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Dying Herschel maps the fuel for new stars

Paul Sutherland, Feature writer
Jun 14, 2013, 23:00 UTC

Sen—Something of a sad moment for infrared astronomy has come with the death of the European Space Agency’s powerful space telescope Herschel.

Vital helium coolant has already run out. And when its thrusters’ fuel tanks are finally exhausted on Monday, the giant eye on the sky will be switched off for ever.

But the work of the hugely successful instrument, with its 3.5 metre mirror, nearly 2 million km from Earth in the depths of space, continue to bring exciting discoveries.

The latest to be revealed is that the amount of cool hydrogen gas within our Milky Way galaxy that forms a reservoir that will fuel new stars has been hugely underestimated.

Herschel mapped the location of the clouds of invisible gas molecules and found that there was nearly a third more than had been known and it extends further out from the centre of the galaxy than had been thought.

In the past, astronomers have looked for carbon monoxide, which exists alongside the hydrogen gas, to discover where these supplies of star-making material lie in the Milky Way.

But this CO tracer molecule does not show the way to all such matter in our galaxy because there is no CO in regions where the hydrogen is just beginning to pool. By using Herschel to map ionized carbon (C+), astronomers were able to locate additional reservoirs of the gas.

Jorge Pineda, of NASA’s Jet Propulsion Laboratory (JPL), in California, is lead author of a new paper about the findings published in the journal Astronomy and Astrophysics. He said: “There is an enormous additional reservoir of material available to form new stars that we couldn’t identify before.”

Herschel, which was launched in 2009 to study the birth of stars and how galaxies evolve, was sensitive to far-infrared and sub-millimetre light. This Sen writer was fortunate enough to see it when it was being prepared for the mission in a clean room at Friedrichshafen, Germany.

Labelled layers within Milky Way

The reservoir of gas newly found by Herschel is picked out in this diagram. Credit: ESA/NASA/JPL-Caltech

JPL’s William Langer was principal investigator of the Herschel project to map the gas in our galaxy. He said: “We had to go to space to solve this mystery because our atmosphere absorbs the specific radiation we wanted to detect.

“We also needed to see far-infrared light to pinpoint the location of the gas. For both these reasons, Herschel was the only telescope for the job.”

The first step in a star’s birth comes when clouds of gas are squeezed together so that atoms fuse into molecules. Under the pull of gravity, it collects and becomes denser, eventually reaching a dense enough state for nuclear fusion to take place and the star to shine.

Astronomers were previously stymied in their bid to observe the earliest stage in this process because of the lack of the CO to mark out the invisible cold hydrogen.

Langer said: “Ultraviolet light destroys the carbon monoxide. In the space between stars, where the gas is very thin, there is not enough dust to shield molecules from destruction by ultraviolet light.”

However, ionized carbon is found in these vast but relatively empty spaces and was used by Herschel to track down the hydrogen molecules. For the first time, the telescope provided a dramatically improved map of its location and abundance within the Milky Way.