.
Three Earth-like planets have been discovered around Proxima Centauri. These are Proxima Centauri b, c, and d.
Proxima Centauri b
Proxima b is a super-Earth, which means it is more massive than our planet, Earth, but lighter than the ice giant planets (Uranus and Neptune). It is thought to be a rocky planet since it has 1.27 times the Earth’s mass. It is located at 0.0485 AU from its host star. Since it is very close to the star, this planet is likely tidally locked to Proxima Centauri. It has an orbital period of 11.2 days.
Proxima Centauri c
Proxima Centauri c is an exoplanet candidate that has around 7 times the Earth’s mass. This super-Earth is located at about 1.49 AU from the host star. Since Proxima Centauri is a red dwarf, its habitable zone is also closer. Given this distance, the chance of habitability on this exoplanet is not very high.
Proxima Centauri d
Proxima Centauri d is a sub-Earth or an exoplanet that is less massive than Earth. It is the nearest known planet to its host star, located roughly 0.029 AU away. With this distance, it only takes 5.1 to complete a trip around its orbit.
Centaurus Constellation (The Centaur)
The constellation Centaurus is notable because it is home to the closest stellar neighbor of the Sun—the Alpha Centauri star system. This star represents the right front hoof of the celestial centaur.
Spanning an area of 1,060 square degrees, the celestial centaur is the 9th largest among the 88 modern constellations of today. Centaurus was even larger in the past because the constellations of Lupus, Circinus, and Crux used to be part of it.
Aside from the prominent Alpha Centauri system, the Centaurus constellation also has other notable stars. One of which is Beta Centauri, the second-brightest point in the constellation. Interestingly, it is also a triple star system with B-type spectral types.
Another noteworthy star in the constellation is V766 Centauri. This is a yellow hypergiant star and is one of the largest stars ever known.
Centaurus is a rich field of deep-sky objects. Omega Centauri, the most luminous globular cluster in the Milky Way galaxy, is located in this constellation. It also contains a planetary nebula called the Blue Planetary (NGC 3918).
Some of the galaxies in the vicinity of the celestial centaur are the spiral galaxy NGC 4622, irregular galaxy NGC 5253, the barred spiral NGC 4945, and the lenticular galaxy NGC 5102. One of the most notable objects in the constellation is the magnificent Centaurus A galaxy (NGC 5128). This close radio galaxy is characterized by its active galactic nucleus.
In Greek mythology, Centaurus is associated with the centaur—a half-horse, half-human creature. It is said that the constellation is the wise centaur Chiron. He tutored many heroes including Heracles and Jason. In the story, Chiron was placed in the sky after Heracles accidentally shot him with a poisoned arrow.
Where Can You See Alpha Centauri in the Night Sky?
The star system of Alpha Centauri is in the southern constellation of Centaurus. This large constellation is visible between the latitudes +25° (northern hemisphere) and −90° (southern hemisphere). It is circumpolar in the southern sky which means it is visible all year long.
Below are the coordinates of the stars that make up this star system.
|Rigil Kent||14h 39m 36s||−60° 50′ 02”|
|Toliman||14h 393m 35s||−60° 50′ 15”|
|Proxima Centauri||14h 29m 42s||−62° 40′ 46”|
The surrounding star and constellations are also helpful in locating Alpha Centauri in the night sky. These neighboring constellations are:
How and When to Find Alpha Centauri?
The star system of Alpha Centauri AB culminates around April 24 at midnight and on June 8 at 9 pm. It is the time when it is easiest to spot because it is at its highest point in the sky. It is circumpolar to observers from latitude 29°S to the south of it. On the other hand, because of its southerly location, it never rises above the horizon from the mid-northern latitudes.
The red dwarf Proxima Centauri is located about four Full Moons away from Alpha Centauri AB. It is not visible to the naked eye and can only be seen using at least a medium-sized telescope.
We can use the famous asterism of the Southern Cross as a way to locate Alpha Centauri. It is made up of the stars Acrux (Alpha Crucis), Mimosa (Beta Crucis), Gacrux (Gamma Crucis), Imai (Delta Crucis), and Ginan (Epsilon Crucis). This sky pattern represents the Crux constellation which is surrounded by Centaurus on three sides.
To visualize, the small constellation of Crux is located just below Centaurus, between its front and back legs. Alpha Centauri and Hadar (Beta Centauri) are called Southern Pointers because they point to the real Southern Cross, as opposed to the False Cross in the Vela and Carina constellations.
Alpha Centauri in History & Mythology
In Greek mythology, the constellation of Centaurus was placed in the sky after he was wounded by the arrow of Heracles. The constellations of Crux, Circinus, and Lupus were part of it before.
Before it was separated, the Southern Cross was located on one of the hind legs of the celestial centaur. It was said that this may have been where Chiron was wounded by the arrow that eventually caused his death.
The stars Alpha Centauri and Beta Centauri represented the sacred sharks of the Ngarrindjeri people in South Australia. In the story, it is said that these two sharks were going after a stingray that was formed by the Southern Cross.
To the Boorong aboriginal people, Alpha and Beta Centaur are called the Bermbermgle brothers. These two were known for their bravery. They were said to hunt the giant emu which is associated with the Coalsack Nebula.
Our relatively close distance from the Alpha Centauri system makes it the most probable destination if we ever venture into robotic interstellar travel. The major challenge for this is fuel and the long travel.
One solution that scientists find is by using solar sail technology. These very light probes will be powered by sunlight hitting their large mirrors—just like how a boat sails using the wind. This technology offers low-cost operations as well as a long operating lifetime. They can travel at a maximum speed that is around 10% of the speed of light.
The Breakthrough Initiatives and European Southern Observatory (ESO) are making collaborative efforts to search for habitable planets in the system. They helped in upgrading the VLT Imager and Spectrometer for mid-Infrared on the Very Large Telescope in Chile. With this, we can expect to discover more possible exoplanet destinations in our closest neighbor.
Frequently Asked Questions About Alpha Centauri
1. What is the difference between Alpha Centauri and Proxima Centauri?
Proxima Centauri is a part of the Alpha Centauri triple star system. To know the difference, it is important to get to know the stars that make up the Alpha Centauri system. These stars are:
- Binary Star Alpha Centauri A (Rigil Kentaurus)
- Alpha Centauri B (Toliman)
- Alpha Centauri C (Proxima Centauri)
In this triple star system, Alpha Centauri A and B make up a binary star system where they orbit around a common center of mass. Alpha Centauri C or Proxima Centauri is located further from the two and orbits around them.
Alpha Centauri is the closest star system to us. But if we look at them individually, the closest star is Proxima Centauri.
2. How far away is Alpha Centauri?
When talking about the distance of Alpha Centauri, it is important to note that it is not a single star but a system made up of three stars. The close binary, Alpha Centauri A and B, is located roughly 4.37 light-years from the solar system.
Proxima Centauri is a bit closer to us, lying 4.25 light-years away. This distance is equivalent to about 40 trillion kilometers or roughly 25 trillion miles. Using astronomical units or the distance between the Sun and Earth, our nearest neighbor is about 268,770 AU away.
3. How long will it take to travel to Alpha Centauri?
Looking at the distance, traveling to Alpha Centauri would take less than five years if we could travel at the speed of light. However, no man-made spacecraft has been made to travel this fast.
If Voyager 1 and 2 are heading towards Alpha Centauri, it would take them tens of thousands of years to reach the star system. If Apollo 11 were to go there at the same speed as it traveled to the Moon, it would take around 43,000 to reach its destination.
The New Horizons spacecraft would travel approximately 78,000 years if it were traveling toward this star system. It will take even longer to walk to Proxima Centauri, which would take about 215 million years!
Researchers are now eyeing a new concept that strengthens the possibility of interstellar travel—the solar sail. Solar sails are tiny probes that are could travel at about 10% of the speed of light. If it becomes successful, the probe can reach the Alpha Centauri system in about 20 years, within the human lifetime.
4. Will Alpha Centauri become a black hole?
No, none of the three stars that make up the star system will turn into a black hole. Stars need to be very massive to leave black holes at the end of their lives.
Alpha Centauri A and B are just about as massive as the Sun while Proxima Centauri is much less massive. These main-sequence stars need to have more than 20 times the Sun’s mass to possibly produce black holes.
The closest black hole to Earth is dubbed “the Unicorn,” located 1,500 light-years in the Monoceros constellation.
The (α Centauri): https://exoplanets.nasa.gov/system/resources/detail_files/104_eso1241ew.jpeg
Rigil Kentaurus: https://www.nasa.gov/sites/default/files/styles/full_width_feature/public/thumbnails/image/hubble_friday_09022016.jpg (Image credit: ESA/NASA)
Proxima Centauri: https://www.nasa.gov/sites/default/files/styles/946xvariable_height/public/potw1343a.jpg?itok=GzeqPQsH (From Hubble Space Telescope)
Other Characteristics: https://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Alpha_Centauri_relative_sizes.svg/1024px-Alpha_Centauri_relative_sizes.svg.png
Proxima Centauri b: https://exoplanets.nasa.gov/exoplanet-catalog/7167/proxima-centauri-b/#:~:text=Proxima%20Centauri%20b%20is%20a,discovery%20was%20announced%20in%202016.
Proxima Centauri d: https://upload.wikimedia.org/wikipedia/commons/thumb/6/67/Artist%E2%80%99s_impression_of_Proxima_d_%28close-up%29.jpg/1024px-Artist%E2%80%99s_impression_of_Proxima_d_%28close-up%29.jpg
Centaurus Constellation (The Centaur): https://upload.wikimedia.org/wikipedia/commons/thumb/5/59/Centaurus_IAU.svg/800px-Centaurus_IAU.svg.png
Centaurus A galaxy (NGC 5128): https://upload.wikimedia.org/wikipedia/commons/thumb/d/d2/ESO_Centaurus_A_LABOCA.jpg/800px-ESO_Centaurus_A_LABOCA.jpg
Where Can You See in the Night Sky?: https://stellarium-web.org/
How and When to Find?: https://stellarium-web.org/
The Future (solar sail): https://upload.wikimedia.org/wikipedia/commons/thumb/a/af/IKAROS_solar_sail.jpg/1024px-IKAROS_solar_sail.jpg