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Stars and stellar evolution
The glorious end of stellar life

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 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.

Nebula gallery

A glowing gallery of planetary nebulae is presented here. Clockwise, from top left: the Cat's Eye Nebula, the Ant Nebula, the Butterfly Nebula, the Hourglass Nebula, the Little Ghost Nebula, and the Helix Nebula.

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The gaseous debris glows like a fluorescent design, producing objects with striking shapes and names like "Cat's Eye" and "Hourglass." Astronomers have recorded more than 1000 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 was released into space by supernova explosions.

Supernova explosions may be more powerful, but the light show from the death of ordinary stars is 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 are thought to occur only once every century or so in galaxies like ours. Ordinary stars, on the other hand, die at an average rate of about one per 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.)

An uneasy peace

Like humans, sun-like stars are born, live their lives, and then 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, 17,000,000° C engine powers the Sun. The engine is constantly converting hydrogen to helium (in a process 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 maintain an uneasy truce. It is like two people arm wrestling to a draw. The weight of a star's outer layers pushes against its inner layers. At the same time, heat generated in its 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.

The golden years

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.

Once the hydrogen in the star's core 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. Its most glorious days are yet to come, however.

As the core continues to contract, the star's energy production increases. To cope with this energy increase, the star swells up to 200 times its normal diameter and becomes about 3000 times more luminous. The result is a red giant star, formed over a period of about 1 billion years.

The battle between gravity and pressure continues in the red giant, forcing the core to contract until it begins to fuse helium into carbon. The resulting energy release allows the core to expand, and the rate of energy production then drops.

A star's final moments

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 last about a year, the surface layers expand and cool. Each time this happens some of the stellar exterior is flung into space and carried away in a "slow wind," traveling at about 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 140,000° C, compared with about 6100° C for the surface of a sun-like 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 1000 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. The various colors in images of planetary nebulae are associated with these glowing gases. From far away, the former layers of the star appear as a glowing planetary nebula, about 1000 times the size of our solar system. The fluorescent light of planetary nebulae lasts for about 10,000 years.

Eventually, the core stops ejecting gas into space. The dying star is on the path to becoming a slowly fading white dwarf — a hot, Earth-sized fossil. 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.


"Tales of … The glorious end of stellar life" presents the story of stellar evolution of sun-like stars. It chronicles the death of the star and formation of the planetary nebulae. This selection originally appeared as background information for a press release on planetary nebulae.

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9-12, but the material can be adapted for use in other grades at the teacher's discretion
How to use in the classroom

Teachers can use this resource as:

A content reading selection. Teachers should discuss the meaning of unfamiliar vocabulary prior to having students read this selection.

An engagement activity. Have students read the selection. Ask them what might happen to Earth when the Sun dies.

An inquiry tool. Propose a question, such as, "What will happen to the Earth when the Sun dies?" Have students read the selection and write down as many questions as they can about the information in the text.

A source of information. Students can sequence the steps in the lives of Sun-like stars. After reading the selection, have students list or draw the stages in the death of the star and birth of the planetary nebula. Students might include a time period for each stage.

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