The nebulae in our jigsaws: what the NASA photos really show
In our gallery, 4 pictures did not come out of a computer: they are real photos of nebulae, published by NASA. What exactly do they show? Are their colours “real”? Here is a short guided tour, with the credit for each one.
What is a nebula?
A nebula is a vast cloud of gas and dust in space. Some are nurseries, where the gas clumps together until new stars form. Others are leftovers: gas blown off by a star at the end of its life, or the debris of a star that exploded. They are so far away that their light takes hundreds or thousands of years to reach us. Our photos show both kinds.
The Pillars of Creation
The Pillars of Creation are columns of gas and dust in the Eagle Nebula, where stars are being born. Our picture is the version the Hubble Space Telescope took in near-infrared light, which our eyes cannot see. It passes partly through the dust: the pillars become dark silhouettes, and thousands of stars appear behind them. According to NASA, the bluish haze along the edges is material heated by a cluster of young stars, outside the frame, and evaporating into space.
The “Cosmic Cliffs” of Carina
The Carina Nebula is one of the first images from the James Webb Space Telescope, released on 12 July 2022. It looks like mountains under a starry sky: in fact it is the edge of a vast cavity carved into the gas by the radiation and winds of very hot stars, about 7,600 light-years away. The “steam” that seems to rise from the cliffs is hot gas streaming away. This photo, too, was taken in infrared light.
The Helix, what is left of a star
The Helix Nebula, about 650 light-years away in the constellation Aquarius, is a planetary nebula: the gas that a star like the Sun threw off at the end of its life. The small white dot in the centre is what remains of the star, a white dwarf roughly the size of the Earth. The image combines two telescopes: Hubble for visible light and Spitzer for infrared.
The Crab, an explosion seen in 1054
The Crab Nebula is what remains of a supernova, the explosion of a star, recorded by Chinese and Japanese astronomers in 1054. It lies about 6,500 light-years away. Our picture is a mosaic of 24 photos taken by Hubble between 1999 and 2000, and released in 2005.
So, are the colours real?
Yes and no. A telescope does not take colour photos the way a phone does: it takes several black-and-white images, each through a filter that lets through only one kind of light. Astronomers then give each filter a colour to combine them. For the Crab, the blue in the outer filaments shows neutral oxygen, the green ionised sulphur and the red doubly ionised oxygen. For the Helix, green shows ionised hydrogen, blue oxygen and red the molecular hydrogen seen by Spitzer.
In infrared, as with Webb, there is not even a “true colour”: our eyes cannot see that light. It is translated into visible colours, usually blue for the shortest wavelengths through to red for the longest. So the colours are not made up, they carry information; but nobody would see those shades with the naked eye, even through a telescope.
What we retouched
To be clear: on the site and in the HD wallpaper collection, we brightened the colours of these photos and added a few points of light, the small white four-pointed stars. So they are photos retouched to make beautiful jigsaws, not scientific documents. The originals are freely available in NASA's image library (images.nasa.gov): search for them by name.
NASA publishes these images; it does not sponsor or endorse us. Its credit is shown under each picture in the gallery.