AG Carinae (AG Car) is a

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Carina Carina may refer to: Places Australia * Carina, Queensland, a suburb in Brisbane * Carina Heights, Queensland, a suburb in Brisbane * Carina, Victoria, a locality in Mildura Serbia * Carina, Osečina, a village in the Kolubara District ...

. It is classified as a

luminous blue variable Luminous blue variables (LBVs) are rare, massive, evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of th ...

(LBV) and is one of the most luminous stars in the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

. The great distance (20,000

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s) and intervening

dust Dust is made of particle size, fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian processes, aeolian process), Types of volcan ...

mean that the star is not usually visible to the

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or microsc ...

; its

apparent brightness Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light cau ...

varies erratically between magnitude 5.7 and 9.0. In 1914, Harry Edwin Wood announced his discovery that this star, then called CPD−59°2860, is a

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

, based on

photographic plate Photographic plates preceded film as the primary medium for capturing images in photography. These plates, made of metal or glass and coated with a light-sensitive emulsion, were integral to early photographic processes such as heliography, d ...

s taken in 1911 and 1914. It was given its

variable star designation In astronomy, a variable-star designation is a unique identifier given to variable stars. It extends the Bayer designation format, with an identifying label (as described below) preceding the Latin genitive of the name of the constellation in whic ...

, AG Carinae, in 1921.

Description

The star is surrounded by a

nebula A nebula (; or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the Pillars of Creation in ...

of ejected material at 0.4–1.2 pc from the star. The nebula contains around , all lost from the star around 10,000 years ago. There is an 8.8-parsec-wide empty cavity in the interstellar medium around the star, presumably cleared by fast winds earlier in the star's life. AG Carinae is apparently in a transitional phase between a massive class O blue

supergiant Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram, with absolute visual magnitudes between about −3 and −8. The temperatures of supergiant stars range ...

and a

Wolf–Rayet star Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectroscopy, spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very ...

, where it is highly unstable and suffers from erratic pulsations, occasional larger outbursts, and rare massive eruptions. The spectral type varies between WN11 at visual minimum and an early A

hypergiant A hypergiant ( luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term ''hypergiant'' is defined as luminosity class 0 (zero) in the MK ...

at maximum. At visual minimum the star is about and 20,000–24,000 K, while at maximum it is and 8,000 K. The temperature varies at different minima. One study calculated that the bolometric luminosity of AG Carinae decreases during its

S Doradus S Doradus (also known as S Dor) is one of the brightest stars in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way, located roughly 160,000 light-years away. The star is a luminous blue variable, and one of the List ...

-type outbursts, unlike most LBVs which remain at approximately constant luminosity. The luminosity drops from around at visual minimum to around at visual maximum, possibly due to the energy required to expand a considerable fraction of the star. Evolutionary models of the star suggest that it had a low rotation rate for much of its life, but current observations show fairly rapid rotation. Models of LBV progenitors of type IIb supernovae list AG Carinae as matching the final stellar spectrum prior to core collapse, although the models are for stars with 20 to 25 times the mass of the Sun while AG Carinae is thought to be considerably more massive. The initial mass of the star would have been around and is now thought to be .

Distance controversy

Parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

es from data release 1 (DR1) of the

Gaia In Greek mythology, Gaia (; , a poetic form of ('), meaning 'land' or 'earth'),, , . also spelled Gaea (), is the personification of Earth. Gaia is the ancestral mother—sometimes parthenogenic—of all life. She is the mother of Uranus (S ...

mission suggest a much closer distance to AG Carinae and its neighbour Hen 3-519 than previously accepted, around 2,000

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

s. Then both stars would be less luminous than LBVs and it is argued that they would be former

red supergiant Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) and a stellar classification K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelg ...

s whose unusual characteristics are the result of binary evolution. The earlier Hipparcos parallax for AG Carinae had a margin of error larger than the parallax itself and so gave little information about its distance. The distance of 6,000 parsecs is based on assumptions about the properties of LBVs, models of interstellar extinction, and

kinematical In physics, kinematics studies the geometrical aspects of motion of physical objects independent of forces that set them in motion. Constrained motion such as linked machine parts are also described as kinematics. Kinematics is concerned with s ...

measurements. The Gaia DR1 parallax, derived from the combination of the first year of Gaia measurements with Tycho astrometry, is . The Gaia team recommend that a further 0.3 mas

systematic error Observational error (or measurement error) is the difference between a measurement, measured value of a physical quantity, quantity and its unknown true value.Dodge, Y. (2003) ''The Oxford Dictionary of Statistical Terms'', OUP. Such errors are ...

is allowed for (i.e. added to the formal margin of error). A 2017 study argues that the 0.3 mas systematic margin of error can be ignored and that the implied distance to AG Carinae is . In Gaia Data Release 2, the parallax is , suggesting a distance around . A 2019 observation yields a most likely distance of . Gaia Early Data Release 3 gives a parallax of , although with a non-trivial level of excess astrometric noise where there was none in Gaia DR2.

Light curve

Notes

References

External links

2MASS Atlas Image Gallery: Miscellaneous Objects
includes an

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

image of AG Carinae
Hubble Space Telescope zooms to AG Carinae
YouTube {{DEFAULTSORT:AG Carinae Carina (constellation) Luminous blue variables Carinae, AG 094910 053461 CD-59 03430 Wolf–Rayet stars A-type hypergiants