The University Record, April 9, 1996

Watch for the Owl in Michigan's April skies, Teske says

The Owl Nebula flies high in Michigan's April night skies, passing almost straight above us by the middle of the evening, says astronomer Richard Teske. It is located just below the bottom of the Big Dipper's bowl, nestled next to the southernmost of the two stars that form the Dipper's lip.

"The Owl is a luminous globe of rarefied gases 10 light-years across that was formed by a dying star," Teske saya. "At the globe's center, a feeble star -like object glitters---all that remains of the star's innermost core."

Amateur astronomers know the Owl Nebula well. Viewed with a telescope, the nebula appears smoothly round with two large dark vacancies that give the nebula a striking resemblance to an owl's face. An observer comparing it with the full moon would find the Owl Nebula to be about one-tenth of the full moon's width, but much fainter than the moon.

The Owl is one of the largest examples of a common object astronomers call a "planetary nebula." "The term is misleading, for these objects have nothing to do with planets at all," Teske explains. The objects were named by ea rly observers who thought their round, luminescent shapes resembled telescopic views of known planets in our solar system.

"Today we know that planetary nebulae are light-years-wide globes of bright gas, which one astronomer called 'wreaths placed by Nature around dying stars.' Almost 1,000 nebulae have been discovered in the volume of space around and near our sun," Teske says. Astronomers estimate there may be 50 times as many remaining to be discovered in our Milky Way galaxy.

According to Teske, a star reaches the end of its life when no more thermonuclear fuel remains in its deepest interior. Stars like our sun and those heavier than the sun undergo a final thermonuclear spasm just before this end state is reached. During this last stage the star's outer layers are propelled gently into space, revealing the hot, intensely brilliant core of the now expiring star.

Once they are launched upon their final journey, the gaseous layers remain visible for only a short time. The ejected gases recede at about one million miles a day, Teske says, growing more and more tenuous and dim until they dissipate into space and fade from view, perhaps after 50,000 years.

Apparently this final event isn't always a smooth one that produces an expanding, perfect globe of gas. The blank spots in the "eyes" of the Owl Nebula remind us that complicated forces, which astronomers don't yet understand, are at work during the ejection stages. The notion is supported by nicknames of other planetary nebulaethe Hourglass, the Dumbbell, the Helix and the Clown.

The visible edges of the Owl Nebula have now reached five light-years from the star core left behind at its center. The nebula is already one of the largest and faintest of planetary nebulae. It will fade and disappear from view in perhaps another 10,000 years. The compact star core will fade much more slowly, taking a few billion years to blacken and cool.