Astronomers observe the inside of a star for the first time
Karina Toledo/Agência FAPESP/DICYT Through a small telescope installed at the top of a mountain covered by banana trees between the southern Minas Gerais towns of Brazópolis and Piranguçu, a group of scientists has been monitoring a first-of-its-kind astronomic phenomenon whose peak is expected to occur within days: the opening of a hole in the surface of a giant star known as Eta Carinae, allowing discovery of the secrets inside.
In the coming months, data from the observations will be compared with existing theoretical models and may validate or disprove the body of scientific knowledge about the giant stars, says Augusto Damineli, professor at the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) of the University of São Paulo (USP) and coordinator of the international group of 30 scientists and amateur astronomers, six of which are Brazilian.
“The findings will have an indirect influence on our knowledge of stars more than 10 solar masses because they have similar structures. The small ones, like our Sun, are all very different. But it may be that we’ll find something so momentous that it changes even theories about the structure of small stars,” Damineli told the Agência FAPESP. The researcher has devoted more than 20 years to studying the mysteries of the “beauty,” as he fondly refers to the star, with support from FAPESP.
Situated in the constellation Carina at a distance nearly 8,000 light-years from the Earth, which in astronomical parameters is considered “just next door,” Eta Carinae is a star that defies standards. With a size equivalent to 90 solar masses, its diameter can be compared to the distance the Earth travels around the Sun. Its luminosity is one of the highest known to man: nearly 5 million suns.
“Giant stars were common when the Universe was young and had abundant raw materials, but as they began to form, the gases were captured. This type of star has a short life of nearly 3 million years, while the Sun could reach 10 billion years and stars with one-tenth that solar mass could reach 1 trillion. Most of the giant stars exploded early in the beginning of the Universe. Eta Carinae is the dinosaur we have the luck to know in our backyard. By looking at her, we can study the past,” Damineli said.
Discovery of the binary system
While still early in his career at IAG-USP in the late 1980s, the astronomer became excited about some strange phenomena that had been described on Eta Carinae in 1948 and 1960. He then decided to spend time observing the star whenever he went to an observatory.
“My hypothesis was that the more power the star had, the greater the light emission in the ultraviolet spectrum would be. But it’s not possible to observe ultraviolet emissions from Earth because they are degraded in other wavelengths by the atmospheres of the star and the Earth. So, I began to focus on the helium channel,” Damineli explained.
The helium channel or spectral line is nothing more than ultraviolet light absorbed by helium ions found inside the star and re-emitted in a longer wavelength, as visible light, capable of crossing the stellar and terrestrial atmospheres and arriving in sufficient strength to be captured by the 1.6-meter-diameter Perkin-Elmer telescope at the Pico dos Dias Observatory, managed by the National Astrophysics Laboratory (LNA) in Minas Gerais.
“This is a star that is so breathtaking that it would be impossible to observe everything it emits because there is just not enough time. I focused on the helium channel because I knew that any large energy event would be captured by my simple artificial satellite. And that’s what happened in 1989, 1990 and 1991. But in June 1992, that channel began to go out by about 60 suns per night. It’s a severe variation of energy, even for a star the size of Eta Carinae. After a few months, it started shining again,” the researcher recalls.
In comparing his observation data to descriptions of the 1948 and 1960 phenomena, Damineli concluded that the stellar “blackout” that had occurred in some portions of the electromagnetic spectrum repeated every 5.5 years. In a 1996 article published in The Astrophysical Journal, he predicted that a new event would take place the following year.
Not a single foreign colleague was brave enough to write the article with Damineli. They were afraid that the data obtained on the “jungle telescope” of Minas Gerais were imprecise. However, he was correct. The star went out.
“Such a phenomenon could only be explained by the existence of two stars: one small and one big. They rotate and, from time to time, one ends up concealing the other, like an eclipse. I calculated 90 solar masses for the large one and 30 for the small one. That’s where I got the 5.5 years,” the researcher explained.
If, in fact, there are two stars, Damineli argued, their solar winds would collide when they approach the periastron, the closest point between their two orbits, and this would release energy at 10 million degrees Celsius and cause the emission of x-rays.
A group of researchers led by Mike Corcoran of the National Aeronautics and Space Administration (NASA) accepted Damineli’s suggestion and began to point the Rossi X-ray Timing Explorer (RXTE) telescope towards the star nearly every day until, during the blackout, the hypothesis was confirmed.
“Corcoran said: ‘You were wrong! It’s actually 100 million degrees Celsius’, and I responded: ‘Even better!’” Damineli said.
Corcoran won a NASA award for this work in 1999. His data gave consistency to the approach proposed by Damineli, and from that point on, interest from the international community for studies of the giant of the Milky Way, now divided into Eta Carinae A and Eta Carinae B, grew exponentially.
The next eclipse was monitored by an international team at observatories in several countries in the Southern Hemisphere, from where the phenomenon is more visible. The astronomers calculated that, according to the size of the stars, the blackout would last one month. But Eta Carinae surprised its admirers again and took six months to return to normal.
“We realized that the event that occurred every 5.5 years was more complicated than we had thought. In 2003 and 2009, we saw that it always began at the appointed time, so it was in fact an eclipse. But every time it would happen a little differently. There was something extra. My hypothesis was that the wind of one star got caught up with the wind of the other and there was some sort of collapse. Only when the two separated could everything get back to normal,” Damineli said.
The new theory was published in January 2012 in The Astrophysical Journal, with principle author Mairan Teodoro, then a FAPESP doctoral fellow who is now doing a post-doc at NASA.
“Then, young people from several countries came along, the kind of people who were born with a keyboard in their hands, interested in researching the hypothesis of the collapse. They’re experts at the computer and did new calculations and made important changes to the theory I had proposed,” Damineli explained.
The IAG researcher had noticed that, just before the blackout, it was possible to capture the emission of double ionized helium atoms (He++) – something expected only for stars that are much hotter than Eta Carinae.
“For 50 years, everyone’s been saying that Eta Carinae is a cold star, measuring at most 15,000 degrees Celsius, so it probably only had neutral helium atoms on its surface. But based on the data I observed, João Steiner [professor at IAG/USP] showed that one month before the blackout, there was a fulguration equivalent to 5,200 times the light of the sun in extreme ultraviolet that produces He++”, Damineli explained. The data were published in another article of The Astrophysical Journal.
Then, Thomas Madura, research fellow in the Astrophysics Science Division at NASA, in a new article published in 2013 in the Monthly Notices of the Royal Astronomical Society, proposed that the emission of He++ might be explained by the existence of a hole in the atmosphere of the larger star. The ions would come not from the surface, but from the subphotosphere of the star where the temperature is much higher.
“We thought that at any time the two stars would collide, with the small one becoming part of the larger one and allowing its entrails to be seen. For two or three weeks before the blackout, it’s possible to observe this hole, and that is exactly what we are doing now. But when they get to the periastron, the hole will go to the opposite side. It will stay open for some time, but we won’t be able to study it,” Damineli said.
The hole has been monitored from space since early July by NASA’s Hubble and Swift orbital telescopes as well as observatories in New Zealand, Australia, South Africa, Argentina, Chile and Brazil. The phenomenon is expected to be visible until August 1.
“At the Pico dos Dias Observatory, we’re using an even smaller telescope than the one used to discover the existence of the binary system. It is only 60 centimeters in diameter, is entirely manual and its design is from the 19th century. We traded coffee for it in East Germany,” Damineli said.
Its advantage, according to the researcher, is that the equipment – a Zeiss telescope with a 60-cm-diameter collector mirror – is less coveted, which allows prolonged periods of use.
“By using it, I’m able to capture the channels I need and I get the chance to observe Eta Carinae for 60 nights in a row, something impossible to do with a large telescope. On the Hubble, for example, I only get 1.5 hours a month. That’s not much for a phenomenon like this one,” he said.
One of the goals of the researchers is to discover the size and depth of the hole that EtaB is opening on the surface of EtaA and thus confirm the theories about the different ions found in each of the layers that make up the large stars.
“It is the first time we’re observing a star whose skin is cut, allowing us to see the emissions of the subphotosphere. Now we want to stick our heads in and discover what’s underneath,” Damineli said with enthusiasm.
A fading star
The techniques for indirectly observing the binary system, such as analyzing the spectral lines of helium, were necessary because the two main stars and their smaller sisters are covered by a dense cloud of gas and dust, Damineli explained.
Known as Homunculus, the nebula was recently tridimensionally mapped by a group of nine astrophysicists – three of whom are Brazilian. The results were published in early July in the journal Monthly Notices of the Royal Astronomical Society.
The principal author and developer of the SHAPE software utilized in modeling the Homunculus is Wolfgang Steffen, researcher at the Universidad Nacional Autônoma de México (UNAM).
According to the study, the Homunculus is shaped in the form of two lobes, like an hourglass, constituted by a thin crust of dust nearly 15 times the mass of the Sun and 3 trillion km long. The binary system of stars would be where these two lobes meet.
“It is believed that the nebula originated from a huge explosion on EtaA in 1843 and since then has been expanding. There are 15 solar masses traveling at a speed of 650 km per second. Using a simple formula, we’ve calculated that the energy needed to move such an amount of matter would be equivalent to a small supernova,” Damineli said.
Around the Homunculus are signs of an older outburst that would have taken place nearly 2,000 years ago. Indirect observations further suggest that a third outburst may have even occurred just 110 years ago – again contradicting the theories that a supernova represents the definitive end of a star.
“Apparently, Eta Carinae is showing us that stars can partially die. It’s something new that not all astronomers know,” Damineli said. He added, however, that a large amount of nitrogen released during the outburst 171 years ago would be an indication that the star is indeed dying.
“She has one foot in the grave with these three eruptions. We know that she is 2.5 million years old and that these types of stars don’t normally live 3 million years. But these convulsions may anticipate the death. It could happen at any time, but if we miscalculate some 250,000 years, it’s nothing to be ashamed of,” Damineli said.