Extremely Large Telescope

From Wikipedia, the free encyclopedia

Extremely Large Telescope

An artist's impression of the ELT
Alternative names	ELT
Observatory	Cerro Armazones Observatory
Location(s)	Cerro Armazones, Chile
Coordinates	24°35′52″S 70°11′46″WCoordinates: 24°35′52″S 70°11′46″W
Organization	European Southern Observatory
Altitude	3,046 m (9,993 ft)
Observing time	320 nights per year
Built	19 June 2014 --
First light	2024
Telescope style	Extremely large telescope Infrared telescope Nasmyth telescope Optical telescope
Diameter	39.3 m (128 ft 11 in)
Secondary diameter	4.09 m (13 ft 5 in)
Tertiary diameter	3.75 m (12 ft 4 in)
Angular resolution	0.005 arcsecond
Collecting area	978 m2 (10,530 sq ft)
Focal length	743.4 m (2,439 ft 0 in)
Mounting	Nasmyth telescope
Enclosure	Dome
Website	www.eso.org/public/teles-instr/elt/
Location of Extremely Large Telescope

The Extremely Large Telescope (ELT) is an astronomical observatory and the world's largest optical/near-infrared extremely large telescope now under construction. Part of the European Southern Observatory (ESO) agency, it is located on top of Cerro Armazones in the Atacama Desert of northern Chile. The design consists of a reflecting telescope with a 39.3-metre-diameter (130-foot) segmented primary mirror and a 4.2 m (14 ft) diameter secondary mirror, and will be supported by adaptive optics, eight laser guide star units and multiple large science instruments. The observatory aims to gather 100 million times more light than the human eye, 13 times more light than the largest optical telescopes existing in 2014, and be able to correct for atmospheric distortion. It has around 256 times the light gathering area of the Hubble Space Telescope and, according to the ELT's specifications, would provide images 16 times sharper than those from Hubble. The project was originally called the European Extremely Large Telescope (E-ELT), but the name was shortened in 2017.

The ELT is intended to vastly advance astrophysical knowledge by enabling detailed studies of planets around other stars, the first galaxies in the Universe, supermassive black holes, and the nature of the Universe's dark sector, and to detect water and organic molecules in protoplanetary disks around other stars. The facility is expected to take 11 years to construct.

On 11 June 2012, the ESO Council approved the ELT programme's plans to begin civil works at the telescope site, with construction of the telescope itself pending final agreement with governments of some member states. Construction work on the ELT site started in June 2014. By December 2014, ESO had secured over 90% of the total funding and authorized construction of the telescope to start, which will cost around one billion euros for the first construction phase. The first stone of the telescope was ceremonially laid on 26 May 2017, initiating the construction of the dome’s main structure and telescope, with first light being planned for 2024.

History

On 26 April 2010, the European Southern Observatory (ESO) Council selected Cerro Armazones, Chile, as the baseline site for the planned ELT. Other sites that were under discussion included Cerro Macon, Salta, in Argentina; Roque de los Muchachos Observatory, on the Canary Islands; and sites in North Africa, Morocco, and Antarctica.

Early designs included a segmented primary mirror with a diameter of 42 metres (140 feet) and an area of about 1,300 m² (14,000 sq ft), with a secondary mirror with a diameter of 5.9 m (19 ft). However, in 2011 a proposal was put forward to reduce its size by 13% to 978 m², for a 39.3 m (130 ft) diameter primary mirror and a 4.2 m (14 ft) diameter secondary mirror. It reduced projected costs from 1.275 billion to 1.055 billion euros and should allow the telescope to be finished sooner. The smaller secondary is a particularly important change; 4.2 m (14 ft) places it within the capabilities of multiple manufacturers, and the lighter mirror unit avoids the need for high-strength materials in the secondary mirror support spider.

ESO Council meets at ESO headquarters in Garching, 2012.

 

ESO's Director General commented in a 2011 press release that "With the new E-ELT design we can still satisfy the bold science goals and also ensure that the construction can be completed in only 10–11 years." The ESO Council endorsed the revised baseline design in June 2011 and expected a construction proposal for approval in December 2011. Funding was subsequently included in the 2012 budget for initial work to begin in early 2012. The project received preliminary approval in June 2012. ESO approved the start of construction in December 2014, with over 90% funding of the nominal budget secured.

The design phase of the 5-mirror anastigmat was fully funded within the ESO budget. With the 2011 changes in the baseline design (such as a reduction in the size of the primary mirror from 42 m to 39.3 m), in 2017 the construction cost was estimated to be €1.15 billion (including first generation instruments). As of 2014, the start of operations was planned for 2024. Actual construction officially began in early 2017.

ELT foundation work, October 2018.

Planning

Cerro Armazones at night (2010)

ESO focused on the current design after a feasibility study concluded the proposed 100 m (328 ft) diameter, Overwhelmingly Large Telescope, would cost €1.5 billion (£1 billion), and be too complex. Both current fabrication technology and road transportation constraints limit single mirrors to being roughly 8 m (26 ft) per piece. The next-largest telescopes currently in use are the Keck Telescopes, the Gran Telescopio Canarias and the Southern African Large Telescope, which each use small hexagonal mirrors fitted together to make a composite mirror slightly over 10 m (33 ft) across. The ELT uses a similar design, as well as techniques to work around atmospheric distortion of incoming light, known as adaptive optics.

A 40-metre-class mirror will allow the study of the atmospheres of extrasolar planets. The ELT is the highest priority in the European planning activities for research infrastructures, such as the Astronet Science Vision and Infrastructure Roadmap and the ESFRI Roadmap. The telescope underwent a Phase B study in 2014 that included "contracts with industry to design and manufacture prototypes of key elements like the primary mirror segments, the adaptive fourth mirror or the mechanical structure (...) [and] concept studies for eight instruments.”

Design

The ELT will use a novel design with a total of five mirrors. The first three mirrors are curved (non-spherical), and form a three mirror anastigmat design for excellent image quality over the 10 arcminute field of view (one third of the width of the full Moon). The fourth and fifth mirrors are (almost) flat, and provide adaptive optics correction for atmospheric distortions (mirror 4), and tip-tilt correction for image stabilisation (mirror 5). The fourth and fifth mirrors also send the light sideways to one of the Nasmyth focal stations at either side of the telescope structure, allowing multiple large instruments to be simultaneously mounted.

ELT mirror and sensors contracts

Primary mirror

The optical system of the ELT showing the location of the mirrors.

 

The surface of the 39-metre primary mirror will be composed of 798 hexagonal segments, each measuring approximately 1.4 metres across and with 50 mm thickness. Each working day, two segments will be re-coated and replaced to ensure the mirror is always clean and highly reflective.

Edge sensors constantly measure the relative positions of the primary mirror segments and their neighbours. 2394 position actuators (3 for each segment) use this information to support the system, keeping the overall surface shape unchanged against deformations caused by external factors such as wind, temperature changes or vibrations.

In January 2017, ESO awarded the contract for the fabrication of the 4608 edge sensors to the FAMES consortium, which is composed of Fogale and Micro-Epsilon. These sensors can measure relative positions to an accuracy of a few nanometres, the most accurate ever used in a telescope. 

Cast of the first ELT main mirror segments.

 

In May 2017, ESO awarded two additional contracts. One was awarded to Schott AG who will manufacture the blanks of the 798 segments, as well as an additional 133 segments as part of a maintenance set, allowing for the segments to be removed, replaced and cleaned on a rotating basis once the ELT is in operation. The mirror will be cast from the same low-expansion ceramic Zerodur as the existing Very Large Telescope mirrors in Chile. 

The other contract was awarded to the French company, Safran Reosc, a subsidiary of Safran Electronics & Defense. They will receive the mirror blanks from Schott, and polish one mirror segment per day to meet the 7-year deadline. During this process, each segment will be polished until it has no surface irregularity greater than 7.5 nm RMS. Afterwards, Safran Reosc will then mount, test, and complete all optical testing before delivery. This is the second largest contract for the ELT construction and the third-largest contract ESO has ever signed. 

The segment support system units for the primary mirror are designed and produced by CESA (Spain) and VDL (the Netherlands). The contracts signed with ESO also include the delivery of detailed and complete instructions and engineering drawings for their production. Additionally, they include the development of the procedures required to integrate the supports with the ELT glass segments; to handle and transport the segment assemblies; and to operate and maintain them.

Secondary mirror

Play media

This sequence shows the still very hot blank of the ELT secondary mirror being transported to Schott's annealing facility in Mainz, Germany.

Making the secondary mirror is a major challenge as it is highly convex, and aspheric. It is also very large; at 4.2 metres in diameter and weighing 3.5 tonnes, it will be largest secondary ever employed on a telescope and the largest convex mirror ever produced. 

In January 2017, ESO awarded a contract for the mirror blank to Schott AG, who will manufacture it of Zerodur. 

Complex support cells are also necessary to ensure the flexible secondary and tertiary mirrors retain their correct shape and position; these support cells will be provided by SENER.

The pre-formed glass-ceramic blank of the secondary mirror will then be polished, and tested by Safran Reosc. The mirror will be shaped and polished to a precision of 15 nanometres (15 millionths of a millimetre) over the optical surface.

Tertiary mirror

The 3.8-metre concave tertiary mirror, also cast from Zerodur, will be an unusual feature of the telescope. Most current large telescopes, including the VLT and the NASA/ESA Hubble Space Telescope, use just two curved mirrors to form an image. In these cases, a small, flat tertiary mirror is sometimes introduced to divert the light to a convenient focus. However, in the ELT the tertiary mirror also has a curved surface, as the use of three mirrors delivers a better final image quality over a larger field of view than would be possible with a two-mirror design.

Quaternary mirror

The 2.4-metre quaternary mirror is a flat adaptive mirror, and only 2 millimetres thick. With up to 8000 actuators, the surface can be readjusted at very high time frequencies. The deformable mirror will be the largest adaptive mirror ever made, and consists of six component petals, control systems, and voice-coil actuators. The image distortion caused by the turbulence of the Earth’s atmosphere can be corrected in real time, as well as deformations caused by the wind upon the main telescope. The ELT’s adaptive optics system will provide an improvement of about a factor of 500 in the resolution, compared to the best seeing conditions achieved so far without adaptive optics.

The AdOptica consortium, partnered with INAF (Istituto Nazionale di Astrofisica) as subcontractors, are responsible for the design and manufacture of the quaternary mirror, which is to be shipped to Chile by the end of 2022. Safran Reosc will manufacture the mirror shells, and also polish them.

ELT dome and structure

Dome construction

ELT concept.

The ELT dome will have a height of nearly 74 metres from the ground and a diameter of 86 metres, making it the largest dome ever built for a telescope. The dome will have a total mass of around 5000 tonnes, and the telescope mounting and tube structure will have a total moving mass of more than 3000 tonnes. 

For the observing slit, two main designs were under study: one with two sets of nested doors, and the current baseline design, i.e. a single pair of large sliding doors. This pair of doors has a total width of 45.3 m. 

ESO signed a contract for its construction, together with the main structure of the telescopes, with the Italian ACe Consortium, consisting of Astaldi and Cimolai and the nominated subcontractor, Italy's EIE Group. The signature ceremony took place on 25 May 2016 at ESO’s Headquarters in Garching bei München, Germany.

The dome is to provide needed protection to the telescope in inclement weather and during the day. A number of concepts for the dome were evaluated. The baseline concept for the 40m-class ELT dome is a nearly hemispherical dome, rotating atop a concrete pier, with curved laterally opening doors. This is a re-optimisation from the previous design, aimed at reducing the costs, and it is being revalidated to be ready for construction.

One year after signing the contract, and after the laying of the first stone ceremony in May 2017, the site was handed over to ACe, signifying the beginning of the construction of the dome’s main structure.

Astronomical performance

In terms of astronomical performance the dome is required to be able to track about the 1-degree zenithal avoidance locus as well as preset to a new target within 5 minutes. This requires the dome to be able to accelerate and move at angular speeds of 2 degrees/sec (the linear speed is approximately 5 km/h).

The dome is designed to allow complete freedom to the telescope so that it can position itself whether it is opened or closed. It will also permit observations from the zenith down to 20 degrees from the horizon.

Windscreen

With such a large opening, the ELT dome requires the presence of a windscreen to protect the telescope's mirrors (apart from the secondary), from direct exposure to the wind. The baseline design of the windscreen minimises the volume required to house it. Two spherical blades, either side of the observing slit doors, slide in front of the telescope aperture to restrict the wind.

Ventilation and air-conditioning

The dome design ensures that the dome provides sufficient ventilation for the telescope not to be limited by dome seeing. For this the dome is also equipped with louvers, whereby the windscreen is designed to allow them to fulfill their function. 

Computational fluid dynamic simulations and wind tunnel work are being carried out to study the airflow in and around the dome, as well as the effectiveness of the dome and windscreen in protecting the telescope. 

Besides being designed for water-tightness, air-tightness is also one of the requirements as it is critical to minimise the air-conditioning load. The air-conditioning of the dome is necessary not only to thermally prepare the telescope for the forthcoming night but also in order to keep the telescope optics clean. 

The air-conditioning of the telescope during the day is critical and the current specifications permit the dome to cool the telescope and internal volume by 10 °C over 12 hours.

Science goals

Play media

This is the official trailer for the ELT. The design for the ELT shown here is preliminary.

The ELT will search for extrasolar planets — planets orbiting other stars. This will include not only the discovery of planets down to Earth-like masses through indirect measurements of the wobbling motion of stars perturbed by the planets that orbit them, but also the direct imaging of larger planets and possibly even the characterisation of their atmospheres. The telescope will attempt to image Earthlike exoplanets, which may be possible.

Furthermore, the ELT's suite of instruments will allow astronomers to probe the earliest stages of the formation of planetary systems and to detect water and organic molecules in protoplanetary discs around stars in the making. Thus, the ELT will answer fundamental questions regarding planet formation and evolution.

By probing the most distant objects the ELT will provide clues to understanding the formation of the first objects that formed: primordial stars, primordial galaxies and black holes and their relationships. Studies of extreme objects like black holes will benefit from the power of the ELT to gain more insight into time-dependent phenomena linked with the various processes at play around compact objects.

The ELT is designed to make detailed studies of the first galaxies and to follow their evolution through cosmic time. Observations of these early galaxies with the ELT will give clues that will help understand how these objects form and evolve. In addition, the ELT will be a unique tool for making an inventory of the changing content of the various elements in the Universe with time, and to understand star formation history in galaxies.

One of the goals of the ELT is the possibility of making a direct measurement of the acceleration of the Universe's expansion. Such a measurement would have a major impact on our understanding of the Universe. The ELT will also search for possible variations in the fundamental physical constants with time. An unambiguous detection of such variations would have far-reaching consequences for our comprehension of the general laws of physics.

Instrumentation

 
Play media

This video shows engineers adjusting the complex support mechanisms that control the shape and positioning of two of the 798 segments that will form the complete primary mirror of the telescope.

 

The First ELT Instruments.

 

The telescope will have several science instruments. It will be possible to switch from one instrument to another within minutes. The telescope and dome will also be able to change positions on the sky and start a new observation in a very short time. 

Eight different instrument concepts and two post-focal adaptive modules are currently being studied, and three have been selected for first light: MICADO, HARMONI and METIS. The others planning to become available at various points over the following decade. The instruments being designed or studied are:

• CODEX: a narrow-field, R=135 000 optical spectrograph
• EAGLE: a wide-field, multi-channel integral-field near-infrared (NIR) spectrograph, with multi-object adaptive optics
• EPICS: an optical/NIR planet imager and spectrograph with extreme adaptive optics
• HARMONI: a single field, wide-band integral field spectrograph
• METIS: a mid-infrared imager and spectrograph
• MICADO: a diffraction-limited near-infrared camera and spectrograph
• OPTIMOS: a wide-field visual multi-object spectrograph
• SIMPLE: a high-spectral-resolution NIR spectrograph

The two post-focal adaptive optics modules currently being studied are:

• ATLAS: a laser tomography adaptive optics module
• MAORY: a multi-conjugate adaptive optics module

Comparison

Comparison of nominal sizes of apertures of the Extremely Large Telescope and some notable optical telescopes

 

ELT compared to the VLT and the Colosseum

One of the largest optical telescope operating today is the Gran Telescopio Canarias, with a 10.4 m aperture and a light-collecting area of 74 m². Other planned extremely large telescopes include the 25 m/368 m² Giant Magellan Telescope and 30 m/655 m² Thirty Meter Telescope, which are also targeting the beginning of the 2020 decade for completion. These other two telescopes roughly belong to the same next generation of optical ground-based telescopes. Each design is much larger than previous telescopes. Even with the descale to 39.3 m the ELT is significantly larger than both other planned extremely large telescopes. It has the aim of observing the Universe in greater detail than the Hubble Space Telescope by taking images 15 times sharper, although it is designed to be complementary to space telescopes, which typically have very limited observing time available. The ELT's 4.2-metre secondary mirror is the same size as the primary mirror on the William Herschel Telescope, the second largest optical telescope in Europe. 

Name	Aperture diameter (m)	Collecting area (m²)	First light
Extremely Large Telescope (ELT)	39.3	978	2024
Thirty Meter Telescope (TMT)	30	655	2027
Giant Magellan Telescope (GMT)	24.5	368	2024
Southern African Large Telescope (SALT)	11.1 × 9.8	79	2005
Keck Telescopes	10.0	76	1990, 1996
Gran Telescopio Canarias (GTC)	10.4	74	2007
Very Large Telescope (VLT)	8.2	50 (×4)	1998–2000
Notes: Future first-light dates are provisional and likely to change.

The ELT under ideal conditions has an angular resolution of 0.005 arcsecond which corresponds to separating two light sources 1 AU apart from 200 pc distance. At 0.03 arcseconds, the contrast is expected to be 10⁸, sufficient to search for exoplanets. The unaided human eye has an angular resolution of 1 arcminute which corresponds to separating two light sources 30 cm apart from 1 km distance.

Proxima Centauri

From Wikipedia, the free encyclopedia

Proxima Centauri

Proxima Centauri as seen by Hubble
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Centaurus
Pronunciation	UK: /ˌprɒksɪmə sɛnˈtɔːraɪ/, US: /-ˈtɔːri/
Right ascension	14h 29m 42.94853s
Declination	−62° 40′ 46.1631″
Apparent magnitude (V)	10.43 - 11.11
Characteristics
Evolutionary stage	Main sequence red dwarf
Spectral type	M5.5Ve
Apparent magnitude (U)	14.21
Apparent magnitude (B)	12.95
Apparent magnitude (V)	11.13
Apparent magnitude (R)	9.45
Apparent magnitude (I)	7.41
Apparent magnitude (J)	5.357±0.023
Apparent magnitude (H)	4.835±0.057
Apparent magnitude (K)	4.384±0.033
U−B color index	1.26
B−V color index	1.82
V−R color index	1.68
R−I color index	2.04
J−H color index	0.522
J−K color index	0.973
Variable type	UV Ceti ("flare star")
Astrometry
Radial velocity (Rv)	−22.204±0.032 km/s
Proper motion (μ)	RA: −3775.75 mas/yr Dec.: 765.54 mas/yr
Parallax (π)	768.5 ± 0.2 mas
Distance	4.244 ± 0.001 ly (1.3012 ± 0.0003 pc)
Absolute magnitude (MV)	15.60
Orbit
Primary	Alpha Centauri AB
Companion	Proxima Centauri
Period (P)	547000+6600 −4000 yr
Semi-major axis (a)	8700+700 −400 AU
Eccentricity (e)	0.50+0.08 −0.09
Inclination (i)	107.6+1.8 −2.0°
Longitude of the node (Ω)	126±5°
Periastron epoch (T)	+283+59 −41
Argument of periastron (ω) (secondary)	72.3+8.7 −6.6°
Details
Mass	0.1221±0.0022 M☉
Radius	0.1542±0.0045 R☉
Luminosity (bolometric)	0.0017 L☉
Luminosity (visual, LV)	0.00005 L☉
Surface gravity (log g)	5.20±0.23 cgs
Temperature	3042±117 K
Metallicity [Fe/H]	0.21 dex
Rotation	82.6±0.1 days
Rotational velocity (v sin i)	less than 0.1 km/s
Age	4.85 Gyr
Other designations
Alpha Centauri C, CCDM J14396-6050C, GCTP 3278.00, GJ 551, HIP 70890, LFT 1110, LHS 49, LPM 526, LTT 5721, NLTT 37460, V645 Centauri

Proxima Centauri (from Latin, meaning 'nearest [star] of Centaurus'), or Alpha Centauri C, is a red dwarf, a small low-mass star, about 4.244 light-years (1.301 pc) from the Sun in the constellation of Centaurus. It was discovered in 1915 by Robert Innes and is the nearest-known star to the Sun. With a quiescent apparent magnitude of 11.13, it is too faint to be seen with the naked eye. Proxima Centauri forms a third component of the Alpha Centauri system, currently with a separation of about 12,950 AU (1.94 trillion km) and an orbital period of 550,000 years. At present Proxima is 2.18° to the southwest of Alpha Centauri.

Because of Proxima Centauri's proximity to Earth, its angular diameter can be measured directly. The star is about one-seventh the diameter of the Sun. It has a mass about an eighth of the Sun's mass (M_☉), and its average density is about 33 times that of the Sun. Although it has a very low average luminosity, Proxima is a flare star that undergoes random dramatic increases in brightness because of magnetic activity. The star's magnetic field is created by convection throughout the stellar body, and the resulting flare activity generates a total X-ray emission similar to that produced by the Sun. The mixing of the fuel at Proxima Centauri's core through convection and its relatively low energy-production rate mean that it will be a main-sequence star for another four trillion years, or nearly 300 times the current age of the universe.

In 2016, the European Southern Observatory announced the discovery of Proxima Centauri b, planet orbiting the star at a distance of roughly 0.05 AU (7.5 million km) with an orbital period of approximately 11.2 Earth days. Its estimated mass is at least 1.3 times that of the Earth. The equilibrium temperature of Proxima b is estimated to be within the range of where water could exist as liquid on its surface, thus placing it within the habitable zone of Proxima Centauri, although because Proxima Centauri is a red dwarf and a flare star, whether it could support life is disputed. Previous searches for orbiting companions had ruled out the presence of brown dwarfs and supermassive planets.

Observation

In 1915, the Scottish astronomer Robert Innes, Director of the Union Observatory in Johannesburg, South Africa, discovered a star that had the same proper motion as Alpha Centauri. He suggested that it be named Proxima Centauri (actually Proxima Centaurus). In 1917, at the Royal Observatory at the Cape of Good Hope, the Dutch astronomer Joan Voûte measured the star's trigonometric parallax at 0.755″±0.028″ and determined that Proxima Centauri was approximately the same distance from the Sun as Alpha Centauri. It was also found to be the lowest-luminosity star known at the time. An equally accurate parallax determination of Proxima Centauri was made by American astronomer Harold L. Alden in 1928, who confirmed Innes's view that it is closer, with a parallax of 0.783″±0.005″.

Stars closest to the Sun, including Proxima Centauri

 

In 1951, American astronomer Harlow Shapley announced that Proxima Centauri is a flare star. Examination of past photographic records showed that the star displayed a measurable increase in magnitude on about 8% of the images, making it the most active flare star then known. The proximity of the star allows for detailed observation of its flare activity. In 1980, the Einstein Observatory produced a detailed X-ray energy curve of a stellar flare on Proxima Centauri. Further observations of flare activity were made with the EXOSAT and ROSAT satellites, and the X-ray emissions of smaller, solar-like flares were observed by the Japanese ASCA satellite in 1995.^[43] Proxima Centauri has since been the subject of study by most X-ray observatories, including XMM-Newton and Chandra.

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name Proxima Centauri for this star on August 21, 2016 and it is now so included in the List of IAU approved Star Names.

Because of Proxima Centauri's southern declination, it can only be viewed south of latitude 27° N. Red dwarfs such as Proxima Centauri are far too faint to be seen with the naked eye. Even from Alpha Centauri A or B, Proxima would only be seen as a fifth magnitude star. It has an apparent visual magnitude of 11, so a telescope with an aperture of at least 8 cm (3.1 in) is needed to observe it, even under ideal viewing conditions—under clear, dark skies with Proxima Centauri well above the horizon.

In 2018, a superflare was observed from Proxima Centauri, the strongest flare ever seen. The optical brightness increased by a factor of 68 to approximately magnitude 6.8. It is estimated that similar flares occur around five times every year but are of such short duration, just a few minutes, that they have never been observed before.

Physical properties

Proxima Centauri is a red dwarf, because it belongs to the main sequence on the Hertzsprung–Russell diagram and is of spectral class M5.5. M5.5 means that it falls in the low-mass end of M-type stars. Its absolute visual magnitude, or its visual magnitude as viewed from a distance of 10 parsecs (33 ly), is 15.5. Its total luminosity over all wavelengths is 0.17% that of the Sun, although when observed in the wavelengths of visible light the eye is most sensitive to, it is only 0.0056% as luminous as the Sun. More than 85% of its radiated power is at infrared wavelengths. It has a regular activity cycle of starspots.

This illustration shows the comparative sizes of (from left to right) the Sun, α Centauri A, α Centauri B, and Proxima Centauri.

 

The two bright points are the Alpha Centauri system (left) and Beta Centauri (right). The faint red star in the centre of the red circle is Proxima Centauri.

In 2002, optical interferometry with the Very Large Telescope (VLTI) found that the angular diameter of Proxima Centauri was 1.02±0.08 mas. Because its distance is known, the actual diameter of Proxima Centauri can be calculated to be about 1/7 that of the Sun, or 1.5 times that of Jupiter. The star's mass, estimated from stellar theory, is 12.2% M_☉, or 129 Jupiter masses (M_J). The mass has been calculated directly, although with less precision, from observations of microlensing events to be 0.150+0.062
−0.051 M_☉.

The mean density of main-sequence stars increase with decreasing mass, and Proxima Centauri is no exception: it has a mean density of 47.1×10³ kg/m³ (47.1 g/cm³), compared with the Sun's mean density of 1.411×10³ kg/m³ (1.411 g/cm³).

A 1998 study of photometric variations indicates that Proxima Centauri rotates once every 83.5 days. A subsequent time series analysis of chromospheric indicators in 2002 suggests a longer rotation period of 116.6±0.7 days. This was subsequently ruled out in favor of a rotation period of 82.6±0.1 days.

Because of its low mass, the interior of the star is completely convective, causing energy to be transferred to the exterior by the physical movement of plasma rather than through radiative processes. This convection means that the helium ash left over from the thermonuclear fusion of hydrogen does not accumulate at the core, but is instead circulated throughout the star. Unlike the Sun, which will only burn through about 10% of its total hydrogen supply before leaving the main sequence, Proxima Centauri will consume nearly all of its fuel before the fusion of hydrogen comes to an end.

Convection is associated with the generation and persistence of a magnetic field. The magnetic energy from this field is released at the surface through stellar flares that briefly increase the overall luminosity of the star. These flares can grow as large as the star and reach temperatures measured as high as 27 million K—hot enough to radiate X-rays. Proxima Centauri's quiescent X-ray luminosity, approximately (4–16) × 10²⁶ erg/s ((4–16) × 10¹⁹ W), is roughly equal to that of the much larger Sun. The peak X-ray luminosity of the largest flares can reach 10²⁸ erg/s (10²¹ W).

Proxima Centauri's chromosphere is active, and its spectrum displays a strong emission line of singly ionized magnesium at a wavelength of 280 nm. About 88% of the surface of Proxima Centauri may be active, a percentage that is much higher than that of the Sun even at the peak of the solar cycle. Even during quiescent periods with few or no flares, this activity increases the corona temperature of Proxima Centauri to 3.5 million K, compared to the 2 million K of the Sun's corona. Proxima Centauri's overall activity level is considered low compared to other red dwarfs, which is consistent with the star's estimated age of 4.85 × 10⁹ years, since the activity level of a red dwarf is expected to steadily wane over billions of years as its stellar rotation rate decreases. The activity level also appears to vary with a period of roughly 442 days, which is shorter than the solar cycle of 11 years.

Proxima Centauri has a relatively weak stellar wind, no more than 20% of the mass loss rate of the solar wind. Because the star is much smaller than the Sun, the mass loss per unit surface area from Proxima Centauri may be eight times that from the solar surface.

A red dwarf with the mass of Proxima Centauri will remain on the main sequence for about four trillion years. As the proportion of helium increases because of hydrogen fusion, the star will become smaller and hotter, gradually transforming from red to blue. Near the end of this period it will become significantly more luminous, reaching 2.5% of the Sun's luminosity (L_☉) and warming up any orbiting bodies for a period of several billion years. When the hydrogen fuel is exhausted, Proxima Centauri will then evolve into a white dwarf (without passing through the red giant phase) and steadily lose any remaining heat energy.

Distance and motion

Based on a parallax of 768.13±1.04 mas, published in 2014 by the Research Consortium On Nearby Stars, Proxima Centauri is about 4.246 light-years (1.302 pc; 268,500 AU) from the Sun. Previously published parallaxes include 772.33±2.42 mas in the Hipparcos Catalogue (1997) and 768.77±0.37 mas using the Hubble Space Telescope's Fine Guidance Sensors (1999). From Earth's vantage point, Proxima is separated from Alpha Centauri by 2.18 degrees, or four times the angular diameter of the full Moon. Proxima also has a relatively large proper motion—moving 3.85 arcseconds per year across the sky. It has a radial velocity toward the Sun of 22.2 km/s.

Distances of the nearest stars from 20,000 years ago through 80,000 years in the future. Proxima Centauri is in yellow.

Among the known stars, Proxima Centauri has been the closest star to the Sun for about 32,000 years and will be so for about another 25,000 years, after which Alpha Centauri A and Alpha Centauri B will alternate approximately every 79.91 years as the closest star to the Sun. In 2001, J. García-Sánchez et al. predicted that Proxima will make its closest approach to the Sun in approximately 26,700 years, coming within 3.11 ly (0.95 pc). A 2010 study by V. V. Bobylev predicted a closest approach distance of 2.90 ly (0.89 pc) in about 27,400 years, followed by a 2014 study by C. A. L. Bailer-Jones predicting a perihelion approach of 3.07 ly (0.94 pc) in roughly 26,710 years. Proxima Centauri is orbiting through the Milky Way at a distance from the Galactic Centre that varies from 27 to 31 kly (8.3 to 9.5 kpc), with an orbital eccentricity of 0.07.

Orbital plot of Proxima Centauri as presently seen from Earth.

 

Ever since the discovery of Proxima, it has been suspected to be a true companion of the Alpha Centauri binary star system. Data from the Hipparcos satellite, combined with ground-based observations, were consistent with the hypothesis that the three stars are a bound system. For this reason, Proxima is sometimes referred to as Alpha Centauri C. Kervella et al. (2017) used high-precision radial velocity measurements to determine with a high degree of confidence that Proxima and Alpha Centauri are gravitationally bound. Proxima's orbital period around the Alpha Centauri AB barycenter is 547000+6600
−4000 years with an eccentricity of 0.5±0.08; it approaches Alpha Centauri to 4300+1100
−900 AU at periastron and retreats to 13000+300
−100 AU at apastron. At present, Proxima is 12,947 ± 260 AU (1.94 ± 0.04 trillion km) from the Alpha Centauri AB barycenter, nearly to the farthest point in its orbit.

Such a triple system can form naturally through a low-mass star being dynamically captured by a more massive binary of 1.5–2 M_☉ within their embedded star cluster before the cluster disperses. However, more accurate measurements of the radial velocity are needed to confirm this hypothesis. If Proxima was bound to the Alpha Centauri system during its formation, the stars are likely to share the same elemental composition. The gravitational influence of Proxima might also have stirred up the Alpha Centauri protoplanetary disks. This would have increased the delivery of volatiles such as water to the dry inner regions, so possibly enriching any terrestrial planets in the system with this material. Alternatively, Proxima may have been captured at a later date during an encounter, resulting in a highly eccentric orbit that was then stabilized by the galactic tide and additional stellar encounters. Such a scenario may mean that Proxima's planetary companion has had a much lower chance for orbital disruption by Alpha Centauri.

Six single stars, two binary star systems, and a triple star share a common motion through space with Proxima Centauri and the Alpha Centauri system. The space velocities of these stars are all within 10 km/s of Alpha Centauri's peculiar motion. Thus, they may form a moving group of stars, which would indicate a common point of origin, such as in a star cluster.

Though Proxima Centauri is the nearest bona fide star, it is still possible that one or more as-yet undetected sub-stellar brown dwarfs may lie closer.

Planetary system

The Proxima Centauri planetary system

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	≥1.27+0.19 −0.17 M⊕	0.0485+0.0041 −0.0051	11.186	<0 .35="" span="">	—	0.8–1.5 R⊕

The first indications of the exoplanet were found in 2013 by Mikko Tuomi of the University of Hertfordshire from archival observation data. To confirm the possible discovery, the European Southern Observatory launched the Pale Red Dot project in January 2016. On August 24, 2016, the team of 31 scientists from all around the world, led by Guillem Anglada-Escudé of Queen Mary University of London, confirmed the existence of Proxima Centauri b through a peer-reviewed article published by Nature. The measurements were performed using two spectrographs: HARPS on the ESO 3.6 m Telescope at La Silla Observatory and UVES on the 8 m Very Large Telescope at Paranal Observatory. Several attempts to detect a transit of this planet across the face of Proxima Centauri have been made. A transit-like signal appearing on September 8, 2016 was tentatively identified, using the Bright Star Survey Telescope at the Zhongshan Station in Antarctica.

Proxima Centauri b, or Alpha Centauri Cb, is a planet orbiting the star at a distance of roughly 0.05 AU (7.5 million km) with an orbital period of approximately 11.2 Earth days. Its estimated mass is at least 1.3 times that of the Earth. Moreover, the equilibrium temperature of Proxima b is estimated to be within the range where water could exist as liquid on its surface; thus, placing it within the habitable zone of Proxima Centauri.

A second signal in the range of 60 to 500 days was also detected, but its nature is still unclear due to stellar activity.

Prior to this discovery, multiple measurements of the star's radial velocity constrained the maximum mass that a detectable companion to Proxima Centauri could possess. The activity level of the star adds noise to the radial velocity measurements, complicating detection of a companion using this method. In 1998, an examination of Proxima Centauri using the Faint Object Spectrograph on board the Hubble Space Telescope appeared to show evidence of a companion orbiting at a distance of about 0.5 AU. A subsequent search using the Wide Field Planetary Camera 2 failed to locate any companions. Astrometric measurements at the Cerro Tololo Inter-American Observatory appear to rule out a Jovian companion with an orbital period of 2−12 years.

Proxima Centauri, along with Alpha Centauri A and B, was among the "Tier 1" target stars for NASA's now-canceled Space Interferometry Mission (SIM), which would theoretically have been able to detect planets as small as three Earth masses (M_⊕) within two AU of a "Tier 1" target star.

In 2017, a team of astronomers using the Atacama Large Millimeter/submillimeter Array reported detecting a belt of dust orbiting Proxima Centauri at a range of 1−4 AU from the star. This dust has a temperature of around 40 K and has a total estimated mass of 1% of the planet Earth. They also tentatively detected two additional features: a cold belt with a temperature of 10 K orbiting around 30 AU and a compact emission source about 1.2 arcseconds from the star. However, upon further analysis, these emissions were determined to be the result of a large flare emitted by the star in March, 2017. The presence of dust is not needed to model the observations.

Habitability

Pale Red Dot is an international search for an Earth-like exoplanet around the closest star Proxima Centauri.

 

Prior to the discovery of Proxima Centauri b, the TV documentary Alien Worlds hypothesized that a life-sustaining planet could exist in orbit around Proxima Centauri or other red dwarfs. Such a planet would lie within the habitable zone of Proxima Centauri, about 0.023–0.054 AU (3.4–8.1 million km) from the star, and would have an orbital period of 3.6–14 days. A planet orbiting within this zone may experience tidal locking to the star. If the orbital eccentricity of this hypothetical planet is low, Proxima Centauri would move little in the planet's sky, and most of the surface would experience either day or night perpetually. The presence of an atmosphere could serve to redistribute the energy from the star-lit side to the far side of the planet.

Proxima Centauri's flare outbursts could erode the atmosphere of any planet in its habitable zone, but the documentary's scientists thought that this obstacle could be overcome. Gibor Basri of the University of California, Berkeley, mentioned that "no one [has] found any showstoppers to habitability". For example, one concern was that the torrents of charged particles from the star's flares could strip the atmosphere off any nearby planet. If the planet had a strong magnetic field, the field would deflect the particles from the atmosphere; even the slow rotation of a tidally locked planet that spins once for every time it orbits its star would be enough to generate a magnetic field, as long as part of the planet's interior remained molten.

Other scientists, especially proponents of the rare-Earth hypothesis, disagree that red dwarfs can sustain life. Any exoplanet in this star's habitable zone would likely be tidally locked, resulting in a relatively weak planetary magnetic moment, leading to strong atmospheric erosion by coronal mass ejections from Proxima Centauri.

Future exploration

The Sun as seen from the Alpha Centauri system, using Celestia.

Because of the star's proximity to Earth, Proxima Centauri has been proposed as a flyby destination for interstellar travel. Proxima currently moves toward Earth at a rate of 22.2 km/s. After 26,700 years, when it will come within 3.11 light-years, it will begin to move farther away.

If non-nuclear, conventional propulsion technologies are used, the flight of a spacecraft to a planet orbiting Proxima Centauri would probably require thousands of years. For example, Voyager 1, which is now travelling 17 km/s (38,000 mph) relative to the Sun, would reach Proxima in 73,775 years, were the spacecraft travelling in the direction of that star. A slow-moving probe would have only several tens of thousands of years to catch Proxima Centauri near its closest approach, and could end up watching it recede into the distance.

Nuclear pulse propulsion might enable such interstellar travel with a trip timescale of a century, inspiring several studies such as Project Orion, Project Daedalus, and Project Longshot.

Project Breakthrough Starshot aims to reach the Alpha Centauri system within the first half of the 21st century, with microprobes travelling at twenty percent of the speed of light propelled by around 100 gigawatts of Earth-based lasers. The probes will perform a fly-by of Proxima Centauri to take photos and collect data of its planet's atmospheric composition. It will take 4.22 years for the information collected to be sent back to Earth.

From Proxima Centauri, the Sun would appear as a bright 0.4-magnitude star in the constellation Cassiopeia, similar to that of Achernar from Earth.