A star goes through different phases during its development. It does not even always begin to emit light.
White dwarfs are the remains of burnt out suns. After a star becomes a red giant at the end of its life, it casts off its outer layers. Its burnt out core shrinks to about 10,000 kilometers in diameter. Almost the entire mass of the star is now concentrated in it. At first, this remnant of a star still emits white light, which is why it is called a white dwarf. How is it made up? How is it possible that such a great mass could exist in such a small space?
Matter, as we know it on Earth, is made up of atoms with minute nuclei in which most of the mass is concentrated. The negatively charged electrons revolve around the positively charged nucleus at a certain distance, thus keeping the nuclei apart from each other.
Inside a white star, the outer shells of the atoms have collapsed, allowing the nuclei to come closer together. The electrons, which are now free, repel each other creating pressure. This pressure resists the force of gravity and prevents a complete collapse of the white dwarf. A single bucket of this matter would weigh thousands of tons. Volumes that once contained 100 atomic nuclei now contain millions.
White dwarfs are only stable when their mass does not exceed 1.4 times the mass of our Sun. If the remainder of a star has a larger mass, a neutron_star is formed.
Once a white dwarf has shed all of the radiation created by the movements of its electrons, it becomes an inactive, aging star. As its mass has become very small, there are no longer any nuclear reactions in its core. Although the core of a white dwarf still emits a reddish light at first, it becomes small and cold as it cools down. This object, which is known as a black dwarf, is no longer visible on account of its minimal luminosity.
Unlike white and black dwarfs, which are formed at the end of the life of a star, the development of a brown dwarf begins with the birth of a star. These small, star-like objects can be seen as a "miscarriage", as they go through no further development.
Following its formation into a protostar from a contracting cloud of gas and dust, its mass is too small for nuclear reactions to occur within the core. The temperature in the center is not sufficient for an energy transformation.
Brown dwarfs are not visible because of their weak luminosity. For this reason, a positive identification of these theoretical objects has not yet been possible. Two objects that fulfill the criteria for brown dwarfs were discovered in 1995. One of them is thought to orbit the star "Gliese 229", which is about 19 light-years away from us. An object revolving the white dwarf Giclas, about 50 light-years away, was discovered in 1987. It emits infrared light. This could also be a brown dwarf.
