Eta Carinae

Eta Carinae (η Carinae or η Car) is a superbright hypergiant star in the constellation Carina, about 7,500 to 8,000 light-years from the Sun.

Hubble Space Telescope image showing Eta Carinae and the bipolar Homunculus nebula which surrounds the star. The Homunculus was formed in an eruption of Eta Carinae, the light from which reached Earth in 1843. Eta Carinae itself appears as the white patch near the center of the image, where the two lobes of the Homunculus touch.

An image of the Carina Nebula taken by the European Southern Observatory's Very Large Telescope (Ultra high image available by clicking)

Eta Carinae has a superstar at its center, as seen in this image from Chandra. The new X-ray observation shows three distinct structures: an outer, horseshoe-shaped ring about 2 light years in diameter, a hot inner core about 3 light-months in diameter, and a hot central source less than 1 light-month in diameter which may contain the superstar that is responsible for the Homunculus nebula. The outer ring provides evidence of another large explosion that occurred over 1,000 years ago. Credit: Chandra Science Center and NASA

The system contains at least two stars, of which the primary is a luminous blue variable (LBV) that initially had around 150 solar masses, of which it has lost at least 30. A hot supergiant of about 30 solar masses is in orbit around the primary. An enormous thick red nebula surrounding Eta Carinae makes it impossible to see this companion optically, but the dumbell-shaped gas cloud is formed by two stars.

The Eta Carinae system is inside the Homunculus nebula, itself part of the much larger Carina nebula. Together they have a combined luminosity of over five million times the Sun's. It is not visible north of latitude 30°N and is seen moving round the pole south of latitude 30°S. Due to its mass and the stage of its life, it is expected to explode as a supernova in the future.

History

Eta Carinae was first recorded as a 4th magnitude star. It brightened from 1837 to 1856 in an event known as "the Great Eruption". Eta Carinae became the second brightest star in the sky between 11 and 14 March 1843 before fading well below naked eye visibility.

It has brightened since about 1940, peaking above magnitude 4.5 in 2014. Eta Carinae is circumpolar south of latitude 30°S, so it is never seen north of latitude 30°N.

System and properties

This stellar system is currently one of the most massive that can be studied in detail. Until recently, Eta Carinae was thought to be the most massive single star, but in 2005 it was proved to be a binary system. The most massive star in the Eta Carinae multiple star system probably has more than 100 times the mass of the Sun. There are other massive stars which are known to be more luminous and are more massive.

Stars like Eta Carinae produce over a million times as much light as the Sun. They are quite rare — only a few dozen are present in a galaxy the size of the Milky Way. They are assumed to be near or over the Eddington limit. That means the outward pressure of their radiation is almost strong enough to counteract gravity. Stars over 120 solar masses exceed the Eddington limit, and their gravity is barely strong enough to hold in their radiation and gas.

Eta Carinae's chief significance for astrophysics is its giant eruption, which was observed around 1843. In a few years, Eta Carinae produced almost as much visible light as a supernova explosion, but it survived. It was a 'supernova impostor' or 'failed supernova'. Eta Carinae's giant eruption was the prototype for this phenomenon.

One remarkable aspect of Eta Carinae is its changing brightness. It is currently classified as a luminous blue variable (LBV) binary star due to peculiarities in its pattern of brightening and dimming.

The ionizing radiation emitted by the secondary star in Eta Carinae is the major radiation source of the system. Much of this radiation is absorbed by the primary stellar wind.

Related pages

• S Doradus