Alpha Centauri is cool. Everyone knows that, right? It’s the closest star system to our own, one of the brightest visible stars in the night sky, and it even has at least one planet. As star systems go, that’s already pretty cool. Ah, but the coolest thing that we’ve discovered about the star itself — at least the main one, Alpha Centauri A — is in its atmosphere. And in this case, I mean that quite literally.
René Liseau and a team of astronomers using ESA’s Herschel Space Observatory have taken detailed observations of Alpha Cen A, and peered into the star’s atmosphere. What they found was a cool layer just above the star’s surface, just like we see in the sun. Except that this is the first time anyone’s observed this phenomenon in another star.
The visible exterior of the sun is no less complex than Earth’s atmosphere, with several different layers each possessing their own different characteristics. The sun’s “surface” is known as the photosphere, so named because it’s the layer where sunlight is emitted out into space. The photosphere itself, reaching a searing temperature of a little under 6,000 degrees Celsius, isn’t actually a true surface per se. It’s simply the part of the sun which becomes opaque to light, preventing us from looking any deeper inside it in visible light. The sun being essentially a roiling magnetized ball of hot plasma powered by nuclear reactions, it doesn’t have any real surface to speak of.
Above the sun’s photosphere lies the chromosphere — the lowest layer of the sun’s atmosphere, extending a few hundred kilometers upwards, and also the coolest with a somewhat lower temperature of around 4,000°C. Actually, the chromosphere is easily seen during a solar eclipse as the region where bright red flares, colored by hydrogen, leap thousands of kilometers in single bounds. Higher up lies the sun’s corona, stretching millions of kilometers out into space, with a temperature peaking at around 5 million degrees. The reason for this sudden huge rise in temperature? Actually, that’s a question which solar physicists are still working on.
The fascinating part is that before now, we had never properly seen this in any other star. Alpha Cen A is quite a logical place to look for these things, mind you. It’s quite close to being a mirror image of the sun. Similar in mass and temperature, slightly hotter and older, the two stars are both G-type yellow dwarfs. Evidently too, they also have the same sort of atmospheres.
The sun’s corona is thought to be heated to such extreme temperatures by electromagnetic phenomena such as Alfvén waves, and the magnetic reconnections that cause solar flares. However, a complete theory to explain the high temperature of the corona — and the low temperature of the chromosphere — has yet to be devised. Studying the same phenomenon in other stars like Alpha Centauri A may help us to better understand the cause for the different layers in a star’s atmosphere, and what processes create them.
Understanding the atmospheres of stars may also help us to get a better understanding of the dusty disks around stars, from which planets are born. René Liseau, whose work has been published in the journal Astronomy & Astrophysics, said on the matter: “Although it is likely only a small effect, a temperature minimum region in other stars could result in us underestimating the amount of dust present in a cold debris disc surrounding it. But armed with a more detailed picture of how Alpha Centauri A shines, we can hope to make more accurate detections of the dust in potential planet-bearing systems around other Sun-like stars.”
So with one observation, we now have proof that other stars’ atmospheres work the same way as the sun’s, and can hope to better comprehend both stars and the formation of planets elsewhere in our galaxy. Now if that isn’t cool, I don’t know what is!