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Astrophysics

The star of the moment

White dwarf behaves like the most precise optical clock known

Since antiquity, the observation of the movement of the heavenly bodies has been providing clues for man to quantify the passage of time. For centuries, which extended throughout the Middle Ages and the beginning of the age of discovery, calculating the time (and the geographical position) was possible with the assistance of an astrolabe, an astronomical instrument that measures the height of the stars above the horizon. The advent of other, more precise, kinds of clock ended up transforming this piece of equipment into a museum piece. But that does not mean to say that the stars have ceased to be useful for telling the time, as is shown by the recent work of a team of astrophysicists led by Brazilian Kepler de Souza Oliveira Filho, from the Federal University of Rio Grande do Sul (UFRGS).

The researchers discovered that the pulses of brightness emitted by a small dying star from the constellation of Leo Minor, G117-B15A, occur at such regular intervals, but so regular, that they can work like the luminous hands of a stellar clock that is never slow. Well, never slow is a figure of speech, a hyperbole that, in this case, is not all that exaggerated. After all, this stellar chronometer would lose one second every 8.9 million years. “It is the most stable optical clock ever known, more precise than the atomic clocks”, explains Kepler, who published a study on the theme in the last December issue of the American scientific magazine The Astrophysical Journal.

Time and patience were needed to find a celestial object with this particularity, more precisely 31 years observing the tenuous alterations in brilliance of G117-B15A, a rare kind of star, classified technically as a pulsating white dwarf, or ZZ Ceti. The work began in 1974 with Edward Robinson and John McGraw, astrophysicists from the University of Texas and the discoverers of the variability of the star. Five years later, Kepler, who studied for a doctorate under Robinson in the United States, took on the task of recording the pulses of light of G117-B15A. On the occasion, his supervisor advised him that it would take 20 years to perhaps arrive at some interesting detail with this line of research. Even so, the Brazilian laid his bet. The effort paid off. “Nobody would say that the project could work out well”, comments astrophysicist Don Winget, from the University of Texas, the coauthor of the work.

As G117-B15A is only visible from December to March and only in the Northern Hemisphere, the observations were made in the United States, with telescopes of the McDonald Observatory, connected to the University of Texas. Since 1979, once a year, Kepler or some researcher associated with the project spends eight hours running recording the emissions of light from the star, which belongs to our galaxy, the Milky Way, and is 150 light-years from the Earth (one light year is equivalent to about 9.5 trillion kilometers). At the beginning of 2006, recording the pulsing white dwarf will be one of the duties of Barbara Castanheira, a doctoral student of the astrophysicist from UFRGS, who is spending a season in Texas. “If the meteorological conditions allow, I am going to observe this and other stars in two spells during the month of January, the first between the 6th and the 8th, and then from January 23 to 29”, says Barbara.

Due to their peculiarities, the pulsating white dwarfs are candidates for playing the role of optical clocks hung up in space. The astrophysicists estimate that 98% of all stars – in particular the small and medium ones, with little mass, like the Sun – must be transformed one day into a white dwarf, a senile star on the brink of death, which no longer produces energy by means of thermonuclear reactions.

On the way to the end of their existence, these celestial bodies shrink in size and become far denser and colder. Throughout the evolutionary process of contracting and losing heat, some of these stars also show periodic instabilities, that is to say, they emit pulsations that alter their brilliance at fixed intervals, in a rhythmic manner. Because of this cadenced change in their luminosity, they are classified as pulsating white dwarfs. In the case of G117-B15A, a star with an age estimated at 400 million years, the main luminous pulse is triggered every 215 seconds. On the dot. The rate of the luminous emission must decrease as the pulsating white star goes on cooling down. But the demise of the star is such a long drawn out process, capable of extending for billions of years, that it does not affect the reliability of the information.

There are those who say that other kinds of stars, such as certain pulsars, which emit radio waves and X-rays, can be space chronometers even more precise than G117-B15A. Kepler, though, disagrees with this view for one reason: pulsars lack stability and are more subject to perturbations that alter the synchronism of the stellar tick-tock. Besides increasing the knowledge on the evolutionary life of the stars, can the search for a stellar timekeeper have any practical usefulness? “Some scholars how now proposed the creation of a network of clocks of this kind to synchronize time in spacecraft”, comments the Brazilian researcher.

Studying the pulsating white stars may also yield other very concrete findings, such as the location of new planets. They would probably be worlds without any life, seeing that the ZZ Cetis do not offer the characteristics needed to keep in orbit a pleasant haven like the Earth. Even so, they would be planets outside the solar system, one of the most fascinating themes of present-day astrophysics. For the while, Kepler’s team has already discarded the existence of giant worlds, of the size of Jupiter or Saturn, in the neighborhood of G117-B15A. Smaller planets may perhaps exist there. Nobody knows. Only time, always time, will bring the definitive reply.

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