Alpha Centauri 3

4 stars
Š Laro Schatzer (Artwork from Alpha Centauri, used with permission)

Breaking News

In January 2008, astronomers working cooperatively within the Research Consortium on Nearby Stars (RECONS) issued a press release about their research project to estimate the size of the so-called habitable zone around nearby stars (RECONS press release). Based on the latest data that RECONS has collected on stars within 10 parsecs (32.6 light-years) of Sol, the astronomers are carefully estimating what they call the "habitable real estate" around each of the Sun's neighbors, where inner rocky planets like the Earth can support liquid water on their surface. Confirming previous modelling of the Alpha Centauri system, the RECONS astronomers found that Alpha Centauri A and B orbit in such a way that when the light and heat of the two stars was combined, neither star in the innermost AB system significantly changed the size of their respective habitable zones, regardless of where each was currently located in its orbit. Although stars A and B would be expected to interfere with each others' habitable zones, the areas of the available good "habitable real estate" around each star was affected by less than one percent. Not surprisingly, distant Proxima was completely unaffected by the other two stars.

Justin Cantrell,
Todd J. Henry,
RECONS

Larger illustration.

In general, brighter
stars have wider
habitable zones than
dimmer ones (more).

On February 25, 2008, a team of astronomers released a paper on simulation results which support the conclusions of previous studies that multiple-planet systems could have formed in close orbits around both heavy-element rich, Alpha Centauri A and B. Focusing on Star B, their simulations suggest that at least one planet in the one to two Earth-mass range could have formed within orbital distances of 0.5 to 1.5 AUs; of particularly note, the simulations frequently generated a Earth-like planet in or near Star B's habitable zone (where liquid water could exist on the planet's surface). Additional simulation work presented in the paper also indicates that long-term telescopic observations may detect wobbles from such planets using the radial velocity method. Star B, a orange-red dwarf with a relatively calm chromosphere and acoustic p-wave mode oscillations, is an easier target for detecting wobbles from terrestrial planets, possibly within only three years of "high cadence" observations for a 1.8 Earth-mass planet (more from New Scientist and Guedes et al, 2008).

Unknown artist, Planet Quest,
JPL, Caltech, NASA

Larger illustration.

Recent simulations suggest that
an Earth-life planet could have
formed within the habitable
zone around Alpha Centauri B,
which can be detected using
the radial-velocity "wobble"
method (more).

System Summary

Š Akira Fujii / David Malin Images
(Used with permission from
Tim Bedding)

Larger image.

Alpha Centauri is the
brightest star in
Constellation Centaurus.

Sol's three closest stellar neighbors are located in the southeastern corner of Constellation Centaurus, the Centaur. Proxima Centauri (or Alpha Centauri C) is only 4.22 light-years (ly) away (14:29:42.95-62:40:46.14, ICRS 2000.0) but is too dim to be seen with the naked eye. The two bright stars, Alpha Centauri A and B (14:39:36.5-62:50:02.3 and 14:39:35.1-60:50:13.8, ICRS 2000.0), are a little farther away at about 4.36 ly. They form a close binary that is separated "on average" by only about 24 times the Earth-Sun distance -- 23.7 astronomical units (AUs) of an orbital semi-major axis -- which is only slightly greater than the distance between Uranus and the Sun ("Sol"). (See an animation of the orbits of Stars A and B and their potentially habitable zones in this system, with a table of basic orbital and physical characteristics.)

La Silla Observatory, ESO

Larger image.

While Stars A and B form
a relatively close binary,
dim Star C (Proxima) is a
distant companion (more).

In contrast, Proxima (Star "C") is located around 15,000 +/- 700 AUs from A and B. This is so far that Proxima may not be gravitationally bound to Stars A and B and so may leave the system after some million years, and according to Anosova et al (1994), all three stars may be part of a stellar moving group of nearby stars that includes: the triple ADS 10288 (Gl 649.1); the binaries, Gliese 140.1 and 676; and six single stars. A subsequent analysis using the most recent kinematic and radial velocity data available in the literature, however, found Proxima "is quitely likely" to be bound to to Stars A and B based on calculations of the binding energy of Proxima relative to the center of mass of the entire triple system, where its orbital semi-major axis exceeds 10,000 AUs and is "on order the same size as Alpha Centauri AB's Hill radius in the galactic potential" (Wertheimer and Laughlin, 2006). Hence, it is quite likely that all three stars formed together from the same nebula at roughly the same time and so should share a similar elemental composition.

VLTI, ESO

Larger and jumbo images.

Although Stars A and B are
similar to Sol in size, Star C
(Proxima) is not a lot bigger
than the planet Jupiter (more).

Being visible to the naked eye, Alpha Centauri has been known for centuries, if not millennia, although perhaps not as a double star until the 1752 observation of the Abbé [Abbot] Nicholas Louis de La Caille (1713-1762) from the Cape of Good Hope, the southernmost point of Africa, where he was studying the stars of the southern hemisphere with just an half-inch (8x) refractor. Dim Proxima, however, was not discovered until 1915 by Robert Thorburn Ayton Innes (1861-1933) of Edinburgh, Scotland who also was observing from Cape Hope, probably with the 7-inch refractor at the Royal Observatory. If our own Sun, Sol, were viewed from the Alpha Centauri system, it would be located in Cassiopeia near the border with Perseus and about five degrees north of a double cluster near the nebula IC 1805/1848, visible as a bright yellow star that would be almost as bright as Capella (Alpha Aurigae) appears in Earth's night sky.

INES, LAEFF, ESA

Larger illustration.

All three members of the system
(including B, which has not been
depicted between A and Proxima
at left) are main-sequence stars.

Due to their proximity to Sol, the stars of this system have been objects of intense interest among astronomers. Stars A and B have been selected as two of the top 100 target stars for NASA's indefinitely postponed Terrestrial Planet Finder (TPF) to directly image small rocky planets in Earth-type habitable orbits (and so images of Alpha Centauri A and -- basically the same images of -- Alpha Centauri B and their position relative to the Milky Way in Earth's night sky are now available from the TPF-C team). In addition, all three stars are among the "Tier 1" target stars for NASA's optical Space Interferometry Mission (SIM) to detect a planet as small as three Earth-masses within two AUs of its host star (and so some summary system information and images on Stars A, B, and C are available from the SIM Teams). Astronomers are also hoping to use the ESA's Darwin group of infrared interferometers to analyze the atmospheres of rocky planets found in the "habitable zone" (HZ) around all three stars for evidence of Earth-type life (Lisa Kaltenegger, 2005).

JPL, CalTech, NASA

Larger illustration of the TPF.

Astronomers have identified
Stars A and B as prime targets
for NASA's TPF, while all three
stars are targets for NASA's
optical SIM and the ESA's
infrared Darwin missions.

Alpha Centauri A

Rigil Kentaurus ("Foot of the Centaur" in Arabic) is the fourth brightest star in the night sky as well as the brightest star in Constellation Centaurus. Like Sol, it is a yellow-orange main sequence dwarf star of spectral and luminosity type G2 V. It has about 1.105 ą 0.007 times Sol's mass (Guedes et al, 2008) and 1.23 its diameter (ESO; and Demarque et al, 1986), and is about 52 to 60 percent brighter than Sol (ESO; and Demarque et al, 1986). Without consideration of interior seismic constraints, Star A (and B) has been estimated to be older than Sol, from 4.85 billion years in age (ESO), to around 7.6 (+/- around 10 percent) billion years or more -- or 6.8 billion years if it does not have a convective core (Guenther and Demarque, 2000); however, recent interior modeling with seismic constraints suggest that Stars A and B are 5.6 to 5.9 billion years old (Mutlu Yildiz, 2007). Since Alpha Centauri A is very similar to our own Sun, however, many speculate whether it might contain planets that harbor life. According to Weigert and Holman (1997), the distance from the star where an Earth-type planet would be "comfortable" with liquid water is centered around 1.25 AUs (1.2 to 1.3 AUs) -- about midway between the orbits of the Earth and Mars in the Solar System -- with an orbital period of 1.34 years using calculations based on Hart (1979), but more recent calculations based on Kasting et al (1993) allow for a wider "habitable zone."

Research Consortium on Nearby Stars (RECONS)

Cerro Tololo Inter-American Observatory

Infrared image of Alpha Centauri AB, with
diffraction effects from partially closing the
mirror covers of the 1.5-m telescope. (A
Digitized Sky Survey image of Alpha
Centauri may become available at the
Nearby Stars Database, or see one
at Astronomy Picture of the Day.)

The distance separating Alpha Centauri A from its companion star B averages 23.7 AUs (semi-major axis of 17.57" with a HIPPARCOS distance estimate of 4.40 light-years). The stars swings between 11.4 and 36.0 AUs away in a highly elliptical orbit (e= 0.52) that takes almost 80 (79.90) years to complete and are inclined at an angle of 79.23° from the perspective of an observer on Earth (see Pourbaix et al, 2002, or 2000 in the Sixth Catalog of Orbits of Visual Binaries; and Worley and Heintz, 1983). As viewed from a hypothetical planet around either star, the brightness of the other increases as the two approach and decreases as they recede. However, the variation in brightness is considered to be insignificant for life on Earth-type planets around either star. At their closest approach, Stars A and B are almost two AUs farther apart than the average orbital distance of Saturn around the Sun, while their widest separation is still about six AUs farther the average orbital distance of Neptune. (See an animation of the orbits of Stars A and B and their potentially habitable zones in this system, with a table of basic orbital and physical characteristics.)

In a binary system, a planet must not be located too far away from its "home" star or its orbit will be unstable. If that distance exceeds about one fifth of the closest approach of the other star, then the gravitational pull of that second star can disrupt the orbit of the planet. Recent numerical integrations, however, suggest that stable planetary orbits exist: within three AUs (four AUs for retrograde orbits) of either Alpha Centauri A or B in the plane of the binary's orbit; only as far as 0.23 AU for 90-degree inclined orbits; and beyond 70 AUs for planets circling both stars (Weigert and Holman, 1997). Hence, under optimal conditions, either Alpha Centauri A and B could hold four inner rocky planets like the Solar System: Mercury (0.4 AU), Venus (0.7 AU), Earth (1 AU) and Mars (1.5 AUs).

Indeed, the AB system are significantly more enriched (1.7 to 1.8 times) in elements heavier than hydrogen ("high metallicity") than our own Solar System (Chmielewski et al, 1992; Cayrel de Strobel et al, 1991, page 297; Furenlid and Meylan, 1984; and Flannery and Ayres, 1978). Hence, either stars A or B could have one or two "rocky" planets in orbital zones where liquid water is possible. Astronomers are hoping to use NASA's Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for rocky inner planets in the so-called "habitable zone" (HZ) around both Stars A and B. As currently planned, the TPF will include two complementary observatory groups: a visible-light coronagraph to launch around 2014; and a "formation-flying" infrared interferometer to launch before 2020, while Darwin will launch a flotilla of three mid-infrared telescopes and a fourth communications hub beginning in 2015.

Useful star catalogue numbers and designations for Alpha Centauri A include: Alp or Alf Cen A, HR 5459, Gl 559 A, Hip 71683, HD 128620, CP(D)-60 5483, SAO 252838, FK5 538, and LHS 50.

Alpha Centauri B

Š Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)

Alp Cen B is an orange-red dwarf
star, like Epsilon Eridani at left
center of meteor. (A Digitized Sky
Survey image of Alp Cen B
may become available at
the Nearby Stars Database.)

This much dimmer companion star is a main sequence, reddish-orange dwarf (K0-1 V). It appears to have only 93.4 ą 0.7 percent of Sol's mass (Guedes et al, 2008), about 86.5 percent of its diameter, and 45 to 52 percent of its luminosity (ESO; and Johnson and Wright, 1983, page 681). Viewed from a planet at Earth's orbital distance around Alpha Centauri A, this companion B star would provide more light than the full Moon does on Earth as its brightest night sky object, but the additional light at a distance greater than Saturn's orbital distance in the Solar System would not be significant for the growth of Earth-type life. According to Weigert and Holman (1997), the distance from the star where an Earth-type planet would be comfortable with liquid water is centered around 0.73 to 0.74 AU -- somewhat beyond the orbital distance of Venus in the Solar System -- with an orbital period under an Earth year using calculations based on Hart (1979), but more recent calculations based on Kasting et al (1993) allow for a wider habitable zone. Useful catalogue numbers and designations for Alpha Centauri B include: Alp or Alf Cen B, HR 5460, Gl 559 B, Hip 71681, HD 128621, and LHS 51.

Proxima Centauri

Š Steve Quirk (Views from Frog Rock; used with permission)

Apparent motion of Proxima Centauri over 15 years.
A real-color, field image of Proxima by David Malin
is available at Astronomy Picture of the Day.

See close-up images of Proxima from Schultz et al, 1998.

Proxima (Alpha Centauri C) is a very cool and very dim, main sequence red dwarf (M5.5Ve) that appears to have only 0.107 ą 0.021 percent of Sol's mass (Pourbaix et al, 2002) and 14.5 percent of its diameter (ESO press releases of 3/15/03 and 2/22/02; and Doyle and Butler, 1990, page 337). With a visual luminosity that has reportedly varied between 0.000053 and 0.00012 of Sol's (based on a distance of 4.22 light-years)the star is as much as 19,000 times fainter than the Sun, and so if it was placed at the location of our Sun from Earth, the disk of the star would barely be visible. It is chromosperically active with a rotation period of 31.5 +/- 1.5 days and appears to be between five and six billion years old (Guinan and Morgan, 1996).

The star is located roughly a fifth of a light-year from the AB binary pair and, if gravitationally bound to it, may have an orbital period of around half a million years. According to Anosova et al (1994), however, its motion with respect to the AB pair is hyperbolic. Accounting for infrared radiation, the distance from Proxima where an Earth-type planet would be "comfortable" with liquid water is around 0.02 to 0.06 AU (Endl et al, 2003, in pdf) -- much closer than Mercury's orbital distance of about 0.4 AU from Sol -- with an orbital period of two to 16 days. Hence, the rotation of such a planet would probably be tidally locked so that one side would be in perpetual daylight and the other in darkness. Three star spots may have been observed recently with the Hubble Space Telescope (Benedict et al, 1998).

NASA -- larger image

Proxima is a dim red dwarf star, like Gliese
623 A (M2.5V) and B (M5.8Ve) at lower right.
(A Digitized Sky Survey image of Proxima may
become available at the Nearby Stars Database.)

Like many red dwarfs, Proxima is a "Flare Star" that can brighten suddenly to many times its normal luminosity. Its flares can roughly double the star's brightness and occur sporadically from hour to hour. Moreover, more than one flare may be emitting at a time. From May to August 1995, several flares were observed with changes within a time-scale of weeks, and archival data suggests that the star may have a long-term activity cycle (Guinan and Morgan, 1996). Its designated variable star name is V645 Centauri. Other useful catalogue numbers for Proxima include: Gl 551, Hip 70890, and LHS 49.

Arnold O. Benz, Institute of Astronomy, ETH Zurich

High resolution and jumbo images (Benz et al, 1998).

Proxima is a flare star, like UV Ceti (Luyten 726-8 B)
shown flaring at left. UV Ceti is an extreme example
of a flare star that can boost its brightness by five times
in less than a minute, then fall somewhat slower back
down to normal luminosity within two or three minutes
before flaring suddenly again after several hours.

Proxima Centauri B?

Using data collected up to early 1994, astronomers using the Hubble Space Telescope discerned a 77-day variation in the proper motion of Proxima (Benedict et al, 1994). The astrometric perturbations found could be due to the gravitational pull of a large planet with about 80 percent of Jupiter's mass at a 1994 separation from Proxima of about 0.17 AUs -- 17 percent of Earth's orbital distance in the Solar System from the distance, or less than half Mercury's orbital distance. The Hubble astrometry team calculated that the chance of a false positive reading from their data -- same perturbations without a planet -- to be around 25 percent.

Š John Whatmough -- larger image
(Artwork from Extrasolar Visions, used with permission)
Glowing red through gravitational contraction, the candidate brown dwarf companion
to Proxima Centauri is depicted with two moons (one eclipsing the flare star) with
distant Alpha Centauri A and B at upper right, as imagined by Whatmough.

In 1996, another group of astronomers using the Hubble Space Telescope discovered that they might have directly observed a companion to Proxima with the implied brightness of a brown dwarf and an apparent visual separation of only about half the Earth-Sun distance -- 0.5 AU (Schultz et al, 1998). A substellar companion at that distance would imply an orbital period of around a year, or it could be in a highly eccentric orbit with a much greater average distance from Proxima. However, later observations by other astronomers using interferometric astrometry and recent radial velocity data found no evidence to support the existence of a companion greater than 0.8 Jupiter mass with an orbital period around Proxima Centauri of between one and about 2.7 years (Benedict et al, 1999). Proxima has been selected to be one of the Tier 1 target stars for NASA's Space Interferometry Mission (SIM) -- which is planned for launch as early as 2011 -- to detect a planet as small as three Earth-masses within two AUs of its host star.

Life Around a Flare Star

Many dim, red (M) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs make up more than half of all stars in our galaxy. Although flares do occur on our Sun every so often, the amount of energy released in a solar flare is small compared to the total amount of energy that Sol produces. However, a flare the size of a solar flare occurring on a red dwarf star (such as Proxima Centauri) that is more than ten thousand times dimmer than our Sun would emit about as much or more light as the red dwarf does normally.

Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be unlikely because their planets must be located very close to dim red dwarfs to be warmed sufficiently by star light to have liquid water (about 0.007 AU for Proxima), which makes flares even more dangerous around such stars. In any case, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.

Closest Neighbors

The following star systems are located within 10 light-years of Alpha Centauri AB.

Star System	Spectra & Luminosity	Distance (light-years)
Sol	G2 V	4.4
Barnard's Star	M3.8 V	6.5
Ross 154	M3.5 Ve	8.1
Wolf 359	M5.8 Ve	8.3
Sirius 2	A0-1 V DA2-5 /VII	9.5
Epsilon Indi	K3-5 Ve	9.7

Other Information

Try Professor Jim Kaler's Stars site for other information about Rigil Kentaurus (Alpha Centauri A) at the University of Illinois' Department of Astronomy.

Up-to-date technical summaries on these stars can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARCNS pages on stars A and B and star C, the Nearby Stars Database, and the Research Consortium on Nearby Stars (RECONS) list of the 100 Nearest Star Systems. Additional information may be available at Roger Wilcox's Internet Stellar Database.

For more discussion about the suitability of this star system for terrestial life, go to Laro Schatzer's website on Alpha Centauri.

Alpha Centauri is not visible in much of the Northern Hemisphere, as Constellation Centaurus cannot be viewed from middle northern latitudes of around 40 degrees, but should become more easily visible to observers that travel south of the equator. For more information about the stars and other objects in this constellation, go to Christine Kronberg's Centaurus. For an illustration, see David Haworth's Centaurus.

For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.

Note: Thanks to Andrew James for notifying us of updated orbit information for Stars A and B and to Aaron Freed for new calculations of the apparent brightness of Stars A and B on planets orbiting in the water zone of each star.