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The Sun: Main Sequence Star
The sun, like most stars in the universe, is a main sequence star. It is in a state of stable nuclear reaction; that is, it burns steadily, with only small variations in temperature and brightness. Think of this stage as adulthood; it is where a star spends the majority of its life. While all stars reach this stage, the size of the star is what determines how long it lasts. Our sun is a medium-sized, yellow dwarf star that is about halfway through its life cycle of 10 billion years.
Betelguese: Red Giant
Betelgeuse is not just a red giant, but a supergiant, in the constellation of Orion, about 430 light years from our solar system. Though larger and brighter than the sun, even in its main sequence, Betelgeuse is the future of our star. Its core has begun to contract and its outer shell has expanded, cooling the star and giving it its red color. In June 2009, researchers at the University of California, Berkeley announced that Betelgeuse has been steadily shrinking since 1994, though they do not yet know why. Edward Wishnow, research physicist at UC Berkley, says, " ... there are still lots of things we don't know about stars, including what happens as red giants near the ends of their lives."
Helix Neula: Planetary Nebula
A red giant star eventually throws off its outer layers in a bright cloud of plasma and gas, exposing its collapsed inner core and forming a planetary nebula illuminated by the core's ultraviolet radiation. The colorful Helix Nebula, in the constellation Aquarius, is the closest one to our solar system, about 650 light years away. From our vantage point, the helix is round because we are seeing one end of a radiant trillion-mile long tunnel of gas. Unlike a diffuse nebula, in which stars are born, the planetary nebula is a vibrant death shroud.
The Pup: White Dwarf
When the star's shroud has dissipated, only the core remains--a white dwarf. The brightest star in our night sky, Sirius A, or the Dog Star, has a white dwarf companion: Sirius B, nicknamed "the Pup." The white dwarf is the last stage in the life cycle of a medium-sized star like the sun. The Pup is now only about 92 percent of Earth's size, but its mass equals that of the sun. Because the brightness of a star is dependent on its mass, the Pup flares bright though it is in the throes of death.
Black Dwarf
When the Pup finally cools off, possibly billions of years from now, it will cease to shine and will become a black dwarf. It is thought to take a trillion years for a star to reach this stage. However, because our universe is only 13.7 billion years old, the existence of black dwarfs remains hypothetical.