1.1.2 What is in space?
The main bodies travelling in space can be described as follow:
- Stars: They are big luminous balls of hot gas, the closest to us being the sun. Stars are made mostly of hydrogen and helium atoms. The nuclear fusion of hydrogen atoms produce the huge amounts of energy that makes them shine. The temperature of their core reaches several millions degrees Celsius and the hydrogen atoms fuse to form new elements, that is those 92 elements listed in the Mendeleev table.
- Planets: A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals. It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A proto-star forms at the core, surrounded by a rotating proto-planetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies.
- Galaxies: Groups of stars are orbiting a common centre of mass to form a galaxy. The number of stars in a galaxy varies from tens of millions to millions of millions. They extend from 20,000 trillions to 20 quintillions miles across.
- Red and brown dwarfs: Red dwarfs are small stars that are not hot enough to sustain nuclear fusion. They have a mass about 40% of the mass of the sun and their surface temperature is of the order of 3200C and this gives them their reddish appearance. They make up the majority of the stars. Stars that are smaller -about 7% of the mass of the sun- are dimmer; they are known as brown dwarfs. Of course, like the red dwarfs, they cannot sustain nuclear fusion in their core.
- Asteroids and comets: Any piece of metal or rock that orbits a star is known as asteroids or meteoroids. Meteoroids are as small as a boulder while asteroids can be as big as minor or dwarf planets. In the solar system most of them are located in what is known as the asteroid belt, between the orbit of mars and Jupiter. Comets are loose collections of ice, dust or rock that are a few kilometres across. They have entered a solar system and they orbit the star.
- Supernovae: When a star reaches the end of its life and their nuclear fuel -hydrogen- starts to run out, they begin to lack the necessary outward force to counter their gravity. They can start to implode but before they can do it, the atoms in their core are squeezed together creating a repulsive force between them stronger that the gravity force maintaining the star together. This is followed by an explosion throwing out in the space new formed elements.
- Neutron stars and pulsars: When a star with a mass not much bigger that twice the mass of the sun runs out of nuclear fuels it normally explodes in a supernova. However it can also collapse on itself forming a neutron star. Neutron stars, although they have a mass about twice that of the sun, have a diameter of the order of 20 kilometres. As a result they are extremely dense and their surface gravity as high as 3 trillions that on the earth. This causes the protons and electrons of the atoms to combine and form neutrons. Some neutron stars can appear to pulsate as they eject particles that are accelerated near their magnetic poles. One can detect this electromagnetic radiation that seems to blink as the star rotates. These neutron stars are called pulsars.
- Black holes: A black hole is what is left when a large star dies. Lacking nuclear fuel, there is no force left to balance gravity and they collapse on themselves, in theory to no volume or infinite density. There gravitational fields are so high that, within a certain boundary called "event horizon," nothing, not even light, can escape. They have not yet been seen but observations of indirect evidences are proof of their existence.
- Solar wind: In addition to dust, debris and larger objects floating around there is also solar wind so that the space is not a perfect vacuum. Solar wind is a stream of plasma -charged particles- emitted by the outer regions of the stars and consisting mainly of free protons and electrons. Solar winds cause aurora such as the northern lights. They also light up the tails, or comas, of comets.

More generally speaking, the universe is made of 4% of the matter we know, of 23% of unknown invisible "dark matter" and the remaining 73% is made of unknown "dark energy" (according to Einstein well known formula E=mc², energy is matter under another form; and inversely).