The infrared Universe
- What is infrared light?
- Why is infrared light so important?
- What will the JWST be able to observe in the infrared?
It has always been the intention that the James Webb Space Telescope will operate over a wavelength range from the far visible to the mid infrared. This wavelength coverage is different from that of Hubble, which covers the range from the ultraviolet to the near infrared.
Infrared radiation is a type of light.
The human eye is tuned to detect only visible light, but there are other types of light, invisible to the human eye: gamma rays, X-rays, ultraviolet light, infrared light, microwaves and radio waves.
All these types of light are called electromagnetic radiation and each belongs to a specific part of the electromagnetic spectrum. Electromagnetic radiation moves through space as a wave, with a velocity of 300 000 kilometres per second. The different parts of the electromagnetic spectrum correspond to different wavelength ranges. The energy associated with each type of radiation depends on wavelength, and shorter wavelengths mean higher energies. Ultraviolet light, for instance, is more energetic and has a shorter wavelength than visible light, which is, in turn, more energetic than the longer wavelength infrared radiation.
In daily life, infrared radiation has numerous uses: in remote controls, police speed measurement systems, night-vision binoculars, alarm systems, auto-focus cameras, car door locks, to name but a few.
The expansion of the Universe implies that the further back astronomers look in time, the more light from these distant sources is stretched or redshifted out to ever-longer wavelengths. Longer wavelengths mean redder light and, ultimately, visible light coming from the most remote objects in the Universe will be in the infrared when it is received.
Observing infrared light is not easy because all objects in the Universe emit infrared light - also known as thermal radiation. Even telescopes. Optical telescopes are put in dark places for the best results; infrared ones need to be kept as cold as possible so as not to confuse the observations with infrared radiation coming from the instruments and telescope structures themselves. The instruments in the JWST will have to be cooled to -240°C.
A cold Universe
All objects, even the coldest ones - for example an ice cube - emit a certain amount of heat radiation (i.e. infrared radiation). As a matter of fact, `cold' objects, in astronomical terms objects with temperatures up to about 3500°C, emit most of their energy at infrared wavelengths. The cool Universe is therefore best studied in the infrared. Hotter objects, like the Sun (which has a surface temperature of about 6000°C) radiate strongly at more energetic - shorter - wavelengths.
The Universe is full of cool objects, including planets, dust and ageing stars, none of which usually shine brightly in the visible part of the spectrum and could not be observed directly until sensitive infrared detectors came along.
A chemical Universe
The chemical make-up of dust clouds and other interesting regions can be probed by looking at the spectra of the molecules in the observed region. Very often these spectra can only be obtained by observing in the infrared.
The reason is that most atoms and molecules emit radiation with energies that fall in the infrared region. Besides, infrared radiation is typical of cooler objects, such as dust clouds, where more complex compounds such as organic molecules are often found. Infrared astronomy has made many interesting discoveries related to these more complex molecules in space.
A dusty Universe
Dust is the bane of optical astronomers' lives, blocking their view of many interesting objects. The Universe is full of dust, microscopic particles of varied composition - carbon, silicon, water-ice, minerals, frozen carbon monoxide, organic compounds, silicates - the list is almost endless.
The particles can be hard or soft and come in many different shapes, but the particle size is usually less than 1 micron, one millionth of a metre. The wavelength of the visible light is much the same size as many of the dust particles, so visible light is very readily blocked (scattered) by the dust, whereas the longer wavelength infrared radiation passes through unhindered and the dust is therefore invisible to it.
However, the dust itself is also a source of infrared radiation that can be picked up by terrestrial detectors. For example, dust grains around a star absorb the starlight so that the dust begins to warm up and to radiate in the infrared.
This absorption of energetic radiation and reemission at less energetic wavelengths is very efficient so dust clouds emit the majority of their energy at infrared wavelengths.
A distant Universe
Infrared radiation can help us to learn much more about the young, distant Universe. As a result of the Big Bang (the event that marks the beginning of our Universe), the Universe is expanding and most of the galaxies within it are moving away from each other. The more remote a galaxy is, the faster it moves away from us - this is known as Hubble's Law.
As an object moves away from us, the light it emits is redshifted - the wavelength of the light is stretched so that it shifts towards the red part of the spectrum. The more distant the object, the greater the redshift. For distant galaxies this effect can be so large that they are only detectable in the infrared region.
The James Webb Space Telescope (JWST) was formerly known as the Next Generation Space Telescope (NGST).