Al Bustan 
Observatory  

 

When the carbon core forms in a red giant, it contracts and heats up as did the helium core before it. This contraction drives up the rate at which hydrogen is being converted to helium and helium to carbon in shells surrounding the core.

The hydrogen-burning and helium-burning shells produce energy unstability which cause the star to shed mass in a strong stellar wind and they also produce stellar pulsations in radius. As the pulsations grow, the star's outer layers become unstable and they lose small fractions of their material. As this gas cools, it forms dust. As the outer layers thin and the core is bared so that it can emit freely outward, the wind speed increases. Dense shells of gas are formed in this way. The shell sometimes are roundish, but often they extend farther in the plane of the star's equator as a result of denser winds there.

In a very short time (1000 years or so) an entire envelope has been ejected. On this timescale the ejected gas has moved several hundred astronomical units away from the star and has spread out enough to be transparent. A typical temperature of this gas is 10,000 K. We see it shining because it is ionized by ultraviolet radiation from the hot exposed core of the original star, now the central star of a planetary nebula.

We know about 1500 such objects in our galaxy that can be explained by this model, and there may be 10,000 in all. They were named "planetary nebulae" over two hundred years ago (in 1785) by William Herschel. When he discovered the first one in his small telescope, it looked similar to him to the planet Uranus which he also discovered. Both the planet and "planetary nebulae" appeared as small, greenish disks...

Planetary nebulae are exceedingly beautiful objects. Thay area ctually semi-transparent shell of gas. When we look at their edges, we are looking obliqually through the shells of gas, and there is enough gas along our line of sight to be visible. The shapes of the shells show the interactions of the stellar winds that have blown. When we look through the centers of the shells, there is less gas along our line of sight, and the nebulae appear transparent. In the middle of most planetary nebulae, we see the central star from which the nebula was ejected. Central stars are very hot, some as hot as 200,000 K. They are the hottest stars known. They are very luminous, in that they give off a lot of energy per second, but they are so hot that most of the energy they give is in the uktraviolet. They therefore appear faint to the human eye.

Astronomer are particularly interested in planetary nebulae because thay want to study the the various means by which stars can eject mass into interstellar space. Planetary nebulae in distant galaxies are also proving to be important for determining the distance scale of the universe.

Some "famous" planetary nebulae are:

M27 - Dumbell Nebula

M57 - The Ring Nebula

M76 - Little Dumbell Nebula

M97 - Owl Nebula

NGC2392 - Eskimo Nebula or Clown Face Nebula

NGC3242 - Ghost of Jupiter

NGC6543 - Cat's Eye Nebula

NGC7009 - Saturn Nebula

NGC7293 - Helix Nebula