Friday, 18 February 2011

The Glorious End of Stellar Life


Garden-variety stars like our Sun live undistinguished lives in their galactic neighbourhoods, churning out heat and light for billions of years. When these stars reach retirement age, however, they become unique and colourful 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 neighbourhood. 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 aeroplane. 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," travelling 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 snowploughs 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 colours 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.

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