Star Death Leads to Cosmic Enrichment

Star Death Leads to Cosmic Enrichment

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Star Death in the Southern Hemisphere Sky

Stars, like every other object we can see in the universe. have a definite life cycle. They're born in clouds of gas and dust, they "live" their lives, and eventually, they come to an end. This is true for every star we know about, no matter its size or mass. Some very massive stars die in cataclysmic explosions called supernovae. That's not the fate of our star, which will have a more "gentle" ending.

Sun-like stars (those that are around the same mass or age as our Sun) come to the ends of their lives and become planetary nebulae. These are objects in the sky that once appeared almost "planetary" looking to astronomers of a century or more ago who had low-power telescopes compared to today's observatories. They have nothing to do with planets and everything to do with the evolution of certain kinds of stars. Astronomers suspect that our own Sun may end its days as a planetary nebula, if conditions permit. If it does, it will lose much of its mass to space and what remains of the Sun will heat the surrounding cloud of gas and dust and make it glow. To anyone looking at it through a telescope from another planet, the dying Sun will resemble a cosmic ghost.

Observing the Owl Nebula

The European Southern Observatory caught a view of one such ghostly remnant, nicknamed the "Southern Owl" Nebula. The expanding cloud of gas and dust measures about four light-years across and contains materials that once were created inside the star and its atmosphere. Now, those elements (such as hydrogen, helium, carbon, oxygen, nitrogen and others) are being spread to interstellar space, possibly to enrich a new generation of stars.

The Southern Owl (which has the official name of ESO 378-1) is a relatively short-lived phenomenon. It will probably last only a few tens of thousands of years before the cloud dissipates completely. All that will be left is a fading white dwarf star.

What Makes a Planetary Nebula?

For a planetary nebula to form, an aging star must be the right stellar type: it should have a mass less than about eight times that of the Sun. Stars that are more massive will end their lives in dramatic fashion as supernova explosions. They, too, spread their material out, enriching the space between stars (also known as the "interstellar medium").

As the less-massive stars age, they begin to lose their outer layers of gas through the action of stellar winds. The Sun has a stellar wind that we call the "solar wind", which is a gentler version of the tempests emitted by old, dying stars.

After the outer layers of the dying star have dissipated, the remaining hot stellar core heats up, and begins to radiate ultraviolet light. That UV radiation energizes (ionizes) the surrounding gas and causes it to glow.

The Long, Last Breath of the Sun

Once the planetary nebula has faded away, the leftover stellar remnant will burn for another billion years, consuming all its remaining fuel. It will then become a tiny - but hot and very dense - white dwarf that will slowly cool over billions of years. The Sun could produce a planetary nebula several billion years in the future and then spend its twilight years as a white dwarf emitting visible and ulltraviolet light, and even x-ray radiation.

Planetary nebulae play a crucial role in the chemical enrichment and evolution of the universe. Elements are created inside these stars and returned to enrich the interstellar medium. They combine to form new stars, build planets, and - if conditions are right - play a role in the formation and evolution of life. We (and the rest of Earth's life) all owe our existence to the ancient stars that lived and then transformed to become white dwarfs, or blew up as supernovae that scattered their elements to space. This is why we can think of ourselves as "star stuff", or even more poetically-as star dust memories of the ghostly death of a star.

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