Eta Carinae Worksheet

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Key Facts & Information

  • Eta Carinae, which is formally known as Eta Argus, is a stellar system located in the constellation of Carina around 7,500 light-years away from our Sun. The star system consists of at least 2 stars with a combined luminosity greater than 5 million times that of our Sun.
  • When Eta Carinae was a 4th-magnitude star in 1837, it became brighter than Rigel, marking the start of its so-called “Great Eruption”.
  • Eta Carinae became the 2nd brightest star in the night sky in March 1843.
  • It faded below naked-eye visibility after the year 1856.
  • Eta Carinae reached 6th magnitude in 1892 due to a smaller eruption and faded again.
  • In the year 2014, Eta Carinae became brighter with a magnitude of 4.5.

Formation

  • Both of the stars of Eta Carinae appear to be around 3 million years old.
  • It is suggested that the stars originated at the Trumpler 16 open cluster that also resides within the Carina Nebula.

Discovery

  • There is no concrete evidence, but researchers suggest that Eta Carinae was not known before the 17th century.
  • One of the earliest records of the star was made by English astronomer Edmond Halley in 1677 when he recorded the star simply as “Sequens” following or relative to another star within a new constellation, Robur Carolinum.
  • The star also had Bayer designations Eta Roboris Caroli, Eta Argus or Eta Navis.
  • It was known as Eta Carinae in 1879, when the stars of Argo Navis were evenly distributed and given the epithets of the daughter constellations in the Uranometria Argentina of Gould.
  • Eta Carinae is too far south, but it is mapped when the Southern Asterisms were created in the 17th century.
  • It is also part of the Sea and Mountain asterism, together with Carinae, Lambda Centauri, and Lambda Muscae.
  • In Chinese, Eta Carinae is known as “Heaven’s altar” and the Second Star of Sea and Mountain.

Great Eruption

  • Eta Carinae was noted as a 4th magnitude star until 1827 when William Burchell observed its unusual brightness reaching the 1st magnitude.
  • In the 1830s, John Herschel made a series of accurate measurements which show that Eta Carinae had a consistent magnitude of 1.4 until November 1837.
  • On the evening of December 16, 1837, Herschel found out that the star slightly outshined Rigel, and this marked the beginning of the 18-year period known as the Great Eruption.
  • Eta Carinae was as bright as Alpha Centauri in January 2, 1838, before fading slightly over the next three months.
  • In 1843, Herschel noted that the star was as bright as Canopus and has the same size and color as Arcturus.
  • In March 1843, Eta Carinae once again reached its peak and was as bright as Alpha Centauri and Canopus for five days then faded again.
  • In 1844, it remained as bright as Alpha Centauri then faded to as bright as Beta Centauri and brightened again at the end of the year with 0.2 magnitude.
  • Its -0.8 magnitude in 1843 was probably due to the two stars of the star system that were approaching each other.
  • Eta Carinae decreased in magnitude by around 0.1 per year from 1845 to 1856, but with possible rapid large fluctuations.
  • In 2010, Duane Hamacher and David Frew of the Macquarie University showed that this was Eta Carinae during the Great Eruption.
  • Its brightness decreased rapidly from 1857 until by 1886 it had faded below  naked-eye visibility.
  • This was calculated to be the result of the condensation of dust in the ejected material surrounding Eta Carinae rather than an intrinsic change in its luminosity.

Lesser Eruption

  • Eta Carinae once brightened again in 1887 and peaked at about 6.2 magnitude in 1892.
  • At the end of March 1895, it faded rapidly at magnitude 7.5.
  • It has been calculated that the star was suffering 4.3 magnitudes of visual extinctions as a result of the gas and dust ejected in the Great Eruption.
  • This seemed to be due to a smaller copy of the of the Great Eruption expelling much less material.

In The 20th Century

  • Between 1900 to 1940, Eta Carinae appeared to have settled at a constant magnitude of around 7.6.
  • In 1953, it was noted to have brightened again to 6.5 magnitude.
  • Since 1996, the variations in the brightness of Eta Carinae were first identified as having a 5.54-year period, leading to the idea of a binary system.
  • The binary theory was confirmed by the observations of radio, optical, and near-infrared radial velocity and line profile changes, which were referred as a spectroscopic event in the late 1997 and early 1998.
  • In 1998 to 1999, the star was once again visible to the naked eye.
  • In 2014, Eta Carinae reached an apparent magnitude of 4.5, thus making it clear that the 5-year period is not always followed.

Properties

  • The stars in the Eta Carinae system are engulfed by the Homunculus cloud, making it difficult to determine their physical properties.
  • Many consider the primary star to have at least 90 solar masses.
  • Other models suggest that the two stars have between 100–120 solar masses and 30–60 solar masses.
  • According to the star’s eruption and mass transfer, it is estimated that the combined mass of the stars was more than 250 solar masses before the Great Eruption.
  • Most estimates suggest that the primary star had around 200 solar masses, while the secondary star had around 90 solar masses.
  • Some studies suggest that at the peak of the Great Eruption, the expulsion material would have been around 1,400 solar radii.
  • The radius of the primary star is speculated to be around 240 or 60 solar radii, while the secondary is around 14.3 to 23.6 solar radii.
  • Eta Carinae is a strong X-ray and gamma-ray source.
  • Its energy emissions vary during tits orbital cycle.
  • The detected highest energy Gamma rays were above 100 MeV.
  • Radio emissions in the microwave band of Eta Carinae were also detected to vary in strength and distribution over a 5.5-year cycle.
  • Both the primary and secondary stars are emitting enormous amounts of energy.
  • It is 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.
  • The stars complete an orbit around each other once every 5.54 years or so.
  • When they became close to each other, their magnitude reached its peak.
  • The ultraviolet spectrum of the star system shows a lot of emission lines of ionized material.
  • The ionization levels and continuum hot central source require the presence of a temperature of at least 37,000 K.
  • Eta Carinae is the only star to have ultraviolet laser emissions.
  • Most of its electromagnetic radiation is absorbed by the surrounding dust.
  • Eta Carinae is the brightest source in the night sky at mid-infrared wavelengths.
  • The large mass of dust, Homunculus Nebula, has been estimated to have around 20 solar masses and a temperature of about 100 to 150 K.
  • The central region of Homunculus Nebula has an even smaller Little Homunculus from the Lesser eruption.

Location

  • Eta Carinae is located in the constellation of Carina and is engulfed by the Homunculus Nebula.
  • Carina was part of a larger constellation called Argo Navis, which represented the ship of Jason and the Argonauts.
  • The constellation of Carina is very near to the south celestial pole.
  • Due to this, it never sets for most of the observers in the southern hemisphere.
  • Homunculus Nebula is a small emission and reflection nebula mainly composed of the gas ejected during the Great Eruption and the dust that condensed from the debris.
  • It consists of two polar lobes aligned with the rotation axis of Eta Carinae and an equatorial skirt, around 18” long.
  • The lobes of the Homunculus Nebula are considered to be mostly formed due to the initial eruption, rather than being shaped by including previously ejected or interstellar material.
  • Homunculus Nebula has a unique feature, the ability to measure the spectrum of the central object at different latitudes by the reflected spectrum from the different portions of the lobes.
  • These show a polar wind in which the stellar wind is faster and stronger at high latitudes due to the rapid rotation that causes gravity brightening, the oblate spheroid shape of a star due to its fast rotation, towards the poles.
  • Eta Carinae is located within the Carina Nebula in the Carina-Sagittarius arm of the Milky Way.
  • Carina Nebula is visible to the naked eye and shows a complex mix of emission, reflection, and dark nebulosity.
  • The star is known to be at the same distance as the nebula, and its spectrum can be seen reflected off various star clouds in the nebula.
  • The appearance of Carina Nebula changes significantly due to the reduction in ionizing radiation from Eta Varina since the Great Eruption.
  • Before the Great Eruption, Eta Carinae contributed up to 20% of the total ionizing flux for the whole nebula.
  • Eta Carinae lies within the stars of the Trumpler 16.
  • It is an open star cluster in which all other stars are below naked-eye visibility, although WR 25 in an extremely massive luminous star.
  • Trumpler 16 and Trumpler 14 are the two dominant star clusters of the Carina OB1 association, an extended grouping of young luminous stars with a common motion through space.

Did You Know?

  • Eta Carinae is regarded as an impostor supernova, which means that the star actually exploded, but the star was not destroyed.
  • The supernova explosion resulted in the star being shrouded by a cloud of gas and dust.
  • Some speculate that Eta Carinae will collapse and form a black hole.
  • Eta Carinae is one of the most massive stars to be closely studied.
  • According to its estimated dimensions, Eta Carinae would reach the orbit of Jupiter or even surpass it.
  • There are only around 10 stars in the Milky Way that have a higher mass than Eta Carinae. The most massive is named R136a1.

Future

  • The uniqueness of Eta Carinae makes it difficult to identify its definite future.
  • Its future evolution is highly unknown, but the star will certainly continue to lose huge amounts of its mass.
  • The primary star will most likely explode as a supernova and some suggest that it will happen anytime from now to one million years.