defriten
defriten

Ce portail offre des connaissances de base sur le thème de l'astronomie et présente les travaux et coopérations de recherche actuels en Suisse.

Immagine: ESO

Immagini astronomiche

If we look through a telescope into space, we see a lot of grey and black and only occasionally some colour. However, the pictures that can be found online and on this website are often very colourful. Why is that? Two factors play a role - firstly, how the pictures are taken and, secondly, how they are edited afterwards.

Recording celestial objects

The human eye only sees a small part of electromagnetic waves. Cameras in telescopes also capture radiation outside the visible range. Filters are integrated for specific wavelengths so that a separate image is created for each type of radiation. Each telescope is active in a slightly different range, with the James Webb Space Telescope, for example, capturing images from the visible red range to the mid-infrared range. However, telescopes also differ from our eyes in other ways. They can "collect" light for much longer to create an image, as they have a longer exposure time and are more sensitive, enabling them to capture weaker light.

Editing images

Depending on the wavelength, the image is coloured differently during processing. The following colours are often used in this process, with short-wave radiation coloured blue, radiation in the medium wavelength range coloured green and long-wave radiation coloured red. Images in the optical range are also usually "embellished" afterwards, as they hardly have any colour. Depending on what they want to find out, researchers superimpose images with different wavelengths or just look at a single one. An example of a combined image is shown below using the Messier 101 spiral galaxy.

  • This image of the Messier 101 spiral galaxy is a compilation of images from the Spitzer Space Telescope, the Hubble Space Telescope and the Chandra X-ray Observatory, each at different wavelengths.
  • The Messier 101 spiral galaxy in red - the infrared view of the Spitzer Space Telescope. Most of this light comes from the fine dust trails of the galaxy. New stars can form in such dense dust clouds. In this image, the dust glows red, having been heated by young, hot stars.
  • The Hubble telescope's view of the visible light of M101 is shown in green. This light mainly comes from the stars.
  • The blue colour shows the view of the Chandra telescope in X-rays on the galaxy. X-rays originate from extremely hot gases, exploded stars and material that collides around black holes.
  • This image of the Messier 101 spiral galaxy is a compilation of images from the Spitzer Space Telescope, the Hubble Space Telescope and the Chandra X-ray Observatory, each at different wavelengths.Immagine: NASA, ESA, CXC, SSC, and STScI1/4
  • The Messier 101 spiral galaxy in red - the infrared view of the Spitzer Space Telescope. Most of this light comes from the fine dust trails of the galaxy. New stars can form in such dense dust clouds. In this image, the dust glows red, having been heated by young, hot stars.Immagine: NASA, Jet Propulsion Lab/Caltech, and K. Gordon (STScI)2/4
  • The Hubble telescope's view of the visible light of M101 is shown in green. This light mainly comes from the stars.Immagine: NASA, ESA, K. Kuntz (JHU), F. Bresolin (University of Hawaii), J. Trauger (Jet Propulsion Lab.), J. Mould (NOAO), Y.-H. Chu (University of Illinois, Urbana) and STScI3/4
  • The blue colour shows the view of the Chandra telescope in X-rays on the galaxy. X-rays originate from extremely hot gases, exploded stars and material that collides around black holes.Immagine: NASA, CXC, and K. Kuntz (JHU)4/4

A video with a very detailed and easy to understand explanation of how the colours in astronomical images are created:

Are the colours in astrophotos real? Why are the images of nebulae, stars and galaxies so colourful?
Blue Marbel - Photo de la Terre prise lors de la mission Apollo 17 en 1972
Immagine: NASA Johnson Space Center Gateway to Astronaut Photography of Earth

The significance of astronomical images goes beyond scientific findings, as they help us humans to better understand our existence in space. The images from space provide an external perspective of the Earth. The Blue Marble photo (see above), which shows the illuminated Earth, was taken during the Apollo 17 mission. The image has been used by numerous environmental movements to show how unique and fragile the Earth is.

This artistic representation shows a view of the Milky Way. An arrow shows the position of our sun.
Immagine: NASA/JPL-Caltech/R. Hurt (SSC)

We cannot grasp the dimensions of the universe in numbers. The enormous indications of kilometres exceed the capacity of our imagination. However, a picture of our Sun within the Milky Way gives us a better idea of how immense these distances are.

Telescopes on Earth cannot capture all forms of radiation. The Earth's atmosphere absorbs certain wavelengths, so that gamma, X-ray or infrared radiation, for example, never reaches terrestrial telescopes. Turbulence in the atmosphere can also lead to disturbances in the images. Telescopes on Earth are also more limited in terms of location and time. They should be located in the most secluded places possible so that no extraneous light from, for example, cities distorts the images. And they can only take pictures at night.

A space telescope has greater options for capturing radiation. But it brings other challenges with it. Building such a telescope and launching it into space is much more complex and expensive than creating a telescope on Earth. The James Webb Telescope, for example, cost around CHF 10 billion to launch into space, while the Extremely Large Telescope, which is stationed on Earth, will cost around CHF 1.3 billion by the time it goes into operation. In addition, space missions are much more critical, because repairs are hardly possible in space.