Andromeda: structure,nucleus,formation

Andromeda

The Andromeda Galaxy (/ænˈdrɒmɪdə/), also known as Messier 31, M31, or NGC 224, is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years) from Earth, and the nearest major galaxy to the Milky Way. Its name stems from the area of the sky in which it appears, the constellation of Andromeda.

The 2006 observations by the Spitzer Space Telescope revealed that the Andromeda Galaxy contains approximately one trillion stars, more than twice the number of the Milky Way’s estimated 200-400 billion stars. The Andromeda Galaxy, spanning approximately 220,000 light years, is the largest galaxy in our Local Group, which is also home to the Triangulum Galaxy and other minor galaxies. Its mass is estimated to be ~0.8-1.5×1012 solar masses , whereas the Milky Way's mass is estimated to be 8.5×1011 solar masses.

The Milky Way and Andromeda galaxies are expected to collide in ~4.5 billion years, merging to form a giant elliptical galaxy or a large disc galaxy. With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects - making it visible to the naked eye on moonless nights, even when viewed from areas with moderate light pollution.

Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi described the Andromeda Galaxy, in his Book of Fixed Stars as a "nebulous smear". Star charts of that period labeled it as the Little Cloud. In 1612, the German astronomer Simon Marius gave an early description of the Andromeda Galaxy based on telescopic observations. The German philosopher Immanuel Kant in 1755 in his work Universal Natural History and Theory of the Heavens conjectured that the blurry spot was an island universe. In 1764, Charles Messier cataloged Andromeda as object M31 and incorrectly credited Marius as the discoverer despite it being visible to the naked eye. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of Andromeda. He believed Andromeda to be the nearest of all the "great nebulae", and based on the color and magnitude of the nebula, he incorrectly guessed that it is no more than 2,000 times the distance of Sirius. In 1850, William Parsons, 3rd Earl of Rosse, saw and made the first drawing of Andromeda's spiral structure.

In 1864, William Huggins noted that the spectrum of Andromeda differs from a gaseous nebula. The spectra of Andromeda displays a continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. Andromeda's spectrum is very similar to the spectra of individual stars, and from this, it was deduced that Andromeda has a stellar nature. In 1885, a supernova (known as S Andromedae) was seen in Andromeda, the first and so far only one observed in that galaxy. At the time Andromeda was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, and was named accordingly; "Nova 1885".

In 1887, Isaac Roberts took the first photographs of Andromeda, which was still commonly thought to be a nebula within our galaxy. Roberts mistook Andromeda and similar spiral nebulae as solar systems being formed.[citation needed] In 1912, Vesto Slipher used spectroscopy to measure the radial velocity of Andromeda with respect to our solar system—the largest velocity yet measured, at 300 kilometres per second (190 mi/s).

Island universe

In 1917, Heber Curtis observed a nova within Andromeda. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred elsewhere in the sky. As a result, he was able to come up with a distance estimate of 500,000 light-years (3.2×1010 AU). He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies.

In 1920, the Great Debate between Harlow Shapley and Curtis took place, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim of the Great Andromeda Nebula being, in fact, an external galaxy, Curtis also noted the appearance of dark lanes within Andromeda which resembled the dust clouds in our own galaxy, as well as historical observations of Andromeda Galaxy's significant Doppler shift. In 1922 Ernst Öpik presented a method to estimate the distance of Andromeda using the measured velocities of its stars. His result placed the Andromeda Nebula far outside our galaxy at a distance of about 450,000 parsecs (1,500,000 ly). Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of Andromeda. These were made using the 2.5-metre (100-in) Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature is not a cluster of stars and gas within our own Galaxy, but an entirely separate galaxy located a significant distance from the Milky Way.

In 1943, Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Baade identified two distinct populations of stars based on their metallicity, naming the young, high-velocity stars in the disk Type I and the older, red stars in the bulge Type II. This nomenclature was subsequently adopted for stars within the Milky Way, and elsewhere. (The existence of two distinct populations had been noted earlier by Jan Oort.) Baade also discovered that there were two types of Cepheid variables, which resulted in a doubling of the distance estimate to Andromeda, as well as the remainder of the Universe.

In 1950, radio emission from the Andromeda Galaxy was detected by Hanbury Brown and Cyril Hazard at Jodrell Bank Observatory. The first radio maps of the galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group. The core of the Andromeda Galaxy is called 2C 56 in the 2C radio astronomy catalog. In 2009, the first planet may have been discovered in the Andromeda Galaxy. This was detected using a technique called microlensing, which is caused by the deflection of light by a massive object.

General

The estimated distance of the Andromeda Galaxy from our own was doubled in 1953 when it was discovered that there is another, dimmer type of Cepheid. In the 1990s, measurements of both standard red giants as well as red clump stars from the Hipparcos satellite measurements were used to calibrate the Cepheid distances.

Formation and history

Andromeda Galaxy was formed roughly 10 billion years ago from the collision and subsequent merger of smaller protogalaxies. This violent collision formed most of the galaxy's (metal-rich) galactic halo and extended disk. During this epoch, star formation would have been very high, to the point of becoming a luminous infrared galaxy for roughly 100 million years. Andromeda and the Triangulum Galaxy had a very close passage 2–4 billion years ago. This event produced high levels of star formation across the Andromeda Galaxy's disk – even some globular clusters – and disturbed M33's outer disk.

Over the past 2 billion years, star formation throughout Andromeda's disk is thought to have decreased to the point of near-inactivity. There have been interactions with satellite galaxies like M32, M110, or others that have already been absorbed by Andromeda Galaxy. These interactions have formed structures like Andromeda's Giant Stellar Stream. A galactic merger roughly 100 million years ago is believed to be responsible for a counter-rotating disk of gas found in the center of Andromeda as well as the presence there of a relatively young (100 million years old) stellar population.[citation needed]

Distance estimate

In 2003, using the infrared surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value and a metallicity correction of −0.2 mag dex−1 in (O/H), an estimate of 2.57 ± 0.06 million light-years (1.625×1011 ± 3.8×109 AU) was derived.

In 2004, using the Cepheid variable method, the distance was estimated to be 2.51 ± 0.13 million light-years (770 ± 40 kpc).

In 2005, an eclipsing binary star was discovered in the Andromeda Galaxy. The binary is two hot blue stars of types O and B. By studying the eclipses of the stars, astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure their absolute magnitude. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at a distance of 2.52×106 ± 0.14×106 ly (1.594×1011 ± 8.9×109 AU) and the whole Andromeda Galaxy at about 2.5×106 ly (1.6×1011 AU). This new value is in excellent agreement with the previous, independent Cepheid-based distance value.

Averaged together, these distance estimates give a value of 2.54×106 ± 0.11×106 ly (1.606×1011 ± 7.0×109 AU). And, from this, the diameter of Andromeda at the widest point is estimated to be 220 ± 3 kly (67,450 ± 920 pc).[original research?] Applying trigonometry (angular diameter), this is equivalent to an apparent 4.96° angle in the sky.

Mass and luminosity estimates

Mass

Mass estimates for the Andromeda Galaxy's halo (including dark matter) give a value of approximately 1.5×1012 M☉ (or 1.5 trillion solar masses) compared to 8×1011 M☉ for the Milky Way. This contradicts earlier measurements, that seem to indicate that Andromeda Galaxy and the Milky Way are almost equal in mass. Even so, Andromeda Galaxy's spheroid actually has a higher stellar density than that of the Milky Way and its galactic stellar disk is about twice the size of that of the Milky Way. The total stellar mass of Andromeda Galaxy is estimated to be between 1.1×1011 M☉., (i.e., around twice as massive as that of the Milky Way) and 1.5×1011 M☉, with around 30% of that mass in the central bulge, 56% in the disk, and the remaining 14% in the halo.

In addition to it, Andromeda Galaxy's interstellar medium contains at least around 7.2×109 M☉ in the form of neutral hydrogen, at least 3.4×108 M☉ as molecular hydrogen (within its innermost 10 kiloparsecs), and 5.4×107 M☉ of dust.

Andromeda Galaxy is surrounded by a large and massive halo of hot gas that is estimated to contain half the mass of the stars in the galaxy. The nearly invisible halo stretches about a million light-years from its host galaxy, halfway to our Milky Way galaxy. Simulations of galaxies indicate the halo formed at the same time as the Andromeda Galaxy. The halo is enriched in elements heavier than hydrogen and helium, formed from supernovae and its properties are those expected for a galaxy that lies in the "green valley" of the Galaxy color–magnitude diagram (see below). Supernovae erupt in Andromeda Galaxy's star-filled disk and eject these heavier elements into space. Over Andromeda Galaxy's lifetime, nearly half of the heavy elements made by its stars have been ejected far beyond the galaxy's 200,000-light-year-diameter stellar disk.

Luminosity

Compared to the Milky Way, the Andromeda Galaxy appears to have predominantly older stars with ages >7×109 years.[clarification needed] The estimated luminosity of Andromeda Galaxy, ~2.6×1010 L☉, is about 25% higher than that of our own galaxy. However, the galaxy has a high inclination as seen from Earth and its interstellar dust absorbs an unknown amount of light, so it is difficult to estimate its actual brightness and other authors have given other values for the luminosity of the Andromeda Galaxy (some authors even propose it is the second-brightest galaxy within a radius of 10 mega-parsecs of the Milky Way, after the Sombrero Galaxy, with an absolute magnitude of around -22.21 or close).

An estimation done with the help of Spitzer Space Telescope published in 2010 suggests an absolute magnitude (in the blue) of −20.89 (that with a color index of +0.63 translates to an absolute visual magnitude of −21.52, compared to −20.9 for the Milky Way), and a total luminosity in that wavelength of 3.64×1010 L☉.

The rate of star formation in the Milky Way is much higher, with Andromeda Galaxy producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of supernovae in the Milky Way is also double that of Andromeda Galaxy.[not in citation given] This suggests that the latter once experienced a great star formation phase, but is now in a relative state of quiescence, whereas the Milky Way is experiencing more active star formation. Should this continue, the luminosity of the Milky Way may eventually overtake that of Andromeda Galaxy.

According to recent studies, the Andromeda Galaxy lies in what in the galaxy color–magnitude diagram is known as the "green valley", a region populated by galaxies like the Milky Way in transition from the "blue cloud" (galaxies actively forming new stars) to the "red sequence" (galaxies that lack star formation). Star formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties to Andromeda Galaxy, star formation is expected to extinguish within about five billion years from the now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between Andromeda Galaxy and the Milky Way.

Structure

A Galaxy Evolution Explorer image of the Andromeda Galaxy. The bands of blue-white making up the galaxy's striking rings are neighborhoods that harbor hot, young, massive stars. Dark blue-grey lanes of cooler dust show up starkly against these bright rings, tracing the regions where star formation is currently taking place in dense cloudy cocoons. When observed in visible light, Andromeda Galaxy’s rings look more like spiral arms. The ultraviolet view shows that these arms more closely resemble the ring-like structure previously observed in infrared wavelengths with NASA’s Spitzer Space Telescope. Astronomers using the latter interpreted these rings as evidence that the galaxy was involved in a direct collision with its neighbor, M32, more than 200 million years ago.

Based on its appearance in visible light, the Andromeda Galaxy is classified as an SA(s)b galaxy in the de Vaucouleurs–Sandage extended classification system of spiral galaxies. However, data from the 2MASS survey showed that Andromeda is actually a barred spiral galaxy, like the Milky Way, with Andromeda's bar oriented along its long axis.

In 2005, astronomers used the Keck telescopes to show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself. This means that the spiral disk of stars in the Andromeda Galaxy is three times larger in diameter than previously estimated. This constitutes evidence that there is a vast, extended stellar disk that makes the galaxy more than 220,000 light-years (67,000 pc) in diameter. Previously, estimates of the Andromeda Galaxy's size ranged from 70,000 to 120,000 light-years (21,000 to 37,000 pc) across.

The galaxy is inclined an estimated 77° relative to the Earth (where an angle of 90° would be viewed directly from the side). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk. A possible cause of such a warp could be gravitational interaction with the satellite galaxies near the Andromeda Galaxy. The Galaxy M33 could be responsible for some warp in Andromeda's arms, though more precise distances and radial velocities are required.

Spectroscopic studies have provided detailed measurements of the rotational velocity of the Andromeda Galaxy as a function of radial distance from the core. The rotational velocity has a maximum value of 225 kilometres per second (140 mi/s) at 1,300 light-years (82,000,000 AU) from the core, and it has its minimum possibly as low as 50 kilometres per second (31 mi/s) at 7,000 light-years (440,000,000 AU) from the core. Further out, rotational velocity rises out to a radius of 33,000 light-years (2.1×109 AU), where it reaches a peak of 250 kilometres per second (160 mi/s). The velocities slowly decline beyond that distance, dropping to around 200 kilometres per second (120 mi/s) at 80,000 light-years (5.1×109 AU). These velocity measurements imply a concentrated mass of about 6×109 M☉ in the nucleus. The total mass of the galaxy increases linearly out to 45,000 light-years (2.8×109 AU), then more slowly beyond that radius.

The spiral arms of the Andromeda Galaxy are outlined by a series of H II regions, first studied in great detail by Walter Baade and described by him as resembling "beads on a string". His studies show two spiral arms that appear to be tightly wound, although they are more widely spaced than in our galaxy. His descriptions of the spiral structure, as each arm crosses the major axis of the Andromeda Galaxy, are as follows§pp1062§pp92:

Since the Andromeda Galaxy is seen close to edge-on, it is difficult to study its spiral structure. Rectified images of the galaxy seem to show a fairly normal spiral galaxy, exhibiting two continuous trailing arms that are separated from each other by a minimum of about 13,000 light-years (820,000,000 AU) and that can be followed outward from a distance of roughly 1,600 light-years (100,000,000 AU) from the core. Alternative spiral structures have been proposed such as a single spiral arm or a flocculent pattern of long, filamentary, and thick spiral arms.

The most likely cause of the distortions of the spiral pattern is thought to be interaction with galaxy satellites M32 and M110. This can be seen by the displacement of the neutral hydrogen clouds from the stars.

In 1998, images from the European Space Agency's Infrared Space Observatory demonstrated that the overall form of the Andromeda Galaxy may be transitioning into a ring galaxy. The gas and dust within the galaxy is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 light-years (2.0×109 AU) (10 kiloparsecs) from the core, nicknamed by some astronomers the ring of fire. This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust, and most of the star formation that is taking place in the Andromeda Galaxy is concentrated there.

Later studies with the help of the Spitzer Space Telescope showed how Andromeda Galaxy's spiral structure in the infrared appears to be composed of two spiral arms that emerge from a central bar and continue beyond the large ring mentioned above. Those arms, however, are not continuous and have a segmented structure.

Close examination of the inner region of the Andromeda Galaxy with the same telescope also showed a smaller dust ring that is believed to have been caused by the interaction with M32 more than 200  million years ago. Simulations show that the smaller galaxy passed through the disk of the Andromeda Galaxy along the latter's polar axis. This collision stripped more than half the mass from the smaller M32 and created the ring structures in Andromeda. It is the co-existence of the long-known large ring-like feature in the gas of Messier 31, together with this newly discovered inner ring-like structure, offset from the barycenter, that suggested a nearly head-on collision with the satellite M32, a milder version of the Cartwheel encounter.

Studies of the extended halo of the Andromeda Galaxy show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally "metal-poor", and increasingly so with greater distance. This evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 100–200 low-mass galaxies during the past 12  billion years. The stars in the extended halos of the Andromeda Galaxy and the Milky Way may extend nearly one-third the distance separating the two galaxies.

Nucleus

M31 is known to harbor a dense and compact star cluster at its very center. In a large telescope it creates a visual impression of a star embedded in the more diffuse surrounding bulge. In 1991, the Hubble Space Telescope was used to image Andromeda Galaxy's inner nucleus. The nucleus consists of two concentrations separated by 1.5 parsecs (4.9 ly). The brighter concentration, designated as P1, is offset from the center of the galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains a black hole measured at 3–5 × 107 M☉ in 1993, and at 1.1–2.3 × 108 M☉ in 2005. The velocity dispersion of material around it is measured to be ≈ 160 km/s.

Chandra X-ray telescope image of the center of Andromeda Galaxy. A number of X-ray sources, likely X-ray binary stars, within the galaxy's central region appear as yellowish dots. The blue source at the center is at the position of the supermassive black hole.

It has been proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an eccentric orbit around the central black hole. The eccentricity is such that stars linger at the orbital apocenter, creating a concentration of stars. P2 also contains a compact disk of hot, spectral class A stars. The A stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1.

While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus is the remnant of a small galaxy "cannibalized" by Andromeda Galaxy, this is no longer considered a viable explanation, largely because such a nucleus would have an exceedingly short lifetime due to tidal disruption by the central black hole. While this could be partially resolved if P1 had its own black hole to stabilize it, the distribution of stars in P1 does not suggest that there is a black hole at its center.

Apparently, by late 1968, no X-rays had been detected from the Andromeda Galaxy. A balloon flight on October 20, 1970, set an upper limit for detectable hard X-rays from the Andromeda Galaxy.

Multiple X-ray sources have since been detected in the Andromeda Galaxy, using observations from the European Space Agency's (ESA) XMM-Newton orbiting observatory. Robin Barnard et al. hypothesized that these are candidate black holes or neutron stars, which are heating the incoming gas to millions of kelvins and emitting X-rays. The spectrum of the neutron stars is the same as the hypothesized black holes but can be distinguished by their masses.

There are approximately 460 globular clusters associated with the Andromeda Galaxy. The most massive of these clusters, identified as Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the Local Group of galaxies. It contains several million stars, and is about twice as luminous as Omega Centauri, the brightest known globular cluster in the Milky Way. Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 to be the remnant core of a dwarf galaxy that was consumed by Andromeda in the distant past. The globular with the greatest apparent brightness is G76 which is located in the south-west arm's eastern half. Another massive globular cluster, named 037-B327 and discovered in 2006 as is heavily reddened by the Andromeda Galaxy's interstellar dust, was thought to be more massive than G1 and the largest cluster of the Local Group; however, other studies have shown it is actually similar in properties to G1.

Unlike the globular clusters of the Milky Way, which show a relatively low age dispersion, Andromeda Galaxy's globular clusters have a much larger range of ages: from systems as old as the galaxy itself to much younger systems, with ages between a few hundred million years to five billion years

In 2005, astronomers discovered a completely new type of star cluster in the Andromeda Galaxy. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that they are much larger—several hundred light-years across—and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters.

In 2012, a microquasar, a radio burst emanating from a smaller black hole, was detected in the Andromeda Galaxy. The progenitor black hole is located near the galactic center and has about 10 {\displaystyle {\begin{smallmatrix}M_{\odot }\end{smallmatrix}}} {\begin{smallmatrix}M_{\odot }\end{smallmatrix}}. Discovered through a data collected by the European Space Agency's XMM-Newton probe, and subsequently observed by NASA's Swift Gamma-Ray Burst Mission and Chandra X-Ray Observatory, the Very Large Array, and the Very Long Baseline Array, the microquasar was the first observed within the Andromeda Galaxy and the first outside of the Milky Way Galaxy.

Satellites

Like the Milky Way, the Andromeda Galaxy has satellite galaxies, consisting of 14 known dwarf galaxies. The best known and most readily observed satellite galaxies are M32 and M110. Based on current evidence, it appears that M32 underwent a close encounter with the Andromeda Galaxy in the past. M32 may once have been a larger galaxy that had its stellar disk removed by M31, and underwent a sharp increase of star formation in the core region, which lasted until the relatively recent past.

M110 also appears to be interacting with the Andromeda Galaxy, and astronomers have found in the halo of the latter a stream of metal-rich stars that appear to have been stripped from these satellite galaxies. M110 does contain a dusty lane, which may indicate recent or ongoing star formation.

In 2006, it was discovered that nine of the satellite galaxies lie in a plane that intersects the core of the Andromeda Galaxy; they are not randomly arranged as would be expected from independent interactions. This may indicate a common tidal origin for the satellites.

 Collision with the Milky Way

The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second (68 mi/s). It has been measured approaching relative to our Sun at around 300 kilometres per second (190 mi/s) as the Sun orbits around the center of our galaxy at a speed of approximately 225 kilometres per second (140 mi/s). This makes the Andromeda Galaxy one of about 100 observable blueshifted galaxies. Andromeda Galaxy's tangential or sideways velocity with respect to the Milky Way is relatively much smaller than the approaching velocity and therefore it is expected to collide directly with the Milky Way in about 4 billion years. A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy or perhaps even a large disc galaxy. Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System in the event of a collision is currently unknown. Before the galaxies merge, there is a small chance that the Solar System could be ejected from the Milky Way or join the Andromeda Galaxy.

 Amateur observing

The Andromeda Galaxy is bright enough to be seen with the naked eye, even with some light pollution. Andromeda is best seen during autumn nights in the Northern Hemisphere, when from mid-latitudes the galaxy reaches zenith (its highest point at midnight) so can be seen almost all night. From the Southern Hemisphere, it is most visible at the same months, that is in spring, and away from our equator does not reach a high altitude over the northern horizon, making it difficult to observe. Binoculars can reveal some larger structures and its two brightest satellite galaxies, M32 and M110. An amateur telescope can reveal Andromeda's disk, some of its brightest globular clusters, dark dust lanes and the large star cloud NGC 206.

references:

Ribas, I.; et al. (2005). "First Determination of the Distance and Fundamental Properties of an Eclipsing Binary in the Andromeda Galaxy". Astrophysical Journal Letters. 635 (1): L37–L40. arXiv:astro-ph/0511045 Freely accessible. Bibcode:2005ApJ...635L..37R. doi:10.1086/499161.

 McConnachie, A. W.; et al. (2005). "Distances and metallicities for 17 Local Group galaxies". Monthly Notices of the Royal Astronomical Society. 356 (4): 979–997. arXiv:astro-ph/0410489 Freely accessible. Bibcode:2005MNRAS.356..979M. doi:10.1111/j.1365-2966.2004.08514.x.

 Jensen, J. B.; et al. (2003). "Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations". Astrophysical Journal. 583 (2): 712–726. arXiv:astro-ph/0210129 Freely accessible. Bibcode:2003ApJ...583..712J. doi:10.1086/345430.

 "SIMBAD-M31". SIMBAD Astronomical Database. Retrieved 2009-11-29.

 Armando, G. P.; et al. (2007). "The GALEX Ultraviolet Atlas of Nearby Galaxies". Astrophysical Journal. 173 (2): 185–255. arXiv:astro-ph/0606440 Freely accessible. Bibcode:2007ApJS..173..185G. doi:10.1086/516636.

 Prajwal R. Kafle; Sanjib Sharma; Geraint F. Lewis; Aaron S. G. Robotham (1 Feb 2018). "The Need for Speed: Escape velocity and dynamical mass measurements of the Andromeda galaxy". Monthly Notices of the Royal Astronomical Society (3): 66. arXiv:1801.03949 Freely accessible. Bibcode:2018MNRAS.tmp...66K. doi:10.1093/mnras/sty082.

 Jorge Peñarrubia; Yin-Zhe Ma; Matthew G. Walker; Alan McConnachie (29 July 2014). "A dynamical model of the local cosmic expansion". Monthly Notices of the Royal Astronomical Society. 433 (3): 2204–2222. arXiv:1405.0306 Freely accessible. Bibcode:2014MNRAS.443.2204P. doi:10.1093/mnras/stu879.

 Chapman, S. C.; et al. (2006). "A kinematically selected, metal-poor spheroid in the outskirts of M31". Astrophysical Journal. 653 (1): 255–266. arXiv:astro-ph/0602604 Freely accessible. Bibcode:2006ApJ...653..255C. doi:10.1086/508599. Also see the press release, "Andromeda's Stellar Halo Shows Galaxy's Origin to Be Similar to That of Milky Way" (Press release). Caltech Media Relations. February 27, 2006. Archived from the original on 9 May 2006. Retrieved 2006-05-24.

 Young, K. (June 6, 2006). "The Andromeda galaxy hosts a trillion stars". New Scientist. Retrieved 2014-10-06.

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 "NASA's Hubble Shows Milky Way is Destined for Head-On Collision". NASA. 31 May 2012. Archived from the original on 4 June 2014. Retrieved 12 July 2012.

 Ueda, Junko; et al. "Cold molecular gas in merger remnants. I. Formation of molecular

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Alpha Cantauri : All facts about ....

Alpha Centauri

(Nearest star from the our solar system)

Alpha Centauri (α Centauri, abbreviated Alpha Cen, α Cen) is the closest star system to the Solar System, being 4.37 light-years (1.34 pc) from the Sun. It consists of three stars: Alpha Centauri A (also named Rigil Kentaurus) and Alpha Centauri B, which form the binary star Alpha Centauri AB, and a small and faint red dwarf, Alpha Centauri C (also named Proxima Centauri), which is loosely gravitationally bound and orbiting the other two at a current distance of about 13,000 astronomical units (0.21 ly). To the unaided eye, the two main components appear as a single point of light with an apparent visual magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and is the third-brightest star in the night sky, outshone only by Sirius and Canopus.

Alpha Centauri A (α Cen A) has 1.1 times the mass and 1.519 times the luminosity of the Sun, while Alpha Centauri B (α Cen B) is smaller and cooler, at 0.907 times the Sun's mass and 0.445 times its visual luminosity. During the pair's 79.91-year orbit about a common centre, the distance between them varies from nearly that between Pluto and the Sun (35.6 AU) to that between Saturn and the Sun (11.2 AU).

Proxima Centauri (α Cen C) is at the slightly smaller distance of 4.24 light-years (1.30 pc) from the Sun, making it the closest star to the Sun, even though it is not visible to the naked eye. The separation of Proxima from Alpha Centauri AB is about 13,000 astronomical units (0.21 ly), equivalent to about 430 times the size of Neptune's orbit. Proxima Centauri b, an Earth-sized exoplanet in the habitable zone of Proxima Centauri, was discovered in 2016.

Nomenclature

α Centauri (Latinised to Alpha Centauri) is the system's Bayer designation. It bore the traditional name Rigil Kentaurus, which is a latinisation of the Arabic name رجل القنطورس Rijl al-Qanṭūris, meaning "Foot of the Centaur".

Alpha Centauri C was discovered in 1915 by the Scottish astronomer Robert Innes, Director of the Union Observatory in Johannesburg, South Africa, who suggested that it be named Proxima Centauri (actually Proxima Centaurus). The name is from Latin, meaning 'nearest  of Centaurus'.

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN states that in the case of multiple stars the name should be understood to be attributed to the brightest component by visual brightness. The WGSN approved the name Proxima Centauri for Alpha Centauri C on 21 August 2016 and the name Rigil Kentaurus for Alpha Centauri A on 6 November 2016. They are now both so entered in the IAU Catalog of Star Names.

Centauri is the name given to what appears as a single star to the naked eye and the brightest star in the southern constellation of Centaurus. At −0.27 apparent visual magnitude (calculated from A and B magnitudes), it is fainter only than Sirius and Canopus. The next-brightest star in the night sky is Arcturus. Alpha Centauri is a multiple-star system, with its two main stars being Alpha Centauri A (α Cen A) and Alpha Centauri B (α Cen B), usually defined to identify them as the different components of the binary α Cen AB. A third companion—Proxima Centauri (or Proxima or α Cen C)—is much further away than the distance between stars A and B, but is still gravitationally associated with the AB system. As viewed from Earth, it is located at an angular separation of 2.2° from the two main stars. Proxima Centauri would appear to the naked eye as a separate star from α Cen AB if it were bright enough to be seen without a telescope. Alpha Centauri AB and Proxima Centauri form a visual double star. Together, the three components make a triple star system, referred to by double-star observers as the triple star (or multiple star), α Cen AB-C.

Together, the bright visible components of the binary star system are called Alpha Centauri AB (α Cen AB). This "AB" designation denotes the apparent gravitational centre of the main binary system relative to other companion star(s) in any multiple star system. "AB-C" refers to the orbit of Proxima around the central binary, being the distance between the centre of gravity and the outlying companion. Some older references use the confusing and now discontinued designation of A×B. Because the distance between the Sun and Alpha Centauri AB does not differ significantly from either star, gravitationally this binary system is considered as if it were one object.

Asteroseismic studies, chromospheric activity, and stellar rotation (gyrochronology), are all consistent with the α Cen system being similar in age to, or slightly older than, the Sun, with typical ages quoted between 4.5 and 7 billion years (Gyr).Asteroseismic analyses that incorporate the tight observational constraints on the stellar parameters for α Cen A and/or B have yielded age estimates of 4.85±0.5 Gyr, 5.0±0.5 Gyr, 5.2–7.1 Gyr, 6.4 Gyr, and 6.52±0.3 Gyr. Age estimates for stars A and B based on chromospheric activity (Calcium H & K emission) yield 4.4–6.5 Gyr, whereas gyrochronology yields 5.0±0.3 Gyr.

Alpha Centauri A

Alpha Centauri A, also known as Rigil Kentaurus, is the principal member, or primary, of the binary system, being slightly larger and more luminous than the Sun. It is a solar-like main-sequence star with a similar yellowish colour, whose stellar classification is spectral type G2 V. From the determined mutual orbital parameters, Alpha Centauri A is about 10 percent more massive than the Sun, with a radius about 22 percent larger. The projected rotational velocity ( v•sin i ) of this star is 2.7±0.7 km/s, resulting in an estimated rotational period of 22 days, which gives it a slightly faster rotational period than the Sun's 25 days. When considered among the individual brightest stars in the sky (excluding the Sun), Alpha Centauri A is the fourth brightest at an apparent visual magnitude of +0.01, being fractionally fainter than Arcturus at an apparent visual magnitude of −0.04.

Alpha Centauri B

Alpha Centauri B is the companion star, or secondary, of the binary system, and is slightly smaller and less luminous than the Sun. It is a main-sequence star of spectral type K1 V, making it more an orange colour than the primary star.Alpha Centauri B is about 90 percent the mass of the Sun and 14 percent smaller in radius. The projected rotational velocity ( v•sin i ) is 1.1±0.8 km/s, resulting in an estimated rotational period of 41 days. (An earlier, 1995 estimate gave a similar rotation period of 36.8 days.) Although it has a lower luminosity than component A, star B emits more energy in the X-ray band. The light curve of B varies on a short time scale and there has been at least one observed flare. Alpha Centauri B at an apparent visual magnitude of 1.33 would be twenty-first in brightness if it could be seen independently of Alpha Centauri A.

Observation

 The two stars of the binary Alpha Centauri AB are too close together to be resolved by the naked eye, as apparent angular separation varies over about 80 years between 2 and 22 arcsec (the naked eye has a resolution of 60 arcsec), but through much of the orbit, both are easily resolved in binoculars or small 5 cm (2 in) telescopes.

In the southern hemisphere, Alpha Centauri forms the outer star of The Pointersor The Southern Pointers, so called because the line through Beta Centauri(Hadar/Agena), some 4.5° west, points directly to the constellation Crux—the Southern Cross. The Pointers easily distinguish the true Southern Cross from the fainter asterism known as the False Cross.

The two bright stars at the lower right are Alpha (right) and Beta Centauri (left, above antenna). A line drawn through them points to the four bright stars of the Southern Cross, just to the right of the dome of La Silla Observatory.

South of about 29° S latitude, Alpha Centauri is circumpolar and never sets below the horizon. Both stars and Crux are too far south to be visible for mid-latitude northern observers. Below about 29° N latitude to the equator (roughly Hermosillo, Chihuahua City in Mexico, Galveston, Texas, Ocala, Florida and Lanzarote, the Canary Islands of Spain) during the northern summer, Alpha Centauri lies close to the southern horizon. The star culminates each year at midnight on 24 April or 9 p.m. on 8 June.

As seen from Earth, Proxima Centauri is 2.2° southwest from Alpha Centauri AB. This is about four times the angular diameter of the Full Moon, and almost exactly half the distance between Alpha Centauri AB and Beta Centauri. Proxima usually appears as a deep-red star of an apparent visual magnitude of 11.1 in a sparsely populated star field, requiring moderately sized telescopes to see. Listed as V645 Cen in the General Catalogue of Variable Stars (G.C.V.S.) Version 4.2, this UV Ceti-type flare star can unexpectedly brighten rapidly by as much as 0.6 magnitudes at visual wavelengths, then fade after only a few minutes. Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes. In August 2015 the largest recorded flares of the star occurred, with the star becoming 8.3 times brighter than normal on 13 August.

Observational history

Alpha Centauri was listed in the 2nd-century star catalog of Ptolemy. He gives the ecliptic coordinates, but texts differ as to whether the ecliptic latitude reads 44° 10′ South or 41° 10′ South. (Presently the ecliptic latitude is 43.5° South but it has decreased by a fraction of a degree since Ptolemy's time due to proper motion.) In Ptolemy's time Alpha Centauri was visible from his city of Alexandria, Egypt, at 31° N, but due to precessionits declination is now –60° 51′ South and it can no longer be seen at that latitude.

English explorer Robert Hues brought Alpha Centauri to the attention of European observers in his 1592 work Tractatus de Globis, along with Canopus and Achernar, noting "Now, therefore, there are but three Stars of the first magnitude that I could perceive in all those parts which are never seene here in England. The first of these is that bright Star in the sterne of Argo which they call Canobus. The second is in the end of Eridanus. The third [Alpha Centauri] is in the right foote of the Centaure."

The binary nature of Alpha Centauri AB was first recognized in December 1689 by astronomer and Jesuit priest Jean Richaud. The finding was made incidentally while observing a passing comet from his station in Puducherry. Alpha Centauri was only the second binary star system to be discovered, preceded by Alpha Crucis.

By 1752, French astronomer Nicolas Louis de Lacaille made astrometric positional measurements using state-of-the-art instruments of that time. Its large proper motion was discovered by Manuel John Johnson, observing from Saint Helena, who informed Thomas Henderson at the Royal Observatory, Cape of Good Hope of it. The parallax of Alpha Centauri was subsequently determined by Henderson from many exacting positional observations of the AB system between April 1832 and May 1833. He withheld his results, however, because he suspected they were too large to be true, but eventually published them in 1839 after Friedrich Wilhelm Bessel released his own accurately determined parallax for 61 Cygni in 1838. For this reason, Alpha Centauri is sometimes considered as the second star to have its distance measured because Henderson's work was not fully recognized at first. (The distance of Alpha is now reckoned at 4.396 ly or 41.59 trillion km.)

Later, John Herschel made the first micrometrical observations in 1834. Since the early 20th century, measures have been made with photographic plates.

By 1926, South African astronomer William Stephen Finsen calculated the approximate orbit elements close to those now accepted for this system. All future positions are now sufficiently accurate for visual observers to determine the relative places of the stars from a binary star ephemeris. Others, like the Belgian astronomer D. Pourbaix (2002), have regularly refined the precision of any new published orbital elements.

Scottish astronomer Robert T. A. Innes discovered Proxima Centauri in 1915 by blinking photographic plates taken at different times during a dedicated proper motion survey. This showed the large proper motion and parallax of the star was similar in both size and direction to those of Alpha Centauri AB, suggesting immediately it was part of the system and slightly closer to Earth than Alpha Centauri AB. Lying 4.24 ly (1.30 pc) away, Proxima Centauri is the nearest star to the Sun. All current derived distances for the three stars are from the parallaxesobtained from the Hipparcos star catalogue (HIP) and the Hubble Space Telescope.

Binary system

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 vs. time  the radial separation of A and B along the line of sight had reached a maximum in 2007 with B being behind A. The orbit is divided here into 80 points, each step refers to a timestep of approx. 0.99888 years or 364.84 days.

With the orbital period of 79.91 years, the A and B components of this binary star can approach each other to 11.2 AU (1.68 billion km), or about the mean distance between the Sun and Saturn; and may recede as far as 35.6 AU (5.33 billion km), approximately the distance from the Sun to Pluto. This is a consequence of the binary's moderate orbital eccentricity e = 0.5179. From the orbital elements, the total mass of both stars is about 2.0 M☉—or twice that of the Sun. The average individual stellar masses are 1.09 M☉ and 0.90 M☉, respectively, though slightly higher masses have been quoted in recent years, such as 1.14 M☉ and 0.92 M☉, or totalling 2.06 M☉. Alpha Centauri A and B have absolute magnitudes of +4.38 and +5.71, respectively. Stellar evolution theory implies both stars are slightly older than the Sun at 5 to 6 billion years, as derived by both mass and their spectral characteristics.

Viewed from Earth, the apparent orbit of this binary star means that its separation and position angle (PA) are in continuous change throughout its projected orbit. Observed stellar positions in 2010 are separated by 6.74 arcsecthrough the PA of 245.7°, reducing to 6.04 arcsec through 251.8° in 2011. The closest recent approach was in February 2016, at 4.0 arcsec through 300°. The observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec. The widest separation occurred during February 1976 and the next will be in January 2056.

In the true orbit, closest approach or periastron was in August 1955, and next in May 2035. Furthest orbital separation at apastron last occurred in May 1995 and the next will be in 2075. The apparent distance between the two stars is rapidly decreasing, at least until 2019.

Proxima Centauri

The much fainter red dwarf Proxima Centauri, or simply Proxima, is about 13,000 astronomical units (AU) away from Alpha Centauri AB. This is equivalent to 0.21 ly or 1.9 trillion km—about 5% the distance between Alpha Centauri AB and the Sun. Due to the large distance between Proxima and Alpha, it was long unknown whether they were gravitationally bound. The true orbital speed is necessarily small, and it was necessary to measure the speeds of Proxima and Alpha with a great precision. Otherwise it was impossible to ascertain whether Proxima is bound to Alpha or whether it is a completely unrelated star that happens to be undergoing a close approach at a low relative speed. Probability suggested that such low speed approaches would be rare and unlikely, but it could not be ruled out.

It was only in 2017 that a paper by Kervella, et al., showed that, based on high precision radial velocity measurements and with a high degree of confidence, Proxima and Alpha Centauri are in fact gravitationally bound. The orbital period of Proxima is approximately 550,000 years, with an eccentricity of 0.50+0.08

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.

Proxima is a red dwarf of spectral class M6 Ve with an absolute magnitude of +15.60, which is only a small fraction of the Sun's luminosity. By mass, Proxima is calculated as 0.123±0.06 M☉ (rounded to 0.12 M☉) or about one-eighth that of the Sun.

Kinematics

 All components of Alpha Centauri display significant proper motions against the background sky, similar to the first-magnitude stars Sirius and Arcturus. Over the centuries, this causes the apparent stellar positions to slowly change. Such motions define the high-proper-motion stars. These stellar motions were unknown to ancient astronomers. Most assumed that all stars were immortal and permanently fixed on the celestial sphere, as stated in the works of the philosopher Aristotle.

Edmond Halley in 1718 found that some stars had significantly moved from their ancient astrometric positions. For example, the bright star Arcturus (α Boo) in the constellation of Boötes showed an almost 0.5° difference in 1800 years, as did the brightest star, Sirius, in Canis Major (α CMa). Halley's positional comparison was Ptolemy's catalogue of stars contained in the Almagest whose original data included portions from an earlier catalogue by Hipparchos during the 1st century BCE. Halley's proper motions were mostly for northern stars, so the southern star Alpha Centauri was not determined until the early 19th century.

Scottish-born observer Thomas Henderson in the 1830s at the Royal Observatory at the Cape of Good Hope discovered the true distance to Alpha Centauri. He soon realized this system was likely to have a high proper motion, In this case, the apparent stellar motion was found using Nicolas Louis de Lacaille's astrometric observations of 1751–1752, by the observed differences between the two measured positions in different epochs.

Distances of the nearest stars from 20,000 years ago until 80,000 years in the future

Combining the 2007 revised data from the Hipparcos Star Catalogue (HIP) for the main binary star components, the average cumulative common proper motion (cpm.) of Alpha Centauri AB is about 6.1 arcmin each century, and 61.4 arcmin or 1.02° each millennium. These motions are about one-fifth and twice, respectively, the diameter of the full Moon. Using spectroscopy the mean radial velocity has been determined to be around 20 km/s towards the Solar System.

Since α Centauri A and B are almost exactly in the plane of the Milky Way as viewed from here, there are many stars behind them. In early May, 2028, α Centauri A will pass between us and a distant red star. There is a 45% probability that an Einstein ring may be observed. Other near conjunctions will also happen in the coming decades. These will allow very accurate measurements of the proper motions of the components and may give information on planets.

Predicted Future Changes

As the stars of Alpha Centauri approach the Solar System, measured common proper motions and trigonometric parallaxes slowly increase. These smaller effects will change until the star system becomes closest to Earth, and begin reversing as the distance increases again. Furthermore, other small changes also occur with the binary star's orbital elements. For example, in the size of the semi-major axis of the orbital ellipse will increasing by 0.03 arcsec per century.. Also the observed position angles of the stars are also subject to small cumulative changes (additional to position angle changes caused by the Precession of the Equinoxes), as first determined by W. H. van den Bos in 1926.

Based on knowing these common proper motions and radial velocities, Alpha Centauri will continue to gradually brighten, passing just north of the Southern Cross or Crux, before moving northwest and up towards the celestial equator and away from the galactic plane. By about 29,700 AD, in the present-day constellation of Hydra, Alpha Centauri will be 1.00 pc or 3.3 ly away., though later calculations suggest 0.90 pc or 2.9 ly in 29,000 AD. Then it will reach the stationary radial velocity (RVel) of 0.0 km/s and the maximum apparent magnitude of −0.86v (which is comparable to present-day magnitude of Canopus). Even during the time of this nearest approach, however, the apparent magnitude of Alpha Centauri will still not surpass that of Sirius, which will brighten incrementally over the next 60,000 years, and will continue to be the brightest star as seen from Earth for the next 210,000 years.

About 28,000 years from now, the Alpha Centauri system will then begin to move away from the Solar System, showing a positive radial velocity. Because of visual perspective, these stars will reach a final vanishing point and slowly disappear among the countless stars of the Milky Way. Here this once bright yellow star will fall below naked-eye visibility. somewhere in the faint present day southern constellation of Telescopium.[citation needed] This unusual location results from the fact that Alpha Centauri's orbit around the galactic centre is highly tilted with respect to the plane of the Milky Way.

Viewed from the Earth in about 6200 AD, the common proper motion of the main binary star Alpha Centaui AB will appear only 23 arcmin north (or two-thirds the diameter of the Moon) of Beta Centauri and form a spectacularly brilliant optical double star. Beta Centauri is in reality far more distant than Alpha Centauri.

Planets

Proxima Centauri b

In August 2016, the European Southern Observatory announced the discovery of a planet slightly larger than the Earth orbiting Proxima Centauri. Proxima Centauri b was found using the radial velocity method, where periodic Doppler shifts of spectral lines of the host star suggest an orbiting object. From these readings, the radial velocity of the parent star relative to the Earth is varying with an amplitude of about 2 metres (6.6 ft) per second. The planet lies in the habitable zone of Proxima Centauri, but it is possible that the planet is tidally locked to the star, resulting in temperature extremes that would be difficult for life to overcome.

Alpha Centauri Bb & Bc

In 2012, a planet around Alpha Centauri B was announced, but in 2015 a new analysis concluded that it almost certainly does not exist and was just a spurious artefact of the data analysis.

Alpha Centauri Bc was first announced in 2013 by Demory et al. It has an estimated orbital period of approximately 12 Earth days – smaller than that of Mercury – with a semimajor axis of 0.10 AU and an eccentricity smaller than 0.24.

Possible detection of another planet

On 25 March 2015, a scientific paper by Demory and colleagues published transit results for Alpha Centauri B using the Hubble Space Telescope for a total of 40 hours. They evidenced a transit event possibly corresponding to a planetary body with a radius around 0.92 R⊕. This planet would most likely orbit Alpha Centauri B with an orbital period of 20.4 days or less, with only a 5 percent chance of it having a longer orbit. The median average of the likely orbits is 12.4 days with an impact parameter of around 0–0.3. Its orbit would likely have an eccentricity of 0.24 or less. Like the probably spurious Alpha Centauri Bb, it likely has lakes of molten lava and would be far too close to Alpha Centauri B to harbour life.

Possibility of additional planets

The discovery of planets orbiting other star systems, including similar binary systems (Gamma Cephei), raises the possibility that additional planets may exist in the Alpha Centauri system. Such planets could orbit Alpha Centauri A or Alpha Centauri B individually, or be on large orbits around the binary Alpha Centauri AB. Because both the principal stars are fairly similar to the Sun (for example, in age and metallicity), astronomers have been especially interested in making detailed searches for planets in the Alpha Centauri system. Several established planet-hunting teams have used various radial velocity or star transit methods in their searches around these two bright stars. All the observational studies have so far failed to find any evidence for brown dwarfs or gas giants.

In 2009, computer simulations showed that a planet might have been able to form near the inner edge of Alpha Centauri B's habitable zone, which extends from 0.5 to 0.9 AU from the star. Certain special assumptions, such as considering that Alpha Centauri A and B may have initially formed with a wider separation and later moved closer to each other (as might be possible if they formed in a dense star cluster) would permit an accretion-friendly environment farther from the star. Bodies around A would be able to orbit at slightly farther distances due to A's stronger gravity. In addition, the lack of any brown dwarfs or gas giants in close orbits around A or B make the likelihood of terrestrial planets greater than otherwise. Theoretical studies on the detectability via radial velocity analysis have shown that a dedicated campaign of high-cadence observations with a 1-meter class telescope can reliably detect a hypothetical planet of 1.8 M⊕ in the habitable zone of B within three years.

Radial velocity measurements of Alpha Centauri B with High Accuracy Radial Velocity Planet Searcher spectrograph ruled out planets of more than 4 M⊕ to the distance of the habitable zone of the star (orbital period P = 200 days).

Current estimates place the probability of finding an earth-like planet around Alpha Centauri A or B at roughly 85%, although this number remains uncertain. The observational thresholds for planet detection in the habitable zones via the radial velocity method are currently (2017) estimated to be about 50 M⊕ for Alpha Centauri A, 8 M⊕ for B, and 0.5 M⊕for Proxima.

Theoretical planets

Early computer-generated models of planetary formation predicted the existence of terrestrial planets around both Alpha Centauri A and B, but most recent numerical investigations have shown that the gravitational pull of the companion star renders the accretion of planets very difficult. Despite these difficulties, given the similarities to the Sun in spectral types, star type, age and probable stability of the orbits, it has been suggested that this stellar system could hold one of the best possibilities for harbouring extraterrestrial life on a potential planet.

In the Solar System both Jupiter and Saturn were probably crucial in perturbing comets into the inner Solar System. Here, the comets provided the inner planets with their own source of water and various other ices. In the Alpha Centauri system, Proxima Centauri may have influenced the planetary disk as the Alpha Centauri system was forming, enriching the area around Alpha Centauri A and B with volatile materials. This would be discounted if, for example, Alpha Centauri B happened to have gas giants orbiting Alpha Centauri A (or conversely, Alpha Centauri A for Alpha Centauri B), or if the stars B and A themselves were able to perturb comets into each other's inner system as Jupiter and Saturn presumably have done in the Solar System. Such icy bodies probably also reside in Oort clouds of other planetary systems, when they are influenced gravitationally by either the gas giants or disruptions by passing nearby stars many of these icy bodies then travel starwards. Such ideas also apply to the close approach of Alpha Centauri or other stars to the Solar System, where in the distant future of our Oort Cloud maybe disrupted enough to see increased numbers of active comets. There is no direct evidence yet of the existence of such an similar Oort cloud around Alpha Centauri AB, and theoretically this may have been totally destroyed during the system's formation.[citation needed]

To be in the star's habitable zone, any suspected planet around Alpha Centauri A would have to be optimally placed about 1.25 AU away [citation needed] – about halfway between the distances of Earth's orbit and Mars's orbit in the Solar System – so as to have similar planetary temperatures and conditions for liquid water to exist. For the slightly less luminous and cooler Alpha Centauri B, the habitable zone would lie closer at about 0.7 AU (100 million km), approximately the distance that Venus is from the Sun.

With the goal of finding evidence of such planets, both Proxima Centauri and Alpha Centauri AB were among the listed "Tier 1" target stars for NASA's Space Interferometry Mission(SIM). Detecting planets as small as three Earth-masses or smaller within two astronomical units of a "Tier 1" target would have been possible with this new instrument. The SIM mission, however, was cancelled due to financial issues in 2010.

Circumstellar discs

Based on observations between 2007 and 2012, a study found a slight excess of emissions in the 24 µm (mid/far-infrared) band surrounding α Centauri AB, which may be interpreted as evidence for a sparse circumstellar disc or dense interplanetary dust. The total mass was estimated to be between 10−7 to 10−6 the mass of the Moon, or 10-100 times the mass of the Solar System's zodiacal cloud. If such a disc existed around both stars, α Centauri A's disc would likely be stable to 2.8 AU, and α Centauri B's would likely be stable to 2.5 AU. This would put A's disc entirely within the frost line, and a small part of B's outer disc just outside.

View from this system

Viewed from near the Alpha Centauri system, the sky would appear very much as it does for an observer on Earth, except that Centaurus would be missing its brightest star. The Sun would be a yellow star of an apparent visual magnitude of +0.5 in eastern Cassiopeia, at the antipodal point of Alpha Centauri's current right ascension and declination, at 02h 39m 35s +60° 50′ (2000). This place is close to the 3.4-magnitude star ε Cassiopeiae. Because of the placement of the Sun, an interstellar or alien observer would find the \/\/ of Cassiopeia had become a /\/\/ shape[note 1]nearly in front of the Heart Nebula in Cassiopeia. Sirius lies less than a degree from Betelgeuse in the otherwise unmodified Orion and with a magnitude of −1.2 is a little fainter than from Earth but still the brightest star in the Alpha Centauri sky. Procyon is also displaced into the middle of Gemini, outshining Pollux, whereas both Vega and Altair are shifted northwestward relative to Deneb (which barely moves, due to its great distance), giving the Summer Triangle a more equilateral appearance.

From Proxima Centauri b

From Proxima Centauri b, Alpha Centauri AB would appear like two close bright stars with the combined apparent magnitude of −6.8. Depending on the binary's orbital position, the bright stars would appear noticeably divisible to the naked eye, or occasionally, but briefly, as a single unresolved star. Based on the calculated absolute magnitudes, the visual apparent magnitudes of Alpha Centauri A and B would be −6.5 and −5.2, respectively.

From a hypothetical A or B planet

An observer on a hypothetical planet orbiting around either Alpha Centauri A or Alpha Centauri B would see the other star of the binary system as an intensely bright object in the night sky, showing a small but discernible disk while near periapse: A up to 210 arc seconds, B up to 155 arc seconds. Near apoapse, the disc would shrink to 60 arc seconds for A, 43 arc seconds for B, being too small to resolve by naked eye. In any case, the dazzling surface brightness could make the discs harder to resolve than a similarly sized less bright object.

For example, some theoretical planet orbiting about 1.25 AU from Alpha Centauri A (so that the star appears roughly as bright as the Sun viewed from the Earth) would see Alpha Centauri B orbit the entire sky once roughly every one year and three months (or 1.3(4) a), the planet's own orbital period. Added to this would be the changing apparent position of Alpha Centauri B during its long eighty-year elliptical orbit with respect to Alpha Centauri A. (The average speed, at 4.5 degrees per Earth year, is comparable in speed to Uranus here. With the eccentricity of the orbit, the maximum speed near periapse, about 18 degrees per Earth year, is faster than Saturn, but slower than Jupiter. The minimum speed near apoapse, about 1.8 degrees per Earth year, is slower than Neptune.) Depending on its and the planet's position on their respective orbits, Alpha Centauri B would vary in apparent magnitude between −18.2 (dimmest) and −21.0 (brightest). These visual apparent magnitudes are much dimmer than the apparent magnitude of the Sun as viewed from the Earth (−26.7). The difference of 5.7 to 8.6 magnitudes means Alpha Centauri B would appear, on a linear scale, 2500 to 190 times dimmer than Alpha Centauri A (or the Sun viewed from the Earth), but also 190 to 2500 times brighter than the full Moon as seen from the Earth (−12.5).

Also, if another similar planet orbited at 0.71 AU from Alpha Centauri B (so that in turn Alpha Centauri B appeared as bright as the Sun seen from the Earth), this hypothetical planet would receive slightly more light from the more luminous Alpha Centauri A, which would shine 4.7 to 7.3 magnitudes dimmer than Alpha Centauri B (or the Sun seen from the Earth), ranging in apparent magnitude between −19.4 (dimmest) and −22.1 (brightest). Thus Alpha Centauri A would appear between 830 and 70 times dimmer than the Sun but some 580 to 6900 times brighter than the full Moon. During the orbital period of such a planet of 0.6(3) a, an observer on the planet would see this intensely bright companion star circle the sky just as humans see with the Solar System's planets. Furthermore, Alpha Centauri A's sidereal period of approximately eighty years means that this star would move through the local ecliptic as slowly as Uranus with its eighty-four year period, but as the orbit of Alpha Centauri A is more elliptical, its apparent magnitude will be far more variable. Although intensely bright to the eye, the overall illumination would not significantly affect climate nor influence normal plant photosynthesis.

An observer on the hypothetical planet would notice a change in orientation to very-long-baseline interferometry reference points commensurate with the binary orbit periodicity plus or minus any local effects such as precession or nutation.

Assuming this hypothetical planet had a low orbital inclination with respect to the mutual orbit of Alpha Centauri A and B, then the secondary star would start beside the primary at "stellar" conjunction. Half the period later, at "stellar" opposition, both stars would be opposite each other in the sky. As a net result, both the local sun and the other star would each be in the sky for half a day, like Sun and Moon are both above the horizon for half a day. But during stellar conjunction, the other star being "new" would be in the sky during daytime, while during the opposition, the other star being "full" would be in the sky for the whole night. In an Earth-like atmosphere, the light of the other star would be appreciably scattered, causing the sky to be perceptibly blue though darker than during daytime, like during twilight or total solar eclipse. Humans could easily walk around and clearly see the surrounding terrain, and reading a book would be quite possible without any artificial light. Over the following half period, the secondary star would be in the sky for a progressively decreasing part of the night (and an increasing part of the day) until at the next conjunction the secondary star would only be in the sky during daytime near the primary star.

From a planet orbiting Alpha Centauri A or B, Proxima Centauri would appear as a fourth to fifth magnitude star, as bright as the faint stars of the constellation of Ursa Minor.

Other names

In modern literature, Rigil Kent (also Rigel Kent and variants;[note 2] /ˈraɪdʒəl ˈkɛnt/) and Toliman, were cited as colloquial alternative names of Alpha Centauri.

Rigil Kent is short for Rigil Kentaurus, which is sometimes further abbreviated to Rigil or Rigel, though that is ambiguous with Beta Orionis, which is also called Rigel. Although the short form Rigel Kent is often cited as an alternative name, the star system is most widely referred to by its Bayer designation Alpha Centauri.

The name Toliman originates with Jacobus Golius' edition of Al-Farghani's Compendium (published posthumously in 1669). Tolimân is Golius' latinization of the Arabic name الظلمانal-Ẓulmān "the ostriches", the name of an asterism of which Alpha Centauri formed the main star.

During the 19th century, the northern amateur popularist Elijah H. Burritt used the now-obscure name Bungula, possibly coined from "β" and the Latin ungula ("hoof").

Together, Alpha and Beta Centauri form the "Southern Pointers" or "The Pointers", as they point towards the Southern Cross, the asterism of the constellation of Crux.

In Standard Mandarin Chinese, 南門 Nán Mén, meaning Southern Gate, refers to an asterism consisting of α Centauri and ε Centauri. Consequently, α Centauri itself is known as 南門二 Nán Mén Èr, the Second Star of the Southern Gate.

To the Australian aboriginal Boorong people of northwestern Victoria, Alpha and Beta Centauri are Bermbermgle, two brothers noted for their courage and destructiveness, who speared and killed Tchingal "The Emu" (the Coalsack Nebula). The form in Wotjobaluk is Bram-bram-bult.

Exploration

Alpha Centauri is envisioned as a likely first target for manned or unmanned interstellar exploration. Crossing the huge distance between the Sun and Alpha Centauri using current spacecraft technologies would take several millennia, though the possibility of nuclear pulse propulsionor laser light sail technology, as considered in the Breakthrough Starshot program, could reduce the journey time to a matter of decades.

Breakthrough Starshot is a proof-of-concept initiative to send a fleet of ultra-fast light-driven nanocraft to explore the Alpha Centauri system, which could pave the way for a first launch within the next generation. An objective of the mission would be to make a fly-by of, and possibly photograph, any planets that might exist in the system. Proxima Centauri b, announced by the European Southern Observatory(ESO) in August 2016, would be a target for the Starshot program.

In January 2017, Breakthrough Initiatives and the ESO entered a collaboration to enable and implement a search for habitable planets in the Alpha Centauri system. The agreement involves Breakthrough Initiatives providing funding for an upgrade to the VISIR (VLT Imager and Spectrometer for mid-Infrared) instrument on ESO's Very Large Telescope (VLT) in Chile. This upgrade will greatly increase the likelihood of planet detection in the system.

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