Infra-Red Astronomy

Our lecture for June 2002 was Infrared Astronomy by Dr Helen Walker from Rutherford Applelton Laboratories.

It has been over 200 years since the astronomer Sir William Herschel started to experiment with "the heating power of coloured rays". He split sunlight into its rainbow of colours using a prism, from bue at one end to red at the other. He was surprised to see that a thermometer placed beyond the visible red end recorded the highest temperature. He correctly reasoned that the thermometer was absorbing invisible radiant heat and called the invisible light 'calorific rays'.

Infrared light or IR lies between the visible and microwave portions of the electromagnetic spectrum. Its range is normally taken to be from 1 micron (a millionth of a metre) to 1 millimetre. 'Near' infrared light is closest in wavelength to the visible and 'far' infrared is closer to microwave wavelengths. To give you an idea of scale: far IR waves are about the size of a pinhead but near IR waves are the size of cells. It is the far IR light that we experience as heat from a fire or sunlight, whereas near IR waves are invisible to our eyes and bodies so can be safely used to control devices such as TVs, hi-fi equipment and computer hardware. Our bodies emit IR at a wavelength of 10 microns (10 millionths of a metre), at normal body temperature, and even huge objects such as stars and galaxies have their own IR signature. The only trouble is that at low altitudes Earth's atmosphere blocks most of the IR radiation. So astronomers have to use telescopes high up on mountains, balloons, aircraft, satellites and spacecraft to take their readings.

Another problem with working in the IR range is that every nearby object is a blinding source of radiation. Great care needs to be taken when pointing an IR telescope so that it is never directed at the Sun, Moon, Earth or any of the bright planets. The other main difficulty is that the radiation detectors need some form of cooling otherwise heat would leak from the surroundings making them blind to any incoming radiation. Present cooling technologies involve using a store of liquid Helium to cool the detecting apparatus but future missions will probably use a combination of passive, mechanical and Helium coolers.

The whole advantage of observing in the infrared instead of visible light is that you can peer through the blankets of dust that can shroud stars or even galaxies from our scrutiny. For example, take M31, the Andromeda galaxy our neighbour at a mere 2 million light years. Often thought of as a sister to our Milky Way, it appears to be and is classed as a spiral galaxy, type Sb. However, when observed in the infrared with an instrument such as IRAS the spiral arms disappear and are replaced by a small nucleus surrounded by a bright ring of star formation 10 kiloparsecs from the centre. In 2007 there are plans for an ESA mission called Herschel, which is a space-based infrared telescope. Using cutting-edge technologies for cooling it will boast the coolest place in the universe at 1.8 Kelvin (-271.35 Celsius!). Well that's until the launch of the Planck mission in the same year that will beat that record by maintaining a chilly 2 tenths of a degree above absolute zero!