A white dwarf is what stars like our Sun become after they have exhausted their nuclear fuel. Near the end of its nuclear burning stage, such a star expels most of its outer material, creating a planetary nebula. Only the hot core of the star remains. This core becomes a very hot young white dwarf, which cools down over the course of the next billion years or so. Many nearby, young white dwarfs have been detected as sources of soft (i.e. lower-energy) X-rays; recently, soft X-ray and extreme ultraviolet observations have become a powerful tool in the study the composition and structure of the thin atmosphere of these stars. A typical white dwarf is half as massive as the Sun, yet only slightly bigger than the Earth. This makes white dwarfs one of the densest forms of matter, surpassed only by neutron stars.
Red Giant
Stars convert hydrogen to helium to produce light and other radiation. As time progresses, the heavier helium sinks to the center of the star, with a shell of hydrogen around this helium center core. The hydrogen is depleted so it no longer generates enough energy and pressure to support the outer layers of the star. As the star collapses, the pressure and temperature rise until it is high enough for helium to fuse into carbon, i.e. helium burning begins. To radiate the energy produced by the helium burning, the star expands into a Red Giant.
Sun
The Sun is a star. It is the source of heat, which sustains life on Earth, and controls our climate and weather. It is the closest star to Earth, and the most closely studied. From it we have learned a great deal about the physical processes, which determine the structure and evolution of stars in general. Only the Sun's outer layers, collectively referred to as the solar 'atmosphere', can be observed directly. There are distinct regions to the solar atmosphere: the photosphere, the chromosphere, and the corona. These three regions have substantially different properties from each other, with regions of gradual transition between them. During the maximum of the cycle, more than 100 sunspots can be seen on the Sun at once. During the minima, the Sun sometimes has no spots at all. This cycle is closely related to the magnetism of the Sun. In fact, it is the changing magnetic field of the Sun, which governs many aspects of solar activity.
Black Holes
Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a " singularity ". As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star. Any emitted photons are trapped into an orbit by the intense gravitational field; they will never leave it. Because no light escapes after the star reaches this infinite density, it is called a black hole. But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. Since black holes are small, and light that would allow us to see them cannot escape, a black hole floating alone in space would be hard, if not impossible, to see.
I think that stars are pretty hott!! (jokin')
What do you think????
