After a star with an initial mass of less than about eight solar masses has completed its core hydrogen burning it becomes a red giant:

• Helium is now burning in the core, producing carbon and oxygen, while hydrogen burns in a thin shell surrounding it.
  
  
• The star brightens by a factor of between 1,000 and 10,000.
  
  
• The outer, hydrogen-rich envelope swells to enormous size. It may become as large as the orbit of the Earth, or even Mars.
  
  
• The surface temperature of this extended envelope drops to about 5,000 to 6,500 degrees Fahrenheit, which is rather cool for stars, and makes the star appear red.
  
  
• A strong wind begins to blow from the star's surface (akin to the Sun's solar wind, but much stronger), and, in the course of the star's red giant life, carries away most of the hydrogen envelope surrounding the star's central core.
  
  
• During the final shedding of its envelope, when the mass loss is the greatest, the star pulsates -- the surface layers expand and then contract in repeating cycles -- with periods from several months to more than a year. Such pulsating stars are called long-period variables.
  
  

The envelope material ejected by the star forms an expanding shell of gas that is known as a planetary nebula.

Planetary nebulae typically have masses of about two-tenths that of the Sun, although some are considerably more massive. They expand at the "modest" speed of about 10 to 20 miles/second (about 35,000 to 70,000 miles/hour) and plow into the surrounding interstellar medium.

Planetary nebulae are illuminated by their central stars and display a variety of often beautiful structures. Some are spherical or helical, others have bipolar shapes, and still others are rather irregularly shaped. In a matter of a few tens of thousands of years, they intermingle with the interstellar medium and disperse.

On average, one planetary nebula comes into existence each year in our galaxy, the Milky Way. About 1,500 have been identified.