COOL LAYER OBSERVED IN THE ATMOSPHERE OF ALPHA CENTAURI

COOL LAYER OBSERVED IN THE ATMOSPHERE OF ALPHA CENTAURI

The European Space Agency's Herschel Space Observatory (launched in 2009 and observing in the far infrared and submillimeter wavelengths) has detected a peculiar pattern of temperature reversal in the atmosphere of one of the nearest stars, Alpha Centauri A. The star, located only 4.37 light-years away, could be a twin of our Sun in terms of age, mass, chemical composition and temperature. (It actually has its own siblings: Alpha Centauri B and a more remote Proxima Centauri.) It turns out that the resemblance is even closer, since the observed pattern of temperature reversal closely resembles what we see in the Sun's atmosphere.

We distinguish three layers in the Sun's atmosphere: the photosphere, the chromosphere and the corona. The photosphere is the visible surface of the Sun. It has a temperature around 5,800K (it varies from 4,500K to 6,000K). This is the layer marked with sunspots and their bright counterparts, solar flares.

Directly above the photosphere lies the chromosphere. It's around 2000 km thick, much less dense than the photosphere, translucent to our eyes, visible only to instruments sensitive to particular hydrogen emission lines. The temperature of the chromosphere varies with the distance from the Sun's surface, first decreasing to only 3,800K and then rising to 35,000K further away.

The corona (Latin for "crown") stretches from the chromosphere outwards. Its thickness varies with the Sun's activity, but it can reach as much as 1.5 million km, or twice the Sun's radius. With density only a fraction (1 trillionth) of that of the photosphere, it's barely there at all. You can see it only during a total solar eclipse.

What's interesting is that spectral emission lines observed in the ghostly light coming from the corona indicate extremely high temperatures, well above 1 million K. There's still some debate about the exact mechanisms responsible for heating of the plasma comprising the corona, but the Sun's magnetic field is thought to play a major role in it.

It's the first time astronomers observed a layer of low-temperature plasma, corresponding to the Sun's chromosphere, in the atmosphere of another star. Further observations, perhaps in other stars, may help explain the origin and the mechanisms behind the peculiar temperature pattern in the solar atmosphere.