The interstellar medium

The space between stars in a galaxy is not empty. This is the interstellar medium. It is filled with a gas of very low density. If low in most of the space that this density is lower than that of the vacuum produced in laboratories on Earth. For comparison, the air around us has a density of 1 g per liter (a liter of air weighs 1 gram) or 10 22 molecules per liter. The vacuum in the laboratory corresponds to 10 -13 g per liter, or 1 million molecules per liter. In the interstellar medium density drops to 10 -21 g per liter, or 100 to 1000 atoms per liter. Although the number of particles per volume is low, the weight of the material between the stars is very important, because the distances, and therefore the volume between the stars are enormous. Accordingly, the interstellar medium represents a fraction of the mass of a galaxy which is often higher than with the sum of the masses of all stars.

See below, see this long ago: the light does not move instantaneously in space. It spreads to 300 000 km per second. Thus a particle of light (a photon) radiation at the surface of the sun at 150 million km dishes for 500 seconds to reach us, or 8 minutes and 20 seconds. Thus the sun as there was 8 minutes and 20 seconds. And they say the sun is 8 minutes and 20 seconds the lights of earth. The more we look away and we see the stars as they were in the past. Thus, the star nearest to Earth is about forty thousand billion kilometers or 40 pétamètres. Its light takes 4.22 years to reach us. The Andromeda galaxy, one of our neighboring rocks, is about 24 000 000 000 000 000 000 km or 24 zettamètres. His light is 2.54 million years to reach us. This means that if we see a star born "now" in Andromeda, it is born there are 2.54 million years. If it is very massive, it is highly likely to be already dead in a supernova explosion as we are witnessing its birth. As the universe has a beginning, we can not observe events that took place before it. The age of the universe is estimated at 13.7 billion years, we can not see farther than 13.7 billion light years is our horizon. This does not mean that the universe is not bigger, it simply means that you can not see beyond. It's already a lot, since this corresponds to 130 000 000 000 000 000 000 000 km or 130 yottamètres.