Brown Dwarfs - A brown dwarf could also be called a failed star. During the process of star formation, some protostars never reach the critical mass required to ignite the fires of nuclear fusion. What remains is a brown dwarf. It is a giant ball of gas that is too massive to be a planet but not massive enough to be a star. They are smaller than the Sun but several times larger than the planet Jupiter.
Brown dwarfs emits no light or heat.
They could account for some of the dark matter suspected to exist in the universe. Variable Stars - A variable star is a star that changes in brightness. These fluctuations can range from seconds to years depending on the type of variable star. Stars usually change their brightness when they are young and when they are old and dying. They are classified as either intrinsic or extrinsic.