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Eta Carinae

There is no doubt that the winter evening skies, from the southern hemisphere, are the most breath–taking views of the Milky Way. Our friends, north of the equator, have much to be jealous about. The view from Scorpius as one sweeps south, along the Milky Way, through Centaurus and onward to the Carina–Vela region has had many American amateurs booking their tickets for Australia. One spectacular region, lying to the west of the Southern Cross, is the Carinae Nebula Complex. This collection of bright and dark nebulae with a number of open star clusters is magnificent to view through binoculars. This was one of the reasons why the central region of this area was chosen for the cover of this publication this year. The other reason was the curious star Eta Carinae. These days it is relatively faint at 7th magnitude. In the 1830’s, while Sir John Herschel was undertaking a survey of the southern sky, it increased in magnitude to become one of the brightest stars in the heavens! At the time, Herschel, working from a temporary observatory in Cape Town, South Africa, was sketching the region in some detail. One of his drawings is reproduced (below right). From this work it is obvious why the prominent dark nebula was called the ‘Keyhole’; the bright star to the left is Eta Carinae. However, as the star started to fade so did the southern region of the Keyhole. Compare the drawing, with the more modern photograph (below left). It is obvious the bright ‘cloud’ defining the lower left rim of the ‘Keyhole’, was a reflection nebula. This confirmed that ‘Eta’ was indeed at a similar distance to the surrounding nebulae. Unfortunately, no photographs exist showing Herschel’s view because photography was still very much in its infancy stages in the 1830’s. It is interesting that one of the reasons given for the establishment of the 48 inch Great Melbourne Telescope was to compare drawings, made with a larger instrument, to those made by Herschel.

It is now known that the brightening of this star was due to Eta Carinae ejecting a significant fraction of its outer surface. The expanding shell cooled and blocked the light from the underlying star, contributing to its fading in the following years. During the last 160 years the shell has continued to expand, at hundreds of kilometres per second, and is now clearly visible in reasonable sized amateur telescopes. Through the ‘scope’, the star may just look ‘fuzzy’, as if one can not quite focus the instrument. The Argentinean astronomer, Ernest Gaviola in 1950, was the first to describe this nebula. He liken it to a ‘Homunculus’ which is a midget or dwarf (he saw a head, legs and even its folded arms). Surrounding the ‘Homunculus’ is an even fainter nebula that is probably the result of an earlier outburst, hundreds of years ago.

The variability in brightness of Eta Carinae was well known before Herschel’s time. It was first recorded by Halley, in 1677, as a 4th magnitude star. It brightened to 2nd magnitude, fading back to 4th, in 1730 and 1801. From about 1820, it steadily increased in brightness, going through a number of minor peaks and fades. Its maximum brilliance was reached in 1843 where it outshone nearby Canopus and was only beaten by Sirius. Sirius is nearby at 9 light years, Canopus is at 200 light years and Eta Carinae is 7000 light years away! There is no doubt that it is a massive star to achieve such a luminosity and be so distant. The shell of gas is probably only a small fraction of the star’s mass. The amount of gas now believed to be contained in the ‘Homunculus’ could make 10 suns like our own. Imagine the size of the underlying star! These massive, ‘mass loss’ objects are now named Wolf-Rayet stars (after the discoverers) and only about 200 are known in the galaxy, despite their brightness. These stars can be 40 times the mass of our own Sun. Not all of these stars show themselves by spectacular behaviour such as Eta Carinae. However, the Wolf-Rayets have a very distinctive temperature and spectra.


Modern Photograph of Eta Carinae	Hershel’s Drawing

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