Continuing from where we left in our previous discussion about James Webb Space Telescope along with its predecessor Hubble Telescope. We have already covered major aspects of the Hubble Telescope in two-part series: Part 1 and Part 2. Today we will understand basic principles on which James Webb Space Telescope is based i.e. Infrared astronomy.

 The James Webb Space telescope under construction. NASA/Desiree Stover
The James Webb Space telescope under construction. NASA/Desiree Stover

Introduction: Atmosphere Sustains Life but Hinders Astronomy

The Universe is full of radiation of all types but most of this does not reach us here on Earth because our atmosphere blocks out many wavelengths of radiation, but permits others through.

Fortunately for life on Earth, the atmosphere blocks out harmful, high-energy radiation like X-rays, gamma rays, and most ultraviolet rays. It also blocks out most infrared radiation, except for a few narrow wavelength ranges that make it through to ground-based infrared telescopes.

Our atmosphere causes another problem – it radiates strongly in the infrared itself, often putting out more infrared radiation than the object in space being observed. This is why ground-based infrared observatories are usually placed near the summits of high mountains to get above as much of the atmosphere as possible.

This is why it is so important to put observatories into space, to get above our atmosphere which prevents so much of this valuable information from reaching us.

Hanle Observatory in Ladakh, India. One of the world's highest observatory for optical and infra-red astronomy
Hanle Observatory in Ladakh, India. One of the world’s highest observatory for optical and infrared astronomy

Infrared Astronomy

Many of the things scientists want to observe in space are far too cold to radiate at optical or shorter wavelengths, but radiate strongly in infrared, for example, the cold atoms and molecules that drift in interstellar space. We need to study these raw materials to understand how stars form and evolve. By observing in the infrared we can study how things get formed, the very early steps because formation processes very often happen in cool and dusty places.
In our own Solar System, cold objects such as comets and asteroids reveal most of their characteristics to us in infrared light.

A radio and infrared image of a section of the Milky Way. Radiation from newborn stars heats cosmic dust that glows in infrared (in violet). Ultraviolet light from these stars gives off radio waves in red. Credit: NRAO/AUI/NSF
A radio and infrared image of a section of the Milky Way. Radiation from newborn stars heats cosmic dust that glows in infrared (in violet). Ultraviolet light from these stars gives off radio waves in red. Credit: NRAO/AUI/NSF

Other things of great interest to astronomy are hidden within or behind vast clouds of gas and dust. These clouds hide stars and planets in the early stages of formation and the powerful cores of active galaxies. The astronomer’s view is blocked because the dust grains are very effective at scattering or absorbing visible light. Longer infrared wavelengths can get through the dust.

The infrared is also the ideal wavelength band to search for direct evidence of giant exoplanets and brown dwarfs – objects that are not quite massive enough to ignite nuclear burning and turn into stars.

A near-infrared image of exoplanet 51 Eridani b captured by the Gemini Planet Imager on 18 December 2014. Credit: J. Rameau (UdeM) and C. Marois (NRC Herzberg).
A near-infrared image of exoplanet 51 Eridani b was captured by the Gemini Planet Imager on 18 December 2014. Credit: J. Rameau (UdeM) and C. Marois (NRC Herzberg).

Infrared is subdivided into four sub-bands:

  • Near infrared, stretching from 0.8 to 3 microns
  • Mid infrared, from 3 to 30 microns
  • Far infrared, from 30 to 200 microns
  • Sub-millimetre infrared, from 200 to 1,000 microns

In each band, we see different sources of radiation.

In the near-infrared, we see light from old stars, like red giants, with surface temperatures around 3,000 Kelvin (2,700ºC).

In the mid-infrared, we see emission from hot dust, with temperatures from 300 to 1,000 Kelvin (30 to 700ºC).

In the far-infrared we tend to see cooler dust, at 30–100 Kelvin (-240 to -170ºC). This is where the thermal emission from dusty galaxies tends to peak.

In the submillimetre, we find that cool objects, with temperatures in the range 3–30 Kelvin (-270 to -240ºC), tend to be prominent. Dusty galaxies at high redshift have the peak of their emission shifted from the far-infrared to the submillimetre.


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