Starquakes offer clues to alien life

Astronomers have been listening in to the seismic upheavals deep within the core of Alpha Centauri, the nearest star to us. And, writes Ian Brown, it sounds remarkably like our own Sun
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The Independent Online

Every fan of science fiction and Trivial Pursuit will immediately name Alpha Centauri as our nearest star. It is not only the nearest, at 4.3 light years, it's also very like our own Sun, itself something of a rarity in the galaxy. Now it seems that, as well as looking like our solar twin, Alpha Centauri even sounds like it too. This is one of the findings of a study this summer on the solar "seismology" of Alpha Centauri, which investigated the "rumblings" coming from deep within the core of this stellar object.

Alpha Centauri, which lies in part of the constellation Centaurus, is especially visible in the southern hemisphere. In fact, it's a double star, consisting of the very Sun-like Alpha Centauri-A and its slightly fainter companion Alpha Centauri-B. A third star, the red dwarf Proxima Centauri, orbits this binary every million years or so, and is at present 0.1 light years closer to us. Proxima is too faint to be seen without a telescope.

Alpha Centauri has held iconic status for thousands of years. It was an object of worship in Ancient Egypt as the first thing visible in the morning sky at the autumn equinox. Several Egyptian temples from the fourth millennium BC appear to be orientated in its direction. Alpha Centauri was known earlier as Rigil Kentaurus, meaning "the foot of the centaur" (the constellation is supposedly named after Chiron, the wise centaur of Ancient Greek myth), but that name was deemed too similar to the brilliant blue-white star Rigel in the constellation of Orion.

During five nights of observation this summer, tiny oscillations in Alpha Centauri-A were detected by two Swiss astronomers, François Bouchy and Fabien Carrier of the Geneva Observatory. They have achieved this using "asteroseismology", that branch of astrophysics that probes the inner properties of stars by measuring the sound waves they emit. Asteroseismology is likely to become an important part of stellar theory, as astronomers are able to learn much about the interior of a star, not just the outer layers visible to telescopes.

Just as geologists study the seismic shock waves generated by earthquakes to learn about the inner structure of Earth, the same technique can be applied to stars. Our own Sun has been probed in this way since the Sixties. However, because they are much fainter, it is much more difficult to detect seismic waves in other stars.

But now Bouchy and Carrier, using the Coralie spectrometer on the 1.2-metre Leonard Euler telescope at La Silla observatory high in the Chilean Andes, have discovered that Alpha Centauri-A pulsates with a seven-minute cycle, very similar to the five-minute cycle detected within our Sun.

The temperature at the Sun's surface (the photosphere) is about 5,500C, but about 15 million C at the solar core. Here, thermonuclear reactions fuse 600 million tons of hydrogen into helium every second. Heat bubbles up from the centre, caused by the enormous energy created. In this so-called "convective zone", the gas is virtually boiling, and hot gas-bubbles rise with a speed close to that of sound.

Just as you can hear water start to boil in a kettle, the same turbulent convection in the Sun creates noise. These sound waves then ripple through to the surface, making it oscillate. This "ringing" is well known in the Sun, where the amplitude and frequency provide astronomers with important insights into the infernal, raging solar interior.

The Alpha Centauri study by Bouchy and Carrier confirms that other stars similar to the Sun are likely to pulsate in much the same way. But asteroseismological searches are difficult because the pulsations are tiny. So great precision is needed.

"We think our results are very significant," Bouchy says. "Nobody thought that these kind of observations could be conducted from the ground. Such oscillations were expected on other solar-like stars but never clearly identified. While oscillations have been detected recently on Procyon and Beta Hydri, these two stars are a bit more evolved than the Sun.

"Alpha Centauri-A is the ideal first target for asteroseismology. It is important to analyse and interpret seismological data to have some preliminary idea of what the stellar parameters are. And observing our nearest neighbour, a multiple system and very bright, has provided us with sufficient parameters for a theoretical model. But we'll need time and several reiterations to allow us to determine the exact radius, luminosity, temperature, chemical composition and age of Alpha Centauri-A. All these parameters will give us a better knowledge of stellar evolution."

The Coralie spectrograph used by Bouchy and Carrier at La Silla has previously been used to detect planets by searching for the tiny wobble of a star being pulled slightly by the counter-gravity of an unseen planetary companion. The same technique measured the oscillations in Alpha Centauri-A. The acoustic waves make the surface of the star periodically pulsate, and the spectra of the stellar surface will show corresponding, though minute, shifts. But these only move with speeds of up to 35cm per second, meaning that such perturbations on the stellar surface only amount to about 40 metres on a star with a radius of 875,000 kilometres.

Observations of other bright, nearby solar-like stars with the Coralie spectrograph are already planned. But the new Harps spectrograph, to be installed at La Silla late next year, will be able to observe stars a hundred times fainter than Coralie can. While it will mostly search for "exo-planets" – those that lie beyond our own solar system – Harps will also be able to undertake an asteroseismological study of about a hundred Sun-like stars, with greater accuracy of the velocity measurements.

Meanwhile, confirmation that Alpha Centauri-A is even more Sun-like than we thought makes it even more intriguing. Is it Sun-like enough to have habitable planets? After scanning the far reaches of the galaxy in our search for extraterrestrial life, could exo-worlds actually be orbiting our nearest interstellar neighbour?

Scientists hope to answer this soon. Remember, Sun-like stars make up only about 4 per cent of the stars in the galaxy, although they still number in the billions. Eighty per cent of stars are red dwarfs like Proxima Centauri, less likely to emit enough heat and light to nurture life-bearing planets.

Alpha Centauri-A may be a "main sequence" G-type star exactly like our Sun (Alpha Centauri-B is a K-type, the next most similar), but unlike the Sun it is a binary star. On average, Alpha Centauri-A and B lie about the same distance from each other as our Sun and Uranus (2.8 billion kilometres) and orbit each other every 80 years. And, while this separation would now theoretically allow each star to retain a family of planets unaffected by each other's gravity, it would have created problems billions of years ago. Any planets trying to form from the disc of interstellar dust surrounding each star might have been pulled apart by the opposing gravitational fields.

Even if single stars like the Sun remain the most likely to have planets, Alpha Centauri must still rank as the first destination pencilled in for any interstellar voyage we might one day undertake. The other nearest Sun-like stars – Tau Ceti, Epsilon Eridani and Epsilon Indi – are around three times further away.

Asteroseismology could also prove important to the search for alien life. "All chemical elements in the universe are manufactured in the stellar interior," points out Bouchy. "Stars are indispensable for life."

Our research will be confined to Earth for the foreseeable future, though. With current technology, it would take even our fastest unmanned probe more than 70,000 years to make the 40 trillion-kilometre trip to our nearest neighbour. To put it another way; it takes light 8.3 minutes to travel the 150,000,000 kilometres separating the Sun and the Earth (one astronomical unit), but 4.3 years to reach Alpha Centauri. Even more simply put, if one astronomical unit was reduced to just one inch, our neighbour would still be more than four miles away.

Sending unmanned probes into interstellar space – even to our nearest neighbour star – will mean waiting a long time before they arrive.