Alpha Centauri

Alpha Centauri is the brightest object in the constellation of Centaurus (top left).
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 versus time, the radial separation of A and B along the line of sight had reached a maximum in 2007, with B being further from Earth than A. The orbit is divided here into 80 points: each step refers to a timestep of approx. 0.99888 years or 364.84 days.
The relative sizes and colours of stars in the Alpha Centauri system, compared to the Sun
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.
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 the Danish 1.54 m telescope at La Silla Observatory in Chile.
Alpha Centauri AB taken in daylight by holding a Canon Powershot S100 in line with the eyepiece of a 110-mm refractor. The photo is one of the best frames of a video. The double star is clearly visible.
View of Alpha Centauri from the Digitized Sky Survey-2
Alpha Centauri A is of the same stellar type G2 as the Sun, while Alpha Centauri B is a K1-type star.
Closest stars to the Sun
Distances of the nearest stars from 20,000 years ago until 80,000 years in the future
Animation showing motion of Alpha Centauri through the sky. (The other stars are held fixed for didactic reasons) "Oggi" means today. "Anni" means years.
The discovery image of Alpha Centauri's candidate Neptunian planet, marked here as "C1".
Looking towards the sky around Orion from Alpha Centauri with Sirius near Betelgeuse, Procyon in Gemini, and the Sun in Cassiopeia generated by Celestia.
Simulated night-sky image with a "W" of stars from Cassiopeia connected by lines, and the Sun, labeled "Sol", as it would appear to the left of the "W"
The Very Large Telescope and Alpha Centauri

For other uses, see Alpha Centauri (disambiguation).

- Alpha Centauri
Alpha Centauri is the brightest object in the constellation of Centaurus (top left).

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Alpha

Alpha Centauri AB is the bright star to the left, which forms a triple star system with Proxima Centauri, circled in red. The bright star system to the right is Beta Centauri.

Proxima Centauri

Small, low-mass star located 4.2465 ly away from the Sun in the southern constellation of Centaurus.

Small, low-mass star located 4.2465 ly away from the Sun in the southern constellation of Centaurus.

Alpha Centauri AB is the bright star to the left, which forms a triple star system with Proxima Centauri, circled in red. The bright star system to the right is Beta Centauri.
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Proxima Centauri is a member of the Alpha Centauri star system, being identified as component Alpha Centauri C, and is 2.18° to the southwest of the Alpha Centauri AB pair.

Hubble Space Telescope image of Sirius A and Sirius B. The white dwarf can be seen to the lower left. The diffraction spikes and concentric rings are instrumental effects.

Sirius

Brightest star in the night sky.

Brightest star in the night sky.

Hubble Space Telescope image of Sirius A and Sirius B. The white dwarf can be seen to the lower left. The diffraction spikes and concentric rings are instrumental effects.
Sirius (bottom) and the constellation Orion (right). The three brightest stars in this image—Sirius, Betelgeuse (top right) and Procyon (top left)—form the Winter Triangle. The bright star at top center is Alhena, which forms a cross-shaped asterism with the Winter Triangle.
The orbit of Sirius B around A as seen from Earth (slanted ellipse). The wide horizontal ellipse shows the true shape of the orbit (with an arbitrary orientation) as it would appear if viewed straight on.
A Chandra X-ray Observatory image of the Sirius star system, where the spike-like pattern is due to the support structure for the transmission grating. The bright source is Sirius B. Credit: NASA/SAO/CXC
Comparison of Sirius A and the Sun, to scale and relative surface brightness
Size comparison of Sirius B and Earth
A bust of Sopdet, Egyptian goddess of Sirius and the fertility of the Nile, syncretized with Isis and Demeter
Sirius midnight culmination at New Year 2022 local solar time
Yoonir, symbol of the universe in Serer religion

This proximity is the main reason for its brightness, as with other near stars such as Alpha Centauri, Procyon and Vega and in contrast to distant, highly luminous supergiants such as Canopus, Rigel or Betelgeuse.(Note that Canopus may be a bright giant) It is still around 25 times more luminous than the Sun.

Binary system of two stars

Binary star

System of two stars that are gravitationally bound to and in orbit around each other.

System of two stars that are gravitationally bound to and in orbit around each other.

Binary system of two stars
Edge-on disc of gas and dust present around the binary star system HD 106906
Algol B orbits Algol A. This animation was assembled from 55 images of the CHARA interferometer in the near-infrared H-band, sorted according to orbital phase.
Artist's conception of a cataclysmic variable system
Artist's impression of the binary star system AR Scorpii
Artist rendering of plasma ejections from V Hydrae
Artist's impression of the sight from a (hypothetical) moon of planet HD 188753 Ab (upper left), which orbits a triple star system. The brightest companion is just below the horizon.
Schematic of a binary star system with one planet on an S-type orbit and one on a P-type orbit
The two visibly distinguishable components of Albireo
Luhman 16, the third closest star system, contains two brown dwarfs.
Planet Lost in the Glare of Binary Stars (illustration)

Orbital periods can be less than an hour (for AM CVn stars), or a few days (components of Beta Lyrae), but also hundreds of thousands of years (Proxima Centauri around Alpha Centauri AB).

Centaurus in the southwestern sky, shortly after sunset.

Centaurus

Bright constellation in the southern sky.

Bright constellation in the southern sky.

Centaurus in the southwestern sky, shortly after sunset.
The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri.
Centaurus in the Firmamentum Sobiescianum of Johannes Hevelius. This image is reversed from what one sees looking at the sky — it is as though one is looking at the "celestial sphere" from the outside.
Centaurus, 1602

Notable stars include Alpha Centauri, the nearest star system to the Solar System, its neighbour in the sky Beta Centauri, and V766 Centauri, one of the largest stars yet discovered.

61 Cygni showing proper motion (movement from our vantage point) at some early 21st century one-year intervals.

61 Cygni

Binary star system in the constellation Cygnus, consisting of a pair of K-type dwarf stars that orbit each other in a period of about 659 years.

Binary star system in the constellation Cygnus, consisting of a pair of K-type dwarf stars that orbit each other in a period of about 659 years.

61 Cygni showing proper motion (movement from our vantage point) at some early 21st century one-year intervals.
A size comparison between the Sun (left), 61 Cygni A (bottom) and 61 Cygni B (upper right).
The orbital motion of component B relative to component A as seen from Earth as well as the true appearance from face-on view. The time steps are approximately 10 years.

His measurement was published only shortly before similar parallax measurements of Vega by Friedrich Georg Wilhelm von Struve and Alpha Centauri by Thomas Henderson that same year.

Depiction of the Crux by João Faras in May 1500

Crux

Constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross.

Constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross.

Depiction of the Crux by João Faras in May 1500
Southern Cross from New Zealand
Deep exposure of Crux, Coalsack Nebula, and IC 2944
Locating the south celestial pole
The constellation Crux as it can be seen by the naked eye
Crux with clouds, from Cape Town
Crux, appearing on a number of flags and insignia

Projecting a line from γ to α Crucis (the foot of the crucifix) approximately 4 1⁄2 times beyond gives a point close to the Southern Celestial Pole which is also, coincidentally, where intersects a perpendicular line taken southwards from the east-west axis of Alpha Centauri to Beta Centauri, which are stars at an alike declination to Crux and of a similar width as the cross, but higher magnitude.

Just-saturated RGB-camera discs

Stellar classification

Classification of stars based on their spectral characteristics.

Classification of stars based on their spectral characteristics.

Just-saturated RGB-camera discs
Main-sequence stars arranged from O to M Harvard classes
The Hertzsprung–Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature.
Montage of false color spectra for main-sequence stars
Guide for Secchi spectral types ("152 Schjellerup" is Y Canum Venaticorum)
Proper motion of stars of early type in ± 200,000 years
The movement of stars of late type around the apex (left) and antapex (right) in ± 200,000 years
The spectrum of an O5V star
B-class stars in the Jewel Box cluster (Credit: ESO VLT)
Class A Vega (left) compared to the Sun (right)
Canopus, an F-type supergiant and the second-brightest star in the night sky
The Sun, a G2 main-sequence star, with dark sunspots
Arcturus, a K1.5 giant compared to the Sun and Antares
UGC 5797, an emission-line galaxy where massive bright blue stars are formed
Hubble Space Telescope image of the nebula M1-67 and the Wolf–Rayet star WR 124 in the center
Artist's impression of an L-dwarf
Artist's impression of a T-dwarf
Artist's impression of a Y-dwarf
Image of the carbon star R Sculptoris and its striking spiral structure
Sirius A and B (a white dwarf of type DA2) resolved by Hubble

There are also giant K-type stars, which range from hypergiants like RW Cephei, to giants and supergiants, such as Arcturus, whereas orange dwarfs, like Alpha Centauri B, are main-sequence stars.

Relation between proper motion and velocity components of an object. A year ago the object was d units of distance from the Sun, and its light moved in a year by angle μ radian/s. If there has been no distortion by gravitational lensing or otherwise then μ = is the distance (usually expressed as annual velocity) transverse (tangential or perpendicular) to line of sight from the Sun. The angle is shaded light blue from the sun to the object's start point and its year later position as if it had no radial velocity. In this diagram the radial velocity happens to be one of the sun and object parting, so is positive.

Proper motion

Astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more distant stars.

Astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more distant stars.

Relation between proper motion and velocity components of an object. A year ago the object was d units of distance from the Sun, and its light moved in a year by angle μ radian/s. If there has been no distortion by gravitational lensing or otherwise then μ = is the distance (usually expressed as annual velocity) transverse (tangential or perpendicular) to line of sight from the Sun. The angle is shaded light blue from the sun to the object's start point and its year later position as if it had no radial velocity. In this diagram the radial velocity happens to be one of the sun and object parting, so is positive.
The celestial north and south poles are above/below CNP, CSP; the origin of all 24 hours of Right Ascension (the measure of absolute celestial east–west position), the March equinox (center of the sun's position then) at the J2000 epoch, is vector V. In red the diagram adds the components of proper motion across the celestial sphere. An ideal time to measure exactly such a small annual shift is at culmination. The culmination of the star is daily reached when the observer (and earth) passes as shown by the blue arrows "beneath" the star. The positive axes of the two components of its usually annually measured or published shift in proper motion are the exaggerated red arrows, note: the right arrows point to the east horizon. One red annotation is subtly shorter as the cosine of a star resting at 0° declination is 1, so such a star's east or west shift would not need to be multiplied by the cosine of its declination. The proper motion vector is μ, α = right ascension, δ = declination, θ = position angle (simply the 90° compliment of declination).
Barnard's Star, showing position every 5 years 1985–2005.
Proper motion of 61 Cygni in one year intervals.

After the Sun and the Alpha Centauri system, it is the nearest known star.

A picture of stars, with a group of appearingly bright blue and white stars. The bright stars together are identified as the asterism Coathanger resembling a coathanger, in the constellation Vulpecula.

Asterism (astronomy)

Observed pattern or group of stars in the sky.

Observed pattern or group of stars in the sky.

A picture of stars, with a group of appearingly bright blue and white stars. The bright stars together are identified as the asterism Coathanger resembling a coathanger, in the constellation Vulpecula.
Some major asterisms on a celestial map (the projection exaggerates the stretching)
The Big Dipper asterism
The "Teapot" asterism in Sagittarius. The Milky Way appears as "steam" coming from the spout.
The "37" or "LE" of NGC 2169, in Orion. It is visible through a pair of binoculars.

The Southern Cross including the first-magnitude stars Acrux and Mimosa, west of the Carina Nebula (one of five first-magnitude deep-sky objects), and with the first-magnitude stars Alpha Centauri (the closest star to the Sun) and Beta Centauri pointing at the cross, distinguishing the cross from less bright and similar asterisms like the Diamond Cross or False Cross.

Artist's conception of Proxima Centauri b as a rocky-like exoplanet, with Proxima Centauri and the Alpha Centauri binary system in the background. The actual appearance of the planet is unknown.

Proxima Centauri b

Artist's conception of Proxima Centauri b as a rocky-like exoplanet, with Proxima Centauri and the Alpha Centauri binary system in the background. The actual appearance of the planet is unknown.
Velocity of Proxima Centauri towards and away from the Earth as measured with the HARPS spectrograph during the first three months of 2016. The red symbols with black error bars represent data points, and the blue curve is a fit of the data. The amplitude and period of the motion were used to estimate the planet's minimum mass.
Overview and comparison of the orbital distance of the habitable zone.
An angular size comparison of how Proxima will appear in the sky seen from Proxima b, compared with how the Sun appears in our sky on Earth. Proxima is much smaller than the Sun, but Proxima b is very close to its star.
Artist's conception of the surface of Proxima Centauri b. The Alpha Centauri AB binary system can be seen in the background, to the upper right of Proxima.

Proxima Centauri b (or Proxima b), sometimes referred to as Alpha Centauri Cb, is an exoplanet orbiting in the habitable zone of the red dwarf star Proxima Centauri, which is the closest star to the Sun and part of the triple star system Alpha Centauri.