Stars to Die for
Ancient astronomers thought the stars were stuck to the black velvet backdrop of space like icy diamonds, permanent and fixed. Present-day astronomers know that stars are born, evolve and die sometimes spectacular deaths--collapsing into black holes that suck in every stray flicker of starlight, or becoming pulsing neutron stars that send out precisely timed radio signals like so many lighthouses in the sky.
For sheer beauty, however, no stellar death holds a candle to the glowing gaseous sculptures that linger in the sky when ordinary stars like the sun finally succumb and die.
Known as “planetary nebulae” because they appeared as cloudy blurs in early telescopes, these interstellar smoke rings are the fossil remains of stars like ours.
As such, they foretell our future: Someday, about 5 billion years hence, our own sun will use up its hydrogen fuel, collapse and begin to puff out glowing rings of gas, much like these, seen in recent images from the Hubble Space Telescope.
The ghostly shapes also take us back to our ultimate origins: The Earth itself and the entire solar system were created from the debris of previous stellar deaths. The carbon atoms that make up our hearts and bones were forged in furnaces of dying stars going off like firecrackers.
The forces that sculpt the glowing shapes are still the subject of some debate. But astronomers believe the story goes as follows:
First, the sun-like star burns up all its hydrogen fuel, fusing the hydrogen into helium in its core and releasing enough energy to prevent the massive star from collapsing under pressure of its enormous gravity. Once the hydrogen is gone and fusion stops, however, the star contracts. Its core heats up, generating enough energy to blow up the star into a so-called red giant. When our sun reaches its red giant phase, it will probably swell so much that it will engulf the Earth.
The star expands, it cools and then contracts, only to pick up more energy and expand again. As the star puffs up and sinks, again and again, it sheds its outer layers, until all that is left are highly compressed ashes--a so-called white dwarf.
Before the star cools completely, however, the remaining hydrogen and helium in its core begin to fuse into heavier elements like carbon. This fusion releases even more energy in the form of ultraviolet light. The heat stirs up an ultra-fast wind that blows more than 3 million mph, plows into the discarded outer layers of the star and sets them glowing.
The result is a fluorescent gas sculpture that is more than just pretty. The intricate shapes of the nebulae record the interaction between the fast, hot winds and the layers of star stuff that puffed out in earlier stages. As such, they give astronomers an inside look at the last gasps of dying stars.
Some of the shapes look like eyes or hourglasses, with symmetrical lobes pushing out on either end. Pinwheel or “S” shapes may be formed by invisible companion stars or even planets that pinch off the flow of gas or pull the star’s outer layers off-center.
Whatever the exact mechanism, the ultimate lesson is as old as the universe itself: Only change is eternal.
