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The enigma of the star Eta Carinae

Brazilian astronomer helps decipher the behavior of a supergiant star, which sometimes stops shining

An enigmatic star, Eta Carinae, seems to unveil part of its secrets from time to time. In 2003, it is expected to show a dramatic reduction in its brightness, in some bands of light energy, according to calculations by the astronomer Augusto Damineli, researcher at the Astronomical and Geophysical Institute (IAG) of the University of São Paulo (USP). Damineli, who foretold a similar phenomenon, which did indeed take place in 1980, reached the conclusion that, over a period he estimates at five and a half years, the brightness of Eta Carinae may fall by the equivalent of 60 suns in a single day. This is a stunning reduction in energy, even for the biggest star in the Milky Way, classified as a luminous blue variable supergiant, whose brightness is 5 million times greater than that of our sun. The interesting thing is that, without using special observation techniques, the brightness fall off will be imperceptible, because it occurs only in certain selected light bands, and almost nothing of it appears in visible light.

Eta Carinae is in the constellation Carina, in the southern sky, to the right of the Southern Cross. In the fall and winter, it can be seen at nightfall, but only through binoculars. It is 7,500 light-years form the earth, the equivalent of 68 quadrillion kilometers, but its brightness is so intense that, if were in the place of Sirius, the brightest star in the shy, it would shine like the full moon.

Eta Carinae became famous in the 19th century for increasing its brightness to the point of rivaling Sirius, the Dog Star. For around a year in 1843, it was visible during the daytime. Then, it covered itself in a cocoon of dust and is now invisible to the naked eye. It was thought that it had exploded to form a supernova. But it had not. Eta Carinae, curiously is still alive. For a 160 years, nobody understood why. Damineli, in discovering the dimming of its brightness, opened up the prospect of finding out.

The behavior of this star has intrigued astronomers at least since July 1827, when the English astronomer William John Burchell, passing through the city of São Paulo, discovered the star’s variability. In a letter to his English colleague John Charles Duncan, Burchell commented that it was strange that Eta Carinae should be much brighter than given in the catalogs and that is should vary from the fourth to the first magnitude, and that, at that time, it was as brilliant as the alpha star of the Southern Cross. Burchell’s observation encouraged John Herschell, at the Observatory of South Africa, to watch the star regularly for decades and to record the giant eruption of 1843.

The first difficulty in understanding Eta Carinae has to do with its mass, which is apparently greater than 120 solar masses, according to Damineli. This would place it outside the acceptable limits for stars. At least in principle, this measure reaches what astronomers call Eddington’s Limit, in tribute to one of the most highly regarded astrophysicists of all time, the Englishman Arthur Stanley Eddington (1882-1944). He foretold that, beyond a given mass, in principle around 100 solar masses, a star could not be sustained. It would be evaporated by its own light radiation, which would produce an outward pressure in this case greater than its gravitational force, an inward force that would keep the star solid.

Why does this not happen to Eta Carinae? We do not know. To make things more complicated, the star is enveloped in a nebula (a huge, dense cloud of gas and cosmic  dust) named as the Homunculus by the Argentine astronomer Henrique Gaviola, of the Observatory of Córdoba. The nebula, that expands at a speed of 2.5 million kilometers an hour (or 700 kilometers a second), has the shape of a small man, hence its name. There are two spherical shells moving in opposite directions produced by the explosion of 1843.

As they cannot observe the central star directly, astronomers studied the Homunculus and discovered a fantastic physical and chemical laboratory in space. The Homunculus forms dust specks ten times bigger than interstellar clouds. Its composition is also unusual, based on silicates, instead of carbon compounds, as it is more common. Analyzing the matter expelled by the star allowed astronomers to measure the incidence of the nuclear reactions going on inside it and provided evidence of the production of a large amount of nitrogen. This observation confirms the theoretical calculations that say that theses large-mass stars are the main source of nitrogen in our galaxy.

Damineli used an indirect method to predict not just the reduction in brightness but also and in particular to postulate that Eta Carinae is made up of a double system and is not just one star, as had previously been believed. The two stars would move in a highly elliptical orbit around a common center of gravity. Every five and a half years, the orbits of the stars move closer together, the so-called periastron, which would cause the falls in brightness.

In a series of projects carried out since 1990, with the support of FAPESP, following the example of the Computer Equipment for Infrared Astronomy Project, underway with funding of R$ 9,800, Damineli used a sort of fingerprint – the so-called spectral lines, of a chemical element produced in abundance in the stars, helium – and measured what physicists call the Doppler effect inside the cloud until reaching the conclusion about the double star. The Doppler effect, so-called in tribute to the Austrian astronomer Cristian Johann Doppler (1803-1853), takes account of the shortening or lengthening of the wave of a source that is approaching or moving away from an observer. The Doppler effect can be commonly seen with an ambulance siren, which sounds sharper when coming toward an observer and deeper when moving away. In the case of the siren, is it the length of the sound waves, rather than the light waves, that undergo distortion because of the movement of the source.

Strong winds
The double star theory proposed by Damineli assumes that the two stars have strong winds, the larger one with a speed of 2.5 million and the smaller one with winds at a speed of 5 million kilometers an hour (700 and 1,300 kilometers a second). Stellar wind is a rain formed by protons and electrons released by an ionized gas, formed by electrically charged particles. According to Damineli, Eta Carinae’s wind has energy a hundred million times greater than that of our sun.

The USP researcher calculates that the collision of these particles would generate temperatures of around 80 million degrees. More recent measurements, he says, suggest temperatures of 60 million degrees, which of consistent with the double star theory. At these temperatures X-rays are emitted that can be captures by orbital telescopes such as the Chandra, so-called in recognition of the America astronomer of Indian origin Subrahmanyan Chandrasekhar (1910-1995).

While he got the first prediction of the collapse of brightness right, proved in January 1980, and he is sure that his binary model will confirm the second prediction in July 2003, Damineli is unable to explain the giant explosion of 1843 which created the Homunculus. In his view, the binary model explains the periodic brightness reduction, through the deep dives and movements of one star in the wind of the other, but it is not enough, at least in principle to explain explosions of that magnitude. According to the researcher, the surprising thing is that the cataclysm of 1843 ejected the equivalent to the   mass of three suns – a great event but which apparently did not disturb the stellar core of Eta Carinae. “Although it was huge, the phenomenon only affected the outer part ”, he says.

Almost all the data which Damineli worked with was obtained using a telescope with a 1.6-meter-diameter mirror set up at the National Astrophysics Laboratory (LNA in the Portuguese acronym) at Brasópolis, in the south of Minas Gerais. The chamber coupled to this telescope allows observation in the infrared spectrum, a wavelength close to the visible light. This technique makes it possible to examine the star through the dust, although in a limited way.

In July 1992, when he observed the reduction in Eta Carinae’s brightness for the first time, Damineli asked for the help of Italian colleagues working with the orbital telescope IUE, operated by the European Space Agency (ESA), the European NASA, which can capture ultraviolet light, a shorter wavelength and thus more powerful than visible or infrared light. But his colleagues refused to point the telescope toward the star, because they had been studying it for 25 years and had never seen any reduction in its brightness. They suggested that Damineli check a possible malfunction of the chamber. “Six months later, when they decided to believe it, the brightness reduction was over”, he says.

Telescope in the jungle – Another European group that had been observing Eta Carinae for 20 years at the European Southern Observatory (ESO), in the Chilean Andes, did not believe in the possibility of the event  suggested by the Brazilian astronomer either. “They wondered how a jungle telescope could reveal what had not been seen in one of the best laboratories in the world”, recalls Damineli. “Well”, he says, “it was simply that the techniques they  were using were not up to it”.

The winds began blowing in the Brazilian astronomer’s favor in 1996, when a group from NASA, headed by Mike Corcoran, accepted Damineli’s suggestion and began pointing the RXTE X-ray telescope at the star practically every day. While he waited for the predicted date of brightness reduction, Corcoran discovered various new phenomena, until, two years later, astounded, he recorded the complete extinguishment of the star in X-rays. Corcoran was awarded a NASA prize for this work in 1999 and his data were consistent with the approach proposed by the Brazilian.

Modern detectors, although able to produce more accurate predictions, would not be enough to protect the Earth from any final explosion in Eta Carinae, which is predicted to happen in 400,000 years time. When this happens – if in fact it does happen – even the Earth, so far away, would be struck by a lethal wave of gamma radiation, the most powerful form of electromagnetic radiation. According to Damineli, all life on the side of the Earth exposed to this source would be wiped out and the destruction would extend through a radius of at least ten thousand light-years from the star. Astronomers are not yet certain that Eta Carinae will end its life in this way. Astronomers like the Swiss André Maeder, of the Geneva Observatory, suggest that Eta Carinae’s continuous loss of matter, including any eruptions like that of 1843, could make the star lose mass dramatically, to the point where it would end its days as a discreet star, with the mass of our sun. Then, it would just be one of many among the approximately 200 billion stars in our galaxy.

• Augusto Damineli is 53 years and has an unusual personal background. Until he was 13, he labored in the land on the outskirts of  Ibiporã, in the interior of the state of Paraná, where he was born. He moved to São Paulo and from 1968 to 1970, he was a worker in the construction and metalworking industries. He took a bachelor’s degree in Physics at the University of São Paulo (USP) in 1973 and did his master’s and doctorate at the USP’s Astronomical and Geophysical Institute (IAG), where he has taught since 1977.
Project: Computer Equipment for the Infrared Astronomy Project
Investment: R$ 9.856