The fact that the space is black, is a thing that we know instinctively since the first time that we look to the night sky, or any photo in which we can see the space. But, if there are supposed to be countless stars in the space, wouldn’t they have to illuminate the immensity of the space as if it were day?
The darkness in the photos
This is a question that we have probably made ourselves after seeing one of the many scientific photographies of one of the astronauts that we have sent to space. In fact, the background of all those images is black. No stars, nebulae, or galaxies appear.
The fantastic cinema has contributed a lot in making us imagine an exterior space full of light and colours. However, the Hubble, with its incredible images of stars, galaxies, and nebulae, also inducts us thinking that one of those sceneries must appear in the photos made in the International Spatial Station (ISS) or any other spacecraft. Nevertheless, in its place, the spatial agencies give us photos of the Moon, Mars, Saturn, or Pluto, in which the background is uniformly black.
The explanation of this phenomenon has to do with photography, not with astronomy. The stars are perfectly visible from space. In fact, it is from where we see them the best. However, the spatial agencies don’t send telescopes to the space to take good photos where many stars appear in the background. In space, the conditions require a quick plugging velocity (that to say, a very low exposition) if we want to take photos of a very near object illuminated by the Sun. On the contrary, take a good photo of the stars or of a very distant object from us requires a slow plugging velocity (that to say, a longer exposition).
The light of the objects in the foreground provokes the background to be black, but, if we want the background to look good, what there is in the foreground will be blur and over-lit. For example, we have to choose between taking a good photo from Saturn, but the background will be black, or the background to look good, but Saturn will get completely white for excessive light. Additionally, the speed at which move the objects that we send to space doesn’t contribute to improving this photographic problem of the black space of the background.
However, from the ISS, with time and a good camera, there have been made very good pictures that permit us to see the Milky Way and many other stars. The spatial telescopes also obtain very good photos. They achieve these good results because they focus on very distant places where there isn’t an object in the foreground that reflects big quantities of light. This not to mention that normally they make photos in wavelengths that aren’t visible for the human eye.
The light of the stars
The answer to why the space is black and isn’t illuminated by its countless stars is because the light travels in a straight line unless it is reflected against an object or curved by a lens. It happens the same thing with a laser. We see the red point that is reflected against a surface, but not the beam of light. Just like the light of the stars, the light of the laser simply travels to where it is pointed, not towards our eyes.
The light of the stars arrives directly to our eyes, without it disperses in any surface or environment. The blue colour of the sky is due to the light that enters the atmosphere, which decomposes and disperses. If the Earth hadn’t an atmosphere, the solar light would arrive directly to our eyes from the solar disc and we wouldn’t receive diffused light. This is what happens in the space: the light that arrives to us hasn’t a place in which it can disperse and it arrives directly to our eyes.