Out of gas
Ken Tapping, May 8, 2013
A few days ago, Herschel ran out of gas, and we finally lost the use of a powerful astronomical tool. Fortunately, over the roughly four-year working life of this orbiting observatory, it produced a flood of new scientific information and discoveries.
Most of the material in the observable universe is dark and cold: gas and dust at temperatures around -260 C. This makes it hard to observe, which is unfortunate because this fascinating stuff is the raw material for making galaxies, stars and planets.
The process starts when some local event, such as the explosive death of an old star destabilizes a cloud of gas and dust, so that it starts to collapse. Unfortunately, this is totally invisible to our eyes or conventional telescopes, especially those located on the Earth’s surface. On the other hand, these cold clouds are warm enough to emit electromagnetic radiation with wavelengths in the millimetre region, which puts it in the twilight zone of the spectrum, between long-wavelength infrared radiation and short wavelength radio waves.
On 14 May 2009 the Herschel space observatory was launched. Named after the British astronomer, it was designed to image the sky in the millimetre wavelength range, and intended to study the formation of stars and galaxies, and the chemical processes taking place in those dark clouds.
The sensitivity of the devices to make measurements and images in astronomy is related to their temperature: the colder the device, the greater the sensitivity. Herschel’s instruments needed the best attainable sensitivity, which meant making them as cold as possible. There is a limit as to how cold things can be. Temperature is a measure of the vibration and motion of atoms. At a temperature of –273 Celsius all that motion has stopped. Since one cannot go slower than stationary, there is no lower temperature, so we call that limit “absolute zero”. Herschel’s detectors were cooled as close as possible to absolute zero. To do this the spacecraft carried 2300 litres of liquid helium. On 29 April the final drops were used up. Herschel’s useful life was over.
During its operational lifetime, Herschel showed the gas and dust clouds in our galaxy, the Milky Way, to be extremely complex. For example, they can occur as complicated networks of thin fibres. The satellite also provided insights as to how these structures can be made to collapse, forming new clouds. It also provided images of embryo stars, still deeply embedded in their birth clouds.
Herschel also showed us stars forming in distant galaxies. Because light from those galaxies takes millions or billions of years to get here, when we observe them we are looking back in time, at a younger universe. Herschel saw stars forming at a prodigious rate: bright, giant blue stars living short lives and then exploding, seeding the universe with the elements needed to make planets, and us.
Herschel’s final mission will be to fire its thrusters to push itself into orbit around the Sun, so that it won’t be a navigation hazard for other space missions.
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