An artist's concept of the Infrared Space Observatory. Photo: ESA

Centuries ago it was commonly believed that comets carried disease in their tails. Nowadays we know the only 'disease' you can get from a comet is a cold - if you stay out too long at night watching it! But these old beliefs were not completely wrong: comet tails do contain an extremely poisonous chemical compound - hydrogen cyanide. Now a team of Dutch and German astronomers using ESA's Infrared Space Observatory and the James Clerk Maxwell Telescope on Hawaii have discovered that this poison can help them to understand the birth of massive stars - its presence is a sign that a massive baby star has begun to warm up.

Annemieke Boonman (Leiden University) and Ronald Stark (Max-Planck-Institut für Radioastronomie, Bonn) studied a massive protostar (a star in the very early stages of development) called GL 2591, located 3000 light-years away. GL 2591 is embedded in a cloud of dust and gas a thousand times larger than our entire Solar System, and it is spewing out powerful jets of hot gas at hundreds of kilometres per second. The team detected hydrogen cyanide deep in the interior of this cloud, and realised that this meant that the massive baby star was already hot at its core.

As Boonman explains, "Detecting large amounts of hydrogen cyanide toward the centre of a massive protostar means that it has already started to warm up. From this information we can determine the degree of evolution, and therefore the age, of the star."

Astronomers now know that GL 2591 is between a few tens of thousands and a hundred thousand years old, which means that in a few hundred thousand years more its birth process will be over and a new star, ten times more massive than our Sun will be shining in the sky. Will anyone be there to see it?

A cold cocoon warming up
Stars are huge balls of hot gas, heated by nuclear fusion processes in their cores. They form within large clouds in galaxies, but their birth process is not yet very well understood. In the case of massive stars, those with at least ten times as much mass as the Sun, scientists know even less since most of the regions in space where massive stars are formed happen to be farther away from Earth than low-mass star-forming clouds. As a result, there is a long list of pending questions regarding how massive stars form.

The toxic compound hydrogen cyanide in a still unborn massive star. Photo: ESA

For instance, when does the star-to-be begin to get 'warm'? The cloud of dust and gas is initially very cold, at about minus 250 degrees Celsius, and obviously it gets warmer as the star-forming process proceeds. In principle, astronomers can trace the increase in temperature by studying the chemical composition of the cloud. As soon as the core of the massive embryo-star reaches room temperature the chemistry in the cloud changes: the existing molecules start to combine, and more complex compounds are formed. So the presence of complex molecules in the cloud tells astronomers that the baby star has begun to warm up.

But there is a technical problem: current instrumentation only permits the detection of complex molecules in the cloud when there are plenty of them, that is, when the chemical changes are well advanced. If astronomers want to mark the true birth of the star's hot core, then they have to identify a molecule that not only needs warm temperature for its synthesis, but that is also much easier to detect than the complex molecules used so far as indicators. The Dutch-German team found that the toxic hydrogen cyanide molecule fits the bill.

A revolution in astro-chemistry
The idea for this approach came when they observed the protostar GL 2591 with ESA's Infrared Space Observatory. ISO, a pioneering telescope for infrared space astronomy, has proven to be a powerful tool for astro-chemists, astronomers who study the chemistry of the Universe. It was the first instrument to be able to detect a whole range of molecules in space which emit only in the infrared, triggering what many astronomers called 'the infrared revolution'.

When the group observed GL 2591 with ISO they detected large amounts of hydrogen cyanide. The astronomers found that this hydrogen cyanide gas was very hot and abundant, and therefore it could be a telltale sign pointing to the existence of a newborn hot core. In April last year the Dutch-German team again observed GL 2591 with the ground-based James Clerk Maxwell Telescope and confirmed that the hydrogen cyanide was located deep in the interior of the cloud.

"We chose hydrogen cyanide because it is one of the few molecules we detected with ISO that is also observable from the ground and present in large amounts in the hot gas. Then we used ground-based observations to exclude the possibility that this compound had been formed by other high temperature phenomena that can occur throughout the cloud and are not related to the hot core," explains Boonman.

"We used a new, highly sensitive instrument (the MPIfR/SRON heterodyne spectrometer) on the James Clerk Maxwell Telescope on Hawaii to observe GL 2591," explains Stark. "The sensitivity of this spectrometer is such that it could reveal the origin of the hydrogen cyanide detected by ISO."

The European Space Agency's infrared space telescope, ISO, operated from November 1995 until May 1998. As an unprecedented observatory for infrared astronomy ISO made nearly 30 000 scientific observations.