Stars are luminous, hot bodies, which are made up of gases, especially hydrogen and helium. They are characterized by different temperatures, colors, diameters, and masses. The color of stars vary as per their surface temperature; the coolest ones (about 3,000 degrees Celsius) appear red, whereas the hottest stars (about 50,000 degrees Celsius) are blue in color. The surface temperature of the Sun is about 5,500 degrees Celsius and it is yellow in color. There are many stars that are much bigger and hotter than the Sun.
Speaking about the stages of a star, it undergoes different phases in its lifespan. Stars that we commonly talk in our day-to-day life originated as nebulae, which are dense clouds of hydrogen molecules and dust particles. As per star astronomy, the life stages of a star are nebula, star, red giant (a stage with less hydrogen fuel, but cooler, brighter and larger than the previous stages), red dwarf (stars are cooler, fainter and smaller than red giant), white dwarf (very small, but hot), supernova (very bright stage) and black hole (an ending stage).
The first white dwarf star discovered is the companion of Sirius. It was identified in 1862, and was found to be the closest one to the Sun. White or degenerative dwarfs are considered to be the end point of a stellar evolution. They are formed after the collapse of the stellar portion, which is the dense and hot central part of a star. It is the stellar core, where the main burning and fusion reactions take place. Since there are no fusion reactions, they have no energy source. In the beginning, they are very hot. In due course of time, they radiate heat and become cooler and fainter. As compared to the previous stages, they are highly stable.
White dwarfs are composed of compact degenerative matter having very high density, which is because of the gravitational effects. Their density is estimated to be 1,000,000 times more than that of the Sun. Their average mass is estimated to be about 0.5 - 10 solar masses. Their diameter is similar to that of the Earth, which is approximately 100 times smaller than that of the Sun. They have a surface temperature of about 8,000 degrees Celsius. Even though they have a high surface temperature, their smaller size results in low brightness, only about 1 percent luminosity of the Sun.
According to astronomy, white dwarfs are of different types, classified based on basis of their mass and composition. Stars with lighter mass lead to the formation of carbon-oxygen type(composed of carbon and oxygen), whereas those with at least 4 solar masses, give rise to neon-oxygen ones (composed of neon and oxygen). Depending upon the main constituent element, they are classified as dA, dB, dO, dAO, and dAB, where 'd' stands for degenerative, 'A' stands for hydrogen, 'B' stands for neutral helium, and 'O' stands for ionized helium.
Over a period of time, white dwarfs will continue to radiate heat. Eventually, the surface temperature will cool down to such an extent that they will no longer be visible, thus leading to formation of a cold black hole.