Garden-variety stars like our Sun live undistinguished lives in their galactic neighborhoods, churning out heat and light for billions of years. When these stars reach retirement age, however, they become unique and colorful works of art.
As ordinary, sun-like stars begin their 30,000-year journey into their twilight years, they swell and glow, shrugging off their gaseous layers until only their small, hot cores remain. The ejected gaseous layers are called planetary nebulae, so named in the 18th century because, through small telescopes, these gas clouds had round shapes similar to distant planets such as Uranus or Neptune.
The gaseous debris glows like a fluorescent design, producing objects with striking shapes and names like "The Cat's Eye" and "The Hourglass." Astronomers have recorded more than 1,000 of them in our galaxy.
Gas released by these dying stars helps create new life. This gas contains new chemical elements, including carbon, which eventually are incorporated into stars and planets. Scientists believe that the carbon found on Earth came, in part, from planetary nebulae billions of years ago. (The rest came from supernova explosions.)
Supernova explosions may be more powerful, but the light show from the death of ordinary stars is a more captivating. As bright as 1 billion suns, supernovae explosions signal the demise of massive stars (roughly 8 solar-masses or more). These powerful blasts occur, though, only once every 30 years in galaxies like ours. The demise of an ordinary star, on the other hand, occurs every year. By understanding how these garden- variety stars live and die, scientists are developing a clearer picture of our Sun's fate. (The Sun will enter its twilight years in another 5 billion years.)
Sun-like stars, like humans, are born, live their lives, and die. A sun- like star's life lasts about 10 billion years. Most of that time is spent in adulthood or the "main sequence" phase, living a blissful life in a suburban galaxy neighborhood. A star's peaceful appearance, however, belies what is happening inside its core where its energy-producing "engine" resides. A highly powerful, self-regulated, 30-million-degree Fahrenheit engine powers the sun. The engine is constantly busy converting hydrogen to helium (called nuclear fusion), which produces the energy necessary to sustain life. The Sun's engine produces the heat that makes the Earth habitable. Energy generated by the core also keeps gravity at bay.
All stars wage a continuous battle against gravity, specifically, the crushing weight of their outer layers. During most of a star's lifetime, pressure and gravity hold an uneasy truce. It is analogous to two people arm wrestling to a draw. The weight of the outer stellar layers pushes against a star's inner layers. At the same time, heat generated in a star's high-metabolism core - by the conversion of hydrogen to helium - produces pressure. This pressure exerts an outward force, like the pressure of gas in a hot air balloon, to combat the inward force of gravity.
As a star ages, it begins to exhaust its supply of hydrogen. When the hydrogen runs out, there is not enough gas pressure inside a star to fight off gravity. A star, then, must make adjustments to keep on running. This signals the beginning of a star's twilight years.
As humans reach their golden years, they retire, take trips, relax. But a sun-like star's senior years are full of drama. It is as if it has ditched its peaceful lifestyle for one last adventure. Once the hydrogen runs out and gravity begins to claim its victory, the core begins to contract and become denser and hotter. At this point a sun-like star has completed 90 to 95 percent of its lifetime. Then the metamorphosis begins with the red giant stage (in which a star swells, to 200 times its normal diameter) and ends with a slowly fading white dwarf (a hot, Earth- sized fossil). One handful of a white dwarf weighs as much as a 747 airplane. A sun-like star spends a fraction of the intervening years (about 10,000) stripping off its outer layers until it uncovers the white dwarf within.
In desperation, the star buys some time for itself by firing up its thermonuclear furnace to convert the remains of hydrogen fusion - helium - into carbon. This process is not particularly productive, buying only about a few hundred million years of life.
Meanwhile, the prolific waste heat from the core is being absorbed in the star's outer layers, causing them to become 3,000 times more luminous, then to expand and, ironically, to cool. A red giant star is formed. This phase lasts about 1 billion years.
Once the helium is exhausted, the core again becomes inactive. The red giant is dying, but the inactive carbon core is still very hot. Surrounding the core are two shells rich in unprocessed hydrogen and helium.
The star's surface pulsates and shudders with seismic energy from the activity of the shells beneath it. With each pulse, which lasts about a year, the surface layers expand and cool. Each time this happens some of the stellar exterior is flung into space and is carried away in a "slow wind," traveling at 10 miles per second. This process continues for a few thousand years until only about two-thirds of the star's mass remains: its carbon-oxygen core.
In a few thousand years, as these last outer layers are stripped off, much hotter inner layers of the star become exposed. Soon only the bare carbon- oxygen core is left. The core's temperature is rising rapidly. Over about 20,000 years, the core's surface temperature leaps to approximately 250,000 degrees Fahrenheit, compared with about 11,000 degrees Fahrenheit for the surface of a sun-like, main-sequence star. The dense carbon-oxygen star is not much larger than Earth.
Ultraviolet light from this intensely hot surface heads into the star's former outer layers, which are still moving outward in space at 10 miles per second. This light is so energetic that it causes the gas to fluoresce - like a fluorescent light bulb - forming the bright planetary nebulae surrounding dying stars.
A new wind, which carries very little mass but lots of energy, is blown outward at 1,000 miles per second (3.6 million mph). The low-density wind races outward and snowplows into the older gas. This so-called "fast wind" helps to sculpt planetary nebulae, creating some strikingly remarkable shapes.
The star's radiation begins to heat the planetary nebula, causing different gases to glow. At first, the nebula appears red because hydrogen gas has been heated. As the exposed stellar surface becomes hotter, the colors shift to green (oxygen) and blue (helium). >From far away, the former layers of the star appear as a glowing planetary nebula, about 1,000 times the size of our solar system. The fluorescent light of planetary nebulae lasts for only about 10,000 years.
Eventually, the core stops ejecting gas into space. The gas expelled earlier ultimately swirls away and merges into the interstellar medium, much as smoke from a train dissipates in our atmosphere. The gas carries traces of newly minted carbon and nitrogen from the atmosphere of the dying star. This material wanders through space until it is drawn into a newly forming star.
Now is a good time to buy real estate on Titan, the largest of Saturn's moons. Land there is dirt cheap. But wait 5 billion years when the sun begins its journey into retirement. As the sun swells and becomes a red giant, life on Earth might get a little uncomfortable: The average temperature on our planet could catapult to a sizzling several thousand degrees Fahrenheit. Then it is time to reach for sunscreen with an S.P.F. of 2,000, or pack up your belongings and take the next space shuttle to a place with a more hospitable climate. That could be Titan, a moon larger than the planet Mercury and about half the size of Earth. Titan is one of the safest bets to colonize because it is far enough from the sun's death rattles, and it has an atmosphere to trap heat.
For those who find searing heat appealing, stick around. Earth will be the place for you. The weather will be fairly predictable. No snowstorms or ice storms. Just extremely hot and dry. The only question is how large will the sun get once it consumes its thermonuclear fuel - hydrogen - and begins expanding. Will the sun swell so much that it engulfs Earth? Or will Earth just barely escape the sun's grasp, only to be scorched by the dying star's prodigious increase in energy output as it fights off death? Scientists speculate about these two possible scenarios.
Sun Swallows Earth
Hot, bright, and foggy. This is the daily forecast if the sun swallows Earth. Right now, Earth and the sun are safely separated by about 93 million miles. But the sun could reach 200 times its present radius during its expansion phase. Earth's atmosphere would quickly evaporate as the planet begins spiraling toward the sun's core, which has heated up to 100 million degrees Fahrenheit. Of course, Earth would burn up before it reaches the core. Our planet's demise could take a few hundred to a few thousand years. But Earth would have company as it travels into the sun. Other planets, such as Venus, would be swallowed up by the giant star.
Earth Barely Escapes the Sun's Grasp
Imagine a bloated, red sun looming in the sky. Temperatures on Earth have catapulted to several thousand degrees Fahrenheit. This is life on the edge, when the sun stops expanding just before reaching our planet.
Of course, barely missing getting swallowed is not much of a consolation. Earth's future still will be unpleasant. Either Earth will eventually evaporate or it will be subjected to a period of unbearable heat followed by an eon of extreme cold. The forecast will hinge on the sun's ultimate distance from Earth. This distance will depend on how much mass the sun loses as it swells during the expansion or red giant phase.
One possibility is that the sun puffs up so much that it almost reaches Earth. Heat from this swelled star scorches our planet's atmosphere, vaporizes vegetation, and boils away its oceans. Earth looks like a wasteland. Because there is no atmosphere, the sky is black. The sun is a huge, red orb that covers half the sky. Daylight is 3,000 times more intense than it is now. The intense heat eventually evaporates Earth.
Another theory is that the bloated sun winds up far enough away from Earth that it does not burn off the atmosphere. This may sound like good news, but it is not. Earth's atmosphere acts like a greenhouse, trapping heat from the enlarged sun.
Regardless of Earth's fate, the sun continues to wither away. A few thousand years after the sun enters its twilight years, it peels off its outer layers, exposing its much hotter inner layers. Eventually the outer 40 percent of the sun's mass will be puffed into space. Soon the sun's carbon- oxygen core is uncovered. The core's surface temperature has risen to 250,000 degrees Fahrenheit, compared with a normal temperature of about 11,000 degrees Fahrenheit. The dense, hot carbon-oxygen star is not much larger than Earth.
Once it retires as a white dwarf, the sun has been reduced to a tiny, bright point of light. This hot cinder gradually cools off, sending Earth into a deep freeze. An icy rain - composed of material floating in Earth's sky - falls on our planet. After billions of years, the glowing cinder that was once our sun burns out.
Won't Happen During a Human's Life Time
If a few opportunistic people decide to videotape these cataclysmic events, they will be very disappointed. The sun's death cannot be recorded during a human's lifetime. Its journey into retirement will take more than a billion years. In fact, once the sun begins to die in another 5 billion years, it will take about 1 billion years for the star to completely expand, and another 10,000 years for it to evolve from a planetary nebula to a fading white dwarf.