Visions from the Atacama

atacamaFormsThe driest non-polar region in the world is Chile’s Atacama Desert. Some areas have gone 173 months without rain. At about a latitude of 20 degrees south of the equator, prevailing southeast trade winds carry moisture into the the eastern slopes of the Andes. But as the warm air ascends,  it encounters lower atmospheric pressure and expands at the expense of its own internal energy and cools. Precipitation ensues. As the remaining dry air descends over the other side of the Andes it compresses as it descends and heats up. Meanwhile the western shoreline is unusually deep, keeping the Pacific waters at that spot quite cold. There is also a cold current from the south, preventing cold onshore winds from delivering any moisture to the area. Dry air and no clouds for years on end are an astronomer’s fantasy, and it is why the Atacama Desert is the location of the European Southern Observatory’s Very Large Telescope facility (VLT). VLT consists of Antu, Kueyen, Melipal and Yepun, four telescopes that can be operated independently or in harmony to achieve a better resolution.

In the foreground VLT’s Antu, Kueyen and Melipal with the Milky Way in the sky.

VLT has been producing a large volume of sharp, beautiful images and noteworthy science. In 2009 VLT revealed that the star Betelgeuse has a vast plume of gas almost as wide as our Solar System and a gigantic bubble boiling on its surface. The star’s atmosphere is apparently constantly stretching out into space and then retracting, losing some material in the process.

Presently (late August, early September) from the southeastern part of North America, Betelgeuse is visible before dawn in the southeastern sky.  A star in Orion’s “right bicep”, it’s a red giant with an inconsistent peachy color.  A red giant that is massive enough to go supernova, Betelgeuse is within our galaxy, only 642 light years away. When it explodes some time within a thousand years, it will be bright enough to be visible on Earth in broad daylight. The plume observed from VLT reveals that Betelgeuse is asymmetric. All red giants shed material, but Betelgeuse is not spewing it out equally in all directions.

Two years later, again using the VLT,  astronomers discovered a surrounding nebula, bigger than Betelgeuse itself, stretching 60 billion kilometres beyond the star’s surface, or approximately 400 times the Earth-Sun distance.  The visible part of the nebula turns out to be made up of silicate and alumina dust. When we looked at the cosmic origins of the chemical elements, we learned that silicon is formed from a massive star capable of generating the necessary temperature and density. A red giant of Betelgeuse’s dimensions satisfies those requirements. With regard to the prominence of aluminum, it suggests that supernovae are not the only source of that element.eso1121a

Another telescope in the same desert is the Atacama Large Millimeter/submillimeter betelgeuse-captured-by-almaArray(ALMA). ALMA has just come up ( in 2017) with the highest-resolution image of Betelgeuse to date. It gives us a clearer look at its asymmetry.

A question that may arise is how can a star that’s approximately 20 times more massive than our sun reach the supernova stage in less than 10 million years, while our star, which although isn’t massive enough to go supernova, is still billions of years away from reaching the red giant stage? A very rough calculation will shed some light on this. Twenty times the mass means that there will also be 20 times the fuel available; however for stars of that size, the luminosity ratio is roughly equivalent to the mass ratio of the main sequence stars raised to a power of 3.5. In other words Betelgeuse has more fuel, but the added heat from the much stronger gravity makes the fuel at the core fuse at a prodigiously higher rate! The time that Betelgeuse spent on the main sequence was only 20 / ( 203.5) of the time that will be spent by our sun. Our sun’s life span on the main sequence is about 10 billion years, but Betelgeuse only spent 10×109[20 / ( 203.5) ] or about 6 million years, if we respect significant figures.

atacamaThe Atacama desert was named after a group of people who settled the northeastern border of the desert at least as far back as 500 AD. The Atacama people, known as atacameños or atacamas, used Rapé  smoking ceremonies, which they believed brought them closer to the gods. It’s fitting that their desert now hosts instruments and minds that try to get closer to distant secrets of the universe.






Other Sources:

Salaris, Maurizio; Santi Cassisi (2005). Evolution of stars and stellar populations. John Wiley & Sons. pp. 138–140

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