Red giants are stars at the end of their life cycle. Stars begin as balls of energy, burning with radiance. As the millennium progress, and the materials at the core of the star burns up, the color changes from white to yellow, finally ending in red. Red giants have exhausted their fuel supply at the core, and have begun to burn the fuel at the outer edges of their shell. Compared to other stars, red giants are huge, and burn at a lower temperature than other stars. But keep in mind that 'lower temperature' is a relative term -- you still couldn't walk on the surface of a red giant, for example, if it had solid mass. The temperature of a typical red giant is around 5,000 Kelvin or lower. For comparison, our own star, the Sun, is around 5,777 Kelvin or 11,000 degrees Fahrenheit. Red giants provide a fascinating glimpse into the future of the cosmos.
Stars consist of gases such as helium and hydrogen. Within the core of a star, hydrogen protons burn to create energy, which we see as light. Once the star runs out of hydrogen protons, what's left in the core are helium atons. The outer layer of the star may still contain hydrogen, but not enough to keep burning as brightly as it once did. As the star runs out of fuel, it loses heat, and as it cools, it shrinks. As the star contracts in on itself, some of the material that was once around the outside begins to fall into the engine or core of the star. But this new material entering the core of the star burns differently. The atoms fuse, rather than burn, and the fusion process creates greater heat than the original burning process. Heat causes things to swell, and stars are no different. As the star gets hotter and hotter, it gets bigger and bigger, leading to the moniker red giant. The red color created by the burning matter and the increased size gave these stars the nickname red giants.
How long does it take a star to turn into a red giant? A long, long time. Our own sun, for example, will take another five billion years before experts believe it will transform into a red giant and swallow the solar system, including Earth and Mars, into its core. By then, let's hope the human race has figured out how to travel in space and has long since moved off the planet. For most stars, the process of turning from a newborn star to a red giant at the end of its lifecycle takes many billions of years. Not all stars, however, will turn into red giants. Some stars simply lack the right fuel to turn into red giants.
To see a red giant star with your own eyes, take a look in the night sky, preferably in late autumn of winter. Find the constellation of Orion, noted for its three stars forming the belt and the red star, Betelgeuse, that forms the shoulder point of the figure of Orion. Betelgeuse is one of the most famous red giants, mostly because it can be seen with the naked eye from Earth. Using a telescope, you can get an even clearer image of a red giant. The Hubble space telescope took pictures of Betelgeuse showing the uneven outer layer of a typical red giant. Betelgeuse is so huge that if it took the place of our own sun, its outer layers would extend as far out into the solar system as the planet Jupiter, encompasses Mercury, Venus, Earth, Mars and Jupiter itself.
Many people hold common misconceptions about red giants. Red giants do not turn into black holes. Instead, as red giants reach the end of their lives as stars, they form planetary nebulae. Early astronomers who viewed nebulae through their telescopes thought that these glowing bodies were additional planets, and so named them planetary nebulae. However, they're actually the remnants of stars. Another misconception about red giants is that all stars follow a neat, orderly march from birth to red giant stage. Some stars simply do not have enough of the right atoms to form the glowing big red ball of light characteristic of a red giant. Other stars burst out of existence in an event called a supernova.
NASA and the experts at the Goddard Space Center provide a pamphlet for educators called "The Exciting End to an Ordinary Star's Life." In this pamplet, they clearly explain how stars progress through their life cycle, how red giants evolve, and what students should know about red giants. The evolutionary path of stars provides a cosmic recycling pattern that uses and reuses stellar matter for the constant cycle of birth to death in the known universe. While it takes billions of years, this cycle provides clues as to the origins of the universe and the fate of our own solar system .