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The Sun and its sisters

Brazilians identify stars similar to the one that warms and lights up the Earth

NASAPast and future: other stars help in understanding the evolution of the SunNASA

Wanted: a new Sun – preferably, right here in the neighborhood; not that the old one is no longer useful. The Sun, which over the last 4.6 billion years has invariably risen every morning and set at the end of the day, has only reached middle age. It should live for another 5 billion years, lighting and warming the Earth and the nearby planets. It is true that not everything will be fine and dandy. In some 300 million to 1 billion years at most, the Sun will start shining more brightly, thereby increasing the temperature on Earth and making life unviable as it is known today. Until then, many will say, there is more than time enough. But there are those who are already starting to think about what to do. After all, according to the astronomers, in the very long-term, humanity will have to leave Earth if it wants to survive.

The nearly 50 international groups dedicated to scanning the skies after a star like ours, of course, do not expect to find a home for their great great great grandchildren’s great great great grandchildren. They are more interested in finding out whether the sun is really a star without equal among the hundreds of billions of stars in the cosmos, or whether, instead, it is an ordinary star, found in droves in this and other galaxies. This doubt comes accompanied by another: are we alone in the Universe or is there life in other worlds?

While they wait for this era of excessive light and heat, teams from Brazil and other countries are investing more than ever in the search for another Sun. Some not too distant candidates have emerged in recent years. Of the 10 whose age is close to that of the Sun, at least four have been identified by researchers working in Rio de Janeiro or São Paulo. Others are likely to appear as data from large stellar surveys and work submitted for publication become publicly available. At least one new star, similar to the Sun, is likely to be presented by astrophysicists José Renan de Medeiros and José Dias do Nascimento Júnior, both from the Federal University of Rio Grande do Norte (UFRN), at the annual meeting of the Brazilian Astronomical Society, at the beginning of September in Passa Quatro, Minas Gerais.

This star, whose name and location in the sky is still being kept secret, was identified among the almost 10,000 that the Corot space telescope, a Franco-Brazilian-European satellite launched in 2006, observed in its first years of operation. There must be at least 20 others like it close to the Sun, the researchers from UFRN calculate, based on data from Corot. These are in addition to the dozens of other similar stars like the Sun that have been mapped out in recent years by other telescopes.

Despite appearing to be a high number, it is not. Very few of these stars have all the same characteristics as the Sun and they are what French astrophysicist, Giusi Cayrel de Strobel called solar twins in the late 1980’s. Only 7% of the stars that are up to 33 light-years from our solar system are similar to the Sun, to the point of having the conditions necessary for the emergence of life on Earth as we know it, according to a survey published in 2006 in Astrobiology by astrophysicists, Gustavo Porto de Mello, from the Federal University of Rio de Janeiro (UFRJ), and Eduardo Del Peloso and Luan Ghez, from the National Observatory.

It was Mello, in fact, who years before had found the star that for a decade was considered the best candidate for the sun’s twin: 18 Scorpii. Described in 1997 by Mello and Lício da Silva, his PhD tutor, 18 Scorpii is a subtle shining star, which is practically invisible to the naked eye and that appears at the top of the Scorpio constellation. It is 46 light-years away from Earth, a distance that could be covered during the lifetime of a human being, should the technology appear to travel at nearly the speed of light, and was identified among 118,000 stars observed by the European Space Agency’s Hipparcos space probe.

The 18 Scorpii is only 5% brighter than the Sun and a little younger, with an age calculated at 4.2 billion years. It’s surface is 12 degrees hotter than the Sun, where the temperature is 5507 degrees Celsius or 5780 Kelvin (K), the preferred temperature scale of physicists. The mass of 18 Scorpii is 1% greater than that of the Sun and its rotation speed is 17% faster: it takes 23 Earth days to rotate once on its axis, while the Sun needs 28.

But not all this similarity has guaranteed 18 Scorpii the title of solar twin. Its chemical composition is slightly different from that of the Sun, Nascimento and Mello found. They compared the concentration of the chemical element lithium, one of nature’s simplest, in five solar twin candidates and noted that the amount of lithium in 18 Scorpii is dozens of times greater than that of the Sun, according to an article published in 2009 in Astronomy and Astrophysics . It is a considerable difference, since, besides indicating the age of the star, the abundance of this chemical element allows for an idea of what goes on inside it.

solThese colossal balls of gas are not calm, as it seems from a distance. The interior of a star is extremely turbulent. From the surface to the center, the electrically charged gas (plasma) becomes denser and hotter. The 5780 K measured on the solar surface – the outermost, visible layer of the sun, responsible for its yellow color – rise gradually to reach 15 million degrees in the heart of the star, where the nuclei of the chemical element hydrogen (the simplest and most abundant in the Universe) combine to produce helium and release energy in the form of highly energetic light particles (photons). The photons, produced by nuclear fusion, follow a tortuous path in which they are absorbed and re-emitted several times by other chemical elements until they manage to cross the 700,000 kilometer thickness of the sun and escape into space about 200,000 years after they were created.

It is now known that the lithium levels of stars like the Sun decrease over time. Some studies have shown that stars with little lithium are less active than those with high levels and suffer fewer explosions, which throw high levels of very energetic radiation onto surrounding planets. For this reason, the researchers concluded, it is unlikely that 18 Scorpii, which is rich in lithium, would house a planetary system that is conducive to life – because it is really different from the Sun or finds itself at a different stage of evolution.

The work that displaced 18 Scorpii as a solar twin helped establish the favoritism of another star, HIP 56948. Identified in 2007 by Peruvian astrophysicist, Jorge Meléndez, currently a professor at the University of São Paulo, this star in the Draco constellation is 47 degrees colder than the Sun and 14% bigger. HIP 56948, which is 220 light-years from Earth, has the same mass and solar chemical composition and is only 100 million years older. Today, this star is number 1 among the 10 main candidates for solar twin and should be investigated by the Seti [Search for Extraterrestrial Intelligence] project, which is looking for signs of life elsewhere in the universe, says Nascimento.

The search for a new Sun gained momentum in 1995 after the Swiss astronomers, Michel Mayor and Didier Queloz, announced they had detected the first planet orbiting a Sun-like star outside our solar system (see Pesquisa FAPESP nº 104). Fifteen years later, there are 479 known extrasolar planets and 43 stars that have more than one planet for company. None of them, however, is identical to the Sun.

Until they find an ideal solar twin, the most natural target for the search for habitable planets, researchers take advantage of stars called solar analogs, which are similar but not identical to the sun, to construct an evolving profile of the star that made life possible on Earth. In this way they expect to understand better what its past was like and what its future is likely to be. The attempt to build an evolving profile of stars like the Sun has received priority in the Corot mission, says José Renan de Medeiros, one of the coordinators of Brazil’s participation in the project.

In April this year Mello, Nascimento and Medeiros added an important part to this map. In partnership with researchers from Spain, England and France, they described a very young sun in the Astrophysical Journal. It is the star, kappa1 Ceti, in the Cetus the Whale constellation, and it has size, mass and temperature that are close to those of the sun, but it is billions of years younger. Kappa1 Ceti is between 400 million and 800 million years old, the age of the Sun when life on Earth emerged and the oceans on Mars evaporated.

If in the distant past the Sun really did look like this star, the evolution models of Earth’s atmosphere may need some adjustments.Kappa1 Ceti emits between two and seven times more highly energetic ultraviolet radiation and 35% less energy than the Sun produces today. This form of radiation is important for controlling the chemical reactions that led to the increase in the concentration of carbon dioxide (CO2) in primitive Earth’s atmosphere and for the warming of the planet.

The energy production of stars oscillates over time and depends on two interlinked phenomena: the speed of rotation and the generation of magnetic fields. As stars are not rigid spheres, their surface moves faster at the equator and more slowly at the poles. The higher the speed of the plasma, the more intense the magnetic fields generated, which, in turn, influence the type of radiation emitted. From time to time, these fields become twisted and entangled in areas of intense activity and energy emission, seen as dark spots on the surface of the stars.

According to the models of stellar evolution, when the Sun appeared there must have been a very high rotation rate; completing a rotation once every three days and emitting hundreds of thousands of times more highly energetic radiation. Over time, it began to spin more slowly and produced more radiation in the visible light band. We know that this seems to be true from the analysis of stars that are younger and older than the Sun, relates Mello, who in a few months is likely to publish a work with data of three stars similar to the Sun in its infancy and data showing how stars should look in billions of years.

Over the last few years, astrophysicist Adriana Válio from Mackenzie Presbyterian University, in São Paulo, has been analyzing activity on the surface of stars similar to the Sun, using an innovative technique that she herself created. When she began participating in the Corot project in 2002, Adriana tried to imagine a way to use the luminosity measured by the satellite to get an idea of what was happening on the surface of stars. She proposed that they take advantage of the eclipse of planets, companions of 20% of the stars with planets observed today, to measure the size of the dark spots on their surface and other indicators of stellar activity.

Adopting the planet as a sort of astronomical ruler, Adriana has been able to measure, with an accuracy never before achieved, the size, temperature, location and length of life of star spots. She also calculated the period of rotation of stars and how much faster they spin at the equator than the poles. With Italian Antonino Lanza, she tested the strategy with Corot 2, a star that is similar to how the Sun must have been when it was 500 million years old, and showed that it works. In August, Adriana presented her analysis of the Corot 6 star, similar to the Sun when it was 3 billion years old, at a symposium of the International Astronomical Union.

From the data about the magnetic field and the rotation of stars similar to the Sun, Nascimento, Meléndez and Mello intend investigating what happens below the stellar surface. We are moving from external quantifiers, such as luminosity and temperature, to internal ones, like the convective zone and magnetic field, says Nascimento, who is optimistic about the quality and quantity of data about solar analogs obtained by Corot and other programs. Over the next 10 years, he calculates, we are likely to find a star identical to the Sun.

Stellar oven
Like every star, the Sun is an immense sphere of plasma; electrically charged gas. In its central region, the core, the temperature is 15 million degrees, and every second 4.5 million tons of material are converted into energy. In the photosphere, the layer that emits most of the visible light, the temperature drops to about 5000 degrees. Magnetic fields help heat the lowest layer of the atmosphere, the chromosphere, and the highest, the crown, which is at 2 million degrees and is the origin of solar winds.

Scientific article
RIBAS, I. et al. Evolution of the solar activity over time and effects on planetary atmospheres. ii. κ1 Ceti, an analog of the Sun when life arose on Earth. The Astrophysical Journal. v. 714. 1o mai. 2010.