Astronomers go to the ends of the Earth to see cosmic carbon

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The edge of the Horsehead nebula, where it touches the empty space outside it, is rich in carbon. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

The carbon cycle is central to life on Earth. It describes how carbon flows between living organisms, and the ocean, atmosphere and rock of our planet, and is driven by the energy from our sun.

But a also exists for our galaxy, and astronomers are opening new windows into space that let us watch this galactic carbon ecosystem in action.

However, the light from carbon in space can be very hard to see because most of it is blocked by the Earth’s atmosphere. But now a new telescope built in one of the most remote regions of our planet is letting us see cosmic carbon in a new light.

Game of millimetres

All elements in the universe emit light with a characteristic fingerprint in the form of emission lines. So just by teasing apart the spectrum of the light received from space, astronomers can determine what elements are out there.

Interstellar carbon comes in several forms. It is sometimes missing an electron, making it ionised. In this state it emits the brightest single spectral line produced by entire galaxies.

Carbon can also be found in atomic form as single atoms. Such atoms reside in the surfaces of molecular clouds, near to the interfaces with atomic gas. Or the carbon can be incorporated into molecules. Here it is primarily found as carbon monoxide, CO, the second most abundant molecule in the universe after hydrogen in the form of H₂.

Carbon monoxide emits in the millimetre portion of the electromagnetic spectrum. This can be readily studied, such as by the Mopra telescope in Australia, which is charting a new map of the molecular clouds of our galaxy.

However, water absorbs the wavelengths of light emitted by ionised or atomic carbon, which makes it hard to see it from here on Earth. This means we must use airborne or space telescopes, which is an expensive proposition.

A small amount of terahertz radiation does penetrate to the ground at the driest locations on the Earth’s surface. One example is the high Altiplano of Chile, where the giant ALMA radio telescope is being built. But the transmission is patchy and the signal variable.

Cold heights

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