Proxima Centauri

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Star
Proxima Centauri
α Cen C
position
The position of Proxima Centauri
AladinLite
Observation
dates equinox :  J2000.0 , epoch : J2000.0
Constellation centaur
Right ascension 14 h 29 m 42.95 s
declination -62 ° 40 ′ 46.2 ″
Apparent brightness 11.13 (11.1-11.3) mag
Typing
Known exoplanets 1
B − V color index 1.82 
U − B color index 1.26 
Spectral class M5.5 Ve
Variable star type BY + UV 
Astrometry
Radial velocity (−22.4 ± 0.5) km / s
parallax (768.5 ± 0.2)  mas
distance (4.244 ± 0.001)  Lj
(1.301 ± 0.001)  pc
Visual absolute brightness M vis 15.49 likes
Proper movement 
Rec. Share: −3781.31 ± 0.09  mas / a
Dec. portion: +769.77 ± 0.19  mas / a
Physical Properties
Dimensions 0.122 ± 0.003  M ☉
radius 0.154 ± 0.005  R ☉
Luminosity

138e-6  L ☉

Effective temperature 3050 ±  100K
Age 4th.85e9  a
Other names
and catalog entries
Bayer name α Cen C
Gliese catalog FY 551 [1]
Hipparcos catalog HIP 70890 [2]
2MASS catalog 2MASS J14294291-6240465 [3]
Other names Proxima Centauri, V645 Centauri

Proxima Centauri ( Latin proxima , nearest '(ending -a in gender congruence to Latin stella , star') and centauri , genitive to centaurus ' Kentaur '), also called V645 Centauri or Alpha Centauri C , is at a distance of about 4.244 Light years (approx. 1.30  pc or 4 · 10 13  km ) the closest known star to the sun . The suffix V645 Centauri follows the rules of naming variable stars and states that it is the 645th variable that was discovered in the constellation Centaur .

Because of its position in the southern sky , the star cannot be observed from Europe, but only from locations south of the 27th parallel north. Since it is inconspicuous with its low apparent magnitude of 11.13  mag , it was only discovered in 1915. Even under good conditions, a telescope with an opening of at least 8 cm is necessary to see it.

Proxima Centauri orbits Alpha Centauri A and Alpha Centauri B within 591,000 years at a distance between 5270 and 12,900 AU . The three stars together form a hierarchical triple star system . The current distance is 0.2 light years, the apparent distance in the sky about 2 °.

Proxima Centauri is the central star of a planetary system . The discovery of its only known planet Proxima Centauri b was announced in August 2016.

Physical Properties

The size and color of the Sun compared to the stars Alpha Centauri A, Alpha Centauri B, and Proxima Centauri
Proxima Centauri (image from the Hubble Space Telescope )

Proxima Centauri is a red dwarf of the spectral class M, i.e. a main sequence star . With the classification M6, it is one of the late M dwarf stars and has a relatively low temperature of 3050 K (approx. 2780 ° C) on its surface ( photosphere ).

Due to the short distance of 4.2 ly, the VLTI 2002 was able to determine the angular diameter to 1.02 ± 0.08  mas (milli-arcseconds) with the help of the optical interferometer , resulting in a diameter of approx. 200,000 km. That is about a seventh the diameter of the Sun or one and a half times that of Jupiter .

Despite its proximity to earth, its apparent brightness is only 11.05 m . This is a hundred times less than the faintest stars visible to the naked eye, the brightness of which is around 6 m . The absolute magnitude is 15.5 M . If Proxima Centauri were the central star of the solar system at the position of the sun, it would occupy 1/50 of its area and, depending on the distance of the full moon, would be around 17 to 27 times as bright as the full moon. The planets would be invisible, with the exception of Venus, which could just about be recognized as an object of the sixth magnitude. The full moon would be a dull red disk with an apparent brightness of −2 m .

The luminosity of Proxima Centauri is 0.014% that of the sun; in the visible range the star shines with 0.0056% of its luminosity. The maximum of the radiation emitted is in the infrared range at a wavelength of 1.2 µm. The chromosphere of this star is active and shows a strong emission of singly ionized magnesium at 280  nm .

The mass of Proxima Centauri corresponds to about 12% of the solar mass (approx. 130 Jupiter masses ). If its mass were a third smaller, hydrogen burning could no longer take place inside it and it would be classified as a brown dwarf . The gravitational force  g on the star's surface is 5.20  log (g / (cm / s²)).

For all main sequence stars, the average density increases with decreasing mass. This relatively small star has an average density of 57 g / cm³ (see also star structure ). For comparison, the sun has an average density of 1.41 g / cm³.

A relatively weak stellar wind emanates from Proxima Centauri . The mass loss is no more than about 20% of the mass loss of the sun due to the solar wind . However, because the star is much smaller, the mass loss per unit of surface area is about eight times that of the sun.

Flare star

Proxima Centauri falls into the Flare Star category because its brightness increases above average from time to time due to magnetic activity. Because of its low mass, the interior of the star is completely convective (the heat generated is transported outward by plasma currents , not radiation). Convection is associated with the creation and transport of a stellar magnetic field . On the surface, the magnetic energy of this field is released by flares, which can more than double the total brightness of the star. This corresponds to an increase in brightness of one magnitude .

These flares can grow to the size of the star and can reach temperatures of up to two million Kelvin . Because of this high temperature, they can emit X-rays with an intensity similar to that of the sun. The maximum power of the X-rays of the largest flares can reach 10 21  W.

About 88% of the surface could be active; this is a much higher proportion than for the sun, even higher than during the highest activity in the sunspot cycle . Even during quiet periods with few or no flares, this activity increases the temperature of the Proxima Centauri corona up to 3.5 million K , while the temperature of the solar corona is only around 2 million K.

The total activity of Proxima is considered to be relatively high compared to other red dwarfs, which does not quite fit the estimated age of the star, as the activity of red dwarfs decreases continuously over billions of years due to the slowing rotation speed.

The proximity of the star allows precise observations of flare activity. They were EXOSAT - and ROSAT - satellite observed and seem to have a cycle of about 400 days.

Proxima Centauri is also a major object of most of the observatories dealing with X-rays, such as the XMM-Newton and the Chandra . In 1980 the Einstein Observatory (High Energy Astronomy Observatory 2) produced an accurate curve of the X-ray energy of such a stellar flare. X-ray emissions from smaller, sun-like flares were observed by the Japanese ASCA satellite in 1995.

Further development

Since Proxima Centauri, like all red dwarfs, has a relatively low energy production and transports both heat and all matter by convection, the helium produced by nuclear fusion is evenly distributed in the star and does not accumulate in the core, as is the case with the sun. Also unlike the Sun, where only about 10% of the available hydrogen fuses before the star leaves the main sequence, Proxima Centauri consumes a much higher proportion before the hydrogen fusion stops.

As the amount of helium increases due to hydrogen burning, the star becomes smaller and hotter, changing its color from red to blue. During this period the star becomes significantly brighter and reaches up to 2.5% of the current solar luminosity. At the same time, the warming of all objects orbiting it increases for a few billion years.

A red dwarf with the mass of Proxima Centauri will linger in the main sequence for about 4 trillion years, much longer than most main sequence stars. That corresponds to 300 times the age of today's universe . When the hydrogen supply is finally exhausted, Proxima Centauri will evolve into a white dwarf without going into the phase of the red giant . Then it will slowly lose its remaining warmth.

Astrometry

Distances of the stars closest to the Sun in a period of 20,000 years in the past to 80,000 years in the future
Position in the equatorial coordinate system : Proxima Centauri is located south of the celestial equator

Galactic orbit

Proxima Centauri orbits the center of the Milky Way at a distance that varies between 8.313 and 9.546 kpc with an eccentricity of 0.069.

Its own movement in the sky, which can be observed from earth, is relatively large at 3.85 ″ ( arc seconds ) annually due to the short distance . In about 500 years it covers the width of a full moon.

distance

By measuring the parallax of 772.3 ± 2.4 mas by Hipparcos and the even more precise value of 768.7 ± 0.3 mas, determined by the Fine Guidance Sensor of the Hubble space telescope , the distance of Proxima Centauri from Earth can be determined to be about 4.2 light years (or 270,000  AU ). In the Gaia DR2 catalog of the Gaia probe , the parallax has been determined to be 768.5 ± 0.2 mas.

Proxima Centauri has been the closest star to the Sun for 32,000 years and will remain so for another 30,000 years until it is replaced by Ross 248 . In about 26,700 years, Proxima Centauri will have reached its closest approach to the sun at a distance of 3.11 ly.

Belonging to the Alpha Centauri system

The affiliation of Proxima Centauri to Alpha Centauri was clarified at the end of 2016.

The angular distance from Proxima to Alpha Centauri in the sky is about 2 degrees (four full moon widths). It is about 12,500 ± 700 AU or 0.2 ly away from this binary star system (1/20 of its distance to the sun). This corresponds to about 1000 times the distance between Alpha Centauri A and Alpha Centauri B or 500 times the distance between Neptune and the sun.

Astrometric measurements such as that of the Hipparcos satellite already suggested that Proxima Centauri is in orbit around the binary star system. According to current measurements, the period of rotation is 591,000 years. That is why it is also known as the Alpha Centauri C. Based on this data, the orbit with a minimum distance of 5270 AU and a maximum distance of 12,900 AU from the inner binary star system would be clearly eccentric. Proxima Centauri would now be near its apocenter (the most distant point in its orbit around Alpha Centauri A and B).

Some radial velocity measurements, e.g. B. in the Gliese catalog, but deviate from the values ​​expected for a bound system, so that it could not be ruled out that it was just a random star encounter . This assumption was supported by simulation calculations which, based on the calculated binding energy of the system, only resulted in a bound system in 44 percent of the examined possibilities.

According to Matthews et al. - taking into account the short distance and the similar airspeed - the chances that the observed arrangement is random were only about 1 in 1,000,000.

Studies from 1994 indicate that Proxima Centauri forms a movement group together with the inner binary star system and nine other star systems. As a result, it would not orbit the pair Alpha Centauri in a bound motion, but its orbit would be hyperbolically disturbed by the binary star system . This means that Proxima Centauri would never make a full orbit around Alpha Centauri A and B.

Surroundings

Seen from Proxima, the binary star system Alpha Centauri A and B appears as a very bright star with an apparent magnitude of −6.80 m . Depending on the position of A and B in their orbits, the binary star would be easy to separate with the naked eye, and then again to be seen as a single star. Alpha Centauri A would appear with a brightness of −6.52 m , whereas B would appear with −5.19 m . After this binary star system and the Sun, Barnard's Arrow Star is the closest neighbor of Proxima Centauri at 6.6 light years. The sun appears from Proxima as a 0.4 m bright star in the constellation Cassiopeia .

From Alpha Centauri, Proxima could only be seen as an inconspicuous star with a brightness of 4.5 m, despite its short distance (a quarter of a light year) . This shows how faint the red dwarf star actually is.

It is conceivable that Proxima Centauri in the pericenter deflects some comets from a spherical comet cloud (similar to the assumed Oort cloud around the solar system), which could be located around the stars Alpha Centauri A and B, and thus any terrestrial planets around the stars A and B could supply water. If Proxima was tied to the Alpha Centauri system during its formation, then it is very likely that the stars are constructed with the same distribution of elements. In addition, the influence of gravity would have stirred the protoplanetary disk Alpha Centauris. This would have promoted the accumulation of ice masses (as well as water ice). A possible terrestrial planet would have been supplied with material.

Search for planets

Upper limits of the mass of a companion
(derived from the radial velocity)
Rotation time
(days) 		Semi-axis
(AE) 		Maximum
mass
(× earth )
3.6-13.8 0.022-0.054 2-3
<100 <0.21 8.5
<1000 <1 16

Proxima Centauri was, together with Alpha Centauri A and B of the priorities for the " Space Interferometry Mission " (SIM) of NASA . Theoretically, SIM could have discovered planets that are at least three times the mass of the earth and orbit their central star within 2 AU. However, the project was discontinued in 2010.

When examined by the Faint Object Spectrograph of the Hubble Space Telescope in 1998, it appeared that a companion orbiting Proxima at a distance of 0.5 AU had been tracked. The subsequent search with the Wide Field Planetary Camera 2 found no more clues.

If Proxima Centauri were orbited by a planet, both would rotate around the common center of gravity , which in the course of each orbit would lead to a weaker or stronger fluctuation of the star's orbit depending on the mass of the companion, which would be recognizable in corresponding deviations. If the orbital plane would be against the line of sight from Earth inclined, these fluctuations would the radial velocity change of Proxima Centauri.

Despite many measurements, however, for a long time no such changes were unequivocally observed, so that initially no massive companions were suspected. After the first indications of a possible planet in 2013, the “Pale Red Dot” project was finally launched with the aim of tracking down earth-like planets around Proxima Centauri. After two years of preparation, the Alpha Centauri system was regularly observed with the HARPS spectrograph at ESO's La Silla Observatory and other terrestrial telescopes in the first half of 2016 . In August 2016, the existence of a planet - provisionally called " Proxima Centauri b " - with a mass of at least 1.3 Earth masses and an orbital period of 11.19 days was confirmed with high significance . Measurements with the ESPRESSO spectrograph from 2020 suggest that the planet has a minimum mass of 1.17 Earth masses. In addition, signals were discovered that allow the conclusion to be drawn to another companion with only a third of the earth's mass. If this were confirmed, it would be the smallest planet to date that would have been discovered using the radial velocity method.

Possibility of life

Models show that a planet whose surface should be above freezing should be no further than 0.032 AU from Proxima Centauri. When a planet orbits this close to a star, the tidal forces would create a bound rotation. One side of the surface would always face the star, which, apart from the brief seasonal fluctuation, would always be seen in the same place in the sky. With this proximity to the central star, a year would be at most 6.3 earth days, which, due to the above-mentioned bound rotation, also corresponds to the sidereal day length of this planet. Even this slow rotation would be enough to create a magnetic field, provided the interior of the planet remained melted. If the magnetic field were too weak, the mass ejections of the corona would massively erode the atmosphere of a planet due to the lack of magnetic deflection.

The flare outbreaks that occur again and again at Proxima Centauri would hardly allow life. Within a few minutes, the star's luminosity could double or triple; A flare observed on March 24, 2017 even reached a thousand times the luminosity compared to the idle state for about 10 seconds. Such flares could destroy the atmosphere of any planet in the habitable zone .

Interstellar journey

Proxima Centauri has often been suggested as the most sensible first destination for interstellar travel because of its short distance , although as a flare star it is a difficult target. At the current distance, a space probe that is as fast as the Voyager 1 space probe at 61,000 km / h, for example , would take around 75,000 years to travel.

With the Longshot project, there is a concept in which Proxima Centauri and the stars Alpha Centauri A and B, 0.2 light years away, could theoretically be reached in about 100 years.

discovery

For a long time Alpha Centauri was thought to be the closest neighbor of the solar system until in 1915 Robert Innes , the then director of the Republic Observatory in Johannesburg , discovered this tiny star in the vicinity of Alpha Centauri by comparing two photo plates and found that both of them have the same proper motion . In 1917 the Dutch astronomer J. Voûte measured the trigonometric parallax at the Royal Observatory on the Cape of Good Hope and found that the star is about as far away as Alpha Centauri and that it was the faintest star known at the time. When it was established that the faint star was a little closer, Innes suggested calling it Proxima Centauri .

In 1951, Harlow Shapley announced that Proxima Centauri was a flare star. Examination of previous photographs showed that the star's brightness was brighter than usual in 8% of the observations. This made him the most active flare star that had been discovered by then.

See also

Remarks

  1. ↑ For the apparent brightness m and the parallax π , the absolute brightness M v is determined from:
  2. ↑ The difference in absolute brightness between Proxima Centauri and the Sun is 15.49 - 4.83 = 10.66. Proxima Centauri at the point of the sun, which appears bright at −26.72 m , would appear −16.06 m bright. The full moon can be bright between −12.5 m to −13.0 m , depending on the distance . Proxima would appear 17 (2.512 ( 16.06-13.0) ) times brighter than the full moon at maximum full moon brightness and 27 (2.512 (16.06-12.5) ) times brighter at minimum full moon brightness . If the full moon were illuminated by Proxima, it would be −1.84 m and −2.34 m bright. Venus reaches a maximum apparent brightness of −4.6 m , so that the brightness of Venus in the same orbit around Proxima Centauri −4, 6 + 10.66 = +6.06 m .
  3. ↑ In astrophysics, surface gravity is expressed in log g. It is the log10 value of the acceleration due to gravity in cgs units, namely the value in cm / s². In the case of Proxima Centauri this is 10 to the power of 5.20, that is 158,490 cm / s² or 1584.9 m / s². That is 161.55 times the force of gravity of the earth, i.e. 9.81 m / s².
  4. ↑ The density ( ρ ) is the quotient of mass per volume. Compared to the sun, the density is:
    =
    = 0.123 x 0.145 -3 x 1.409 g / cm³
    = 56.8 g / cm³

    where is the average density of the sun.

  5. ↑ The coordinates of the sun would be directly opposite Proxima α = 02 h 29 m 42.95 s and δ = + 62 ° 40 ′ 46.14 ″ . The absolute brightness of the sun is 4.83 m . At a distance of 1.295 pc the apparent brightness would be 4.83 - 5 (log 10  0.77199 + 1) = 0.40.2624046.14
  6. ↑ 4.244 ly * (9.46 * 10 ^ 12 km) / 61,000 km / h / 24 h / 365 days ≈ 75,100 a

Web links

Commons : Proxima Centauri  - collection of images, videos and audio files

Individual evidence

Info box:

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This article was added to the list of excellent articles on December 24, 2009 in this version .