Eta Carinae Facts

The Eta Carinae star system is located at around 7.500 light-years / 2.300 parsecs away from the Sun.

Eta Carinae, formally known as Eta Argus, is a stellar system consisting of at least 2 stars located in the constellation of Carina. This star system’s luminosity is more than 5 million times greater than our Sun.

Key Facts & Summary

  • Eta Carinae is the only known star that emits ultraviolet laser emissions.
  • The Eta Carinae star system is located at around 7.500 light-years / 2.300 parsecs away from the Sun.
  • The two main components of Eta Carinae, are named Eta Carinae A and Eta Carinae B.
  • This star system has an extensive history of massive eruptions which propelled it several times in the brightest star charts. It is one of the most unique stars ever studied.
  • The primary star, Eta Carinae A, is a peculiar star similar to a luminous blue variable which initially had 150-200 solar masses, but it has already lost more than 30 solar masses.
  • Eta Carinae A has more than 100 solar masses, its exact mass is difficult to estimate due to eruptions. The same goes for its radius which is estimated at around 240 solar radii.
  • Eta Carinae A is more than 5 million times brighter than our sun and has surface temperatures between 9.400 to 35.200 K.
  • The secondary star, Eta Carinae B, is even more obscure. Most observations point out that it has between 30 to 80 solar masses and a radius between 14.3 to 23.6 solar radii.
  • Eta Carinae B is speculated to be 1 million times brighter than our sun.
  • Both stars are enveloped by a cloud of gas and dust known as the Homunculus nebula.
  • The exact age of these stars is hard to determine, but most speculate that they are around 3 million years old. Thus they are much younger than our sun.
  • Due to eruptions, its visual magnitude has differed across the ages.

Most researchers point out that Eta Carinae was not known before the 17th century. One of the earliest records of Eta Carinae was made by Edmond Halley in 1677.

The star was later named as Eta Roboris, Eta Argus, or Eta Navis. It was known as Eta Carinae after 1879 when the Argo Navis constellation was evenly distributed. The star was noted as a 4th magnitude star until in 1827 when Burchell noted its unusual brightness reaching the 1st magnitude.

Herschel also noted that the star reached a magnitude of 1.4, and was surprised when it outshone Rigel. This event marked the beginning of the 18-year period known as the Great Eruption.

It was as bright as Alpha Centauri in January 1838, and then it started to fade for the following three months. Herschel noted that the star was as bright as Canopus, and was similar in color and size to Arcturus.

In March, it brightened again similarly to Alpha Centauri and Canopus. After five days it began to fade once more. Though most of 1844 the star remained as bright as Alpha Centauri and faded to a brightness similar to Beta Centauri.

In 1843 it reached an apparent magnitude of -0.8 and was likely due to the two stars of the Eta Carinae system, approaching each other. Its brightness decreased from 1845 to 1856 by 0.1 magnitude per year.

Since 1857 it faded below naked-eye visibility before the lesser eruption occurred in 1887. Eta Carinae peaked at a magnitude of 6.2 in 1892 and in 1895 faded rapidly at magnitude 7.5.

Since 1900, the star settled at a constant magnitude of 7.6, until 1940. At the beginning of 1953, the star brightened again to a magnitude of 6.5. Since 1996, the variations in brightness were first identified as having a 5.54-year period.

The binary star system was lately yet surely confirmed. In 1998-9 the star yet again reached naked-eye visibility. In 2014 the star reached an apparent magnitude of 4.5, thus it became clear that the 5 year period is not always followed.


Both stars appear to be at around 3 million years old. It is possible that the two stars formed roughly at the same time. Much of this star system remains unknown. Since Eta Carinae is located in the Carina Nebula, the stars may have originated from there. They are associated with the Trumpler 16 open cluster that resides within the Carina Nebula, so maybe the stars originated from there. 

Regardless, stars form from molecular clouds or nebulas of dust and gas. Gravity pulls the swirling gas and dust together, and as such the stars are born.

Distance, Size, and Mass

The Eta Carinae is located at around 7.500 light-years / 2.300 parsecs away from the Sun. It is currently a 4th magnitude star visible to the naked eye.

Since the stars in the Eta Carinae system are engulfed by the Homunculus cloud, it is difficult to determine their exact physical properties. 

Many consider the primary star as having at least 90 solar masses, other models suggest that the two stars have between 100 to 120 solar masses and 30 to 60 solar masses.

Based upon the star’s eruption and mass transfer, estimates suggest that the combined mass of the stars was more than 250 solar masses before the Great Eruption.

Most estimates point out that the primary star should have around 200 solar masses, while the secondary around 90 solar masses. Their overall sizes are not well defined. Some studies that at the peak of the Great Eruption, the expulsion of the material would have been around 1.400 solar radii.

Regardless, the radius of the primary star is speculated to be around 240 to 60 solar radii while the secondary between 14.3 to 23.6 solar radii.

Other Characteristics

The ultraviolet spectrum of the Eta Carinae star system shows many emission lines of ionized metal. The ionization levels and continuum hot central source, require the existence of a temperature of at least 37.000 K. Eta Carinae is the only known star to have ultraviolet laser emissions.

Most of the star’s electromagnetic radiation is absorbed by the surrounding dust. This star is the brightest source in the night sky at mid-infrared wavelengths.

The large mass of dust named the Homunculus has been estimated to have around 20 solar masses and temperatures of around 100-150 K. The central region of the Homunculus contains an even smaller Little Homunculus, from the lesser eruption.

Eta Carinae is a powerful X-ray and gamma-ray source. The energy emissions vary during its orbital cycle. The highest energy gamma-rays detected were above 100 MeV. There are also many radio emissions in the star’s microwave band that vary in strength and distribution over a 5.5-year cycle.

Both stars are emitting enormous amounts of energy. Beta Carinae A is 5 million times brighter than our sun, while B is 1 million times brighter.

The temperatures are greatly debated. Beta Carinae A has been estimated to have around 9.400 to 35.200 K – this is between 1.6 to 6 times hotter than our sun. Beta Carinae B has surface temperatures of around 37.200 K or 6.4 times hotter than our sun. Eta Carinae A is classified as a luminous blue variable LBV star. It is the most luminous star of this classification.

Eta Carinae B is thought to be a young O-type star while some consider it to be an evolved supergiant star. It’s quite possible that both stars rotate up to 90% of the critical velocity due to their strong stellar winds.

Stellar System

Both stars have a semi-major axis of around 15.4 AU and an orbit eccentricity of 0.9. They complete one orbit around each other once every 5.54 years or so. When they reach their closest point to one another, their magnitude reaches its peak. They have been shown to not always follow their estimated orbit duration.


Eta Carinae is located in the constellation of Carina. Eta Carinae is engulfed by the Homunculus Nebula. This nebula is embedded within the much larger Carina Nebula, a large star-forming  H II region.

Carina was part of a larger constellation called the Argo Navis – this constellation represented the ship of Jason and the Argonauts. The brightest star, Canopus, marked the ship’s keel. The constellation was later divided into three lesser constellations: Carina – the keel – Puppis – the stern – and Vela – the sails.

The constellation of Carina is in the southern sky, very near to the south celestial pole. Because of this, it never sets for most of the southern hemisphere observers. Due to the precession of Earth’s axis, by the year 4.700, the south celestial pole will be in the constellation of Carina.

The Future

Since Eta Carinae is quite a rare star, the prediction of its future is quite a difficult task. Its future evolution is highly uncertain, but the star will certainly continue to lose huge amounts of its mass. The primary star will most likely explode as a supernova at any given time. Some predict it will happen anytime from now to one million years.

Did you know?

  • Eta Carinae is regarded as an impostor supernova. This means that the star actually exploded but this event failed to destroy the star. This resulted in the star being shrouded by a cloud of gas and dust. Eta Carinae will surely go through another supernova event.
  • Some speculate that Eta Carinae’s ultimate fate is to collapse and form a black hole.
  • Eta Carinae is one of the most massive stars ever closely studied.
  • Based upon different estimates of its size, if placed in the solar system, Eta Carinae would reach the orbit of Jupiter or even surpass it.
  • Only around 10 stars in the entire galaxy have a higher mass than Eta Carinae. The most massive is named R136a1.


  1. Wikipedia
  2. Astronomynow
  3. Universetoday

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