All about Eta

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In 1751, the abbe Nicholas-Louis de Lacaille set to work at the Cape of Good Hope cataloguing the stars of the Southern Hemisphere.

Apart from Edmund Halley, who made a cloud-bedevilled expedition to St Helena in 1677, Lacaille was the first European astronomer to observe our skies. Not only did he observe “new” stars those below Europe’s southern horizon but he also defined 14 new constellations and subdivided Ptolemy’s unwieldy Argo Navis into four manageable parts: Carina, Puppis, Vela and Malus the keel, poop, sail, and mast.

Amongst the 10,000 stars Lacaille accurately posi­tioned was Eta Carinae in the great Carina nebula. This bright nebula is the largest naked eye/small telescope object of its type, and lies in one of the richest parts of the Milky Way. Here we are looking along the length of the Sagittarius arm of the galaxy a great sweep of gas, dust and star formation which spirals away from our Sun towards the centre of the galaxy. Apart from the splendour of this area, Lacaille noted nothing remarkable in Carina.

Eighty years later, Sir John Herschel set up an observatory at Feldhausen, at the foot of Table Mountain. Using what had been his father’s favourite instrument, the 18.5 in x 20 ft Herschelian the finest telescope of its time he conducted a complete survey of the southern skies. Local astronomers had already observed that Eta Carinae varied in brightness, and by 1834 it had dimmed in eight years from equalling Beta Centauri (mag. = 0.6), to that of Aldebaran (mag. = 0.9). But during Sir John’s time at the Cape, Eta brightened rapidly to magnitude 0.4, and he made a famous drawing of Eta and the associated Keyhole nebula.

By 1843, Eta had, after another short decline, brightened so much as to outshine Canopus, and was second only to Sirius in brilliance in the sky. Since it  lies at a distance of about 7500 light years from Earth, this means that Eta was about 5,000,000 times brighter than the sun, and at this time it ejected material equal in mass to several suns.

In spite of this unprecedentedly large loss of material, the star appeared to have been unaffected, and continued to shine as energetically as before. However, in late 1845 it started to dim, and 30 years later its magnitude was 6.8, below the limit of naked eye visibility. Thereafter it has behaved irregularly while generally dimming, until today its visual magnitude fluctuates around an unspectacular 8.

But in judging the brightness of Eta Carinae our eyes deceive us, for we are seeing only a very small portion of the total electro­magnetic radiation produced by this star. Our eyes are only sensitive to radiation with wavelengths from about 380 nm (violet) to 780 nm (deep red), but the range of radiation from Eta is from 0.01 nm (extreme X-rays), to 109nm (1 metre—radar/ radio wavelength).

When the brightness at all wavelengths is summed, called the bolometric magnitude, it appears that Eta has dimmed hardly at all, for what we now see is not the star itself but rather the

z hot but cooling envelope of material which it blew off during its explosive outburst in the mid-nineteenth century, and which is now radiating strongly in the infra-red. The “star” Eta Carinae is now the hidden core of a small, bright-emission nebula. The material of the nebula absorbs the star’s light and re-radiates it in the infra-red band. Indeed Eta is the brightest source of infra-red in the sky outside our solar system.

In 1944, the Argentinian astronomer Enrique Gaviola described the Eta nebula as “a shape resembling a homunculus with its head pointing north-west, legs opposite and arms folded over a fat body.” The name “homunculus” has stuck, but the appearance is an artefact of ground-based observation, for, as a recent striking photograph from the Hubble Space Telescope Wide Field Plan­etary Camera shows, the Homunculus is actually two great expanding balls of high-temperature gas with a disc of material forming a collar about the neck joining them. In addition, around and beyond the limits of the bright nebula are the fading remnants of earlier mass ejections. going on?” The star(s) Eta Carinae are hidden from us in the depths of the Homunculus, and the only direct emissions from it/them which we can register are those at the radio end of the spectrum, beyond even infrared.

Since the radiation from the Homunculus is powered entirely by Eta, lying within it, by monitoring and analysing these radiations in conjunction with the star’s radio emissions we can make some reason­ably informed guesses as to what is happening inside the nebula. One of the most powerful tools has been spectroscopy, the analysis of radiant energy wavelength by wavelength. From this we can determine what elements and compounds form the Homunculus, whether they are ionised, their temperature and their velocity towards or away from us, and much more.

Latterly there have been three schools of thought about the nature of the heart of the Homunculus. Eta could be a single supermassive star, 100 and 160 times the mass of the Sun, making it perhaps the most massive star known. Such stars are the final stages in the evolution of massive stars, which end their short (as little as 10,000-year) highly energetic lives with a series of massive eruptions, revealing a naked core of burning helium as they evolve towards core collapse and a final supernova explosion.

Others consider Eta could be a close cluster of about six massive stars, each of 20 to 40 solar masses.

The third school sees Eta as a binary system of two 70-solar-mass stars in which the stars have highly elliptical orbits, but at closest approach are only twice the distance of Mars from the Sun apart. As they approach and swing past one another, their high-velocity solar winds collide to form a hot shock-front which produces an outburst of X-rays, one of the features of the observed spectrum.

Evidence for a binary system was presented by Augusto Damineli, who discovered a five-year periodicity in Eta’s spectral changes. Although there are variable stars, such as the Miras, which pulse with periods of 300 to 600 days, there is no known class of variable stars with a period anywhere near as long as 2000 days. However, periods of this order are easily explained if Eta Carinae is a binary system. Yet even this explanation requires that the orbits of the two stars be aligned in a somewhat unlikely manner.

Observations made at the end of 1997 and during the first quarter of this year may go far to resolving which of the models is the most likely . Thanks to Damineli’s work, observers have for the first time been able to start their observations before the sudden and now known to be periodic changes in Eta’s spectra, so we may soon be able to solve the enigma within the Homunculus.

And it may well be in the nick of time, for the lives of super-massive stars are short, and their demise as sudden as it is violent.