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September 17, 2008 > TechKnow Talk: The Fire Above: the Birth, Life, and Death of the Sun

TechKnow Talk: The Fire Above: the Birth, Life, and Death of the Sun

We have learned to take our sun for granted. It rises each day, nourishes plants, and provides light and warmth. Only when we look directly at a setting sun or fall asleep on the beach are we reminded of the fiery furnace with the power to blind or burn from 93 million miles away. Despite its apparent constancy and eternal nature, the sun did not always exist and will someday exist no more.

What exactly is the sun, how did it form, and what will ultimately become of it? Among the hundreds of billions of stars in our galaxy, our sun is not very special. It is a rather average star, of average size, not very young or very old. Some stars are larger, smaller, hotter, cooler, brighter, dimmer, or have very different life expectancies, but our sun is very much like billions of other run-of-the-mill stars.

The sun is a ball of gas about 835,000 miles in diameter. Its volume is more than a million times that of Earth. Its surface temperature is about 10,000 degrees Fahrenheit (F), but it boasts temperatures near the center of more than 20 million degrees F. In this central region, or core, the tremendous forces of gravity and temperature cause hydrogen atoms to fuse together, creating helium and releasing gamma radiation. This mechanism, known as thermonuclear fusion, produces the heat and light emitted by the sun, and makes life on Earth possible.

Our sun is roughly five billion years old. It may have been born from material remaining from the "big bang" at the creation of the universe, or perhaps more likely from a cloud of gas and dust left over from the violent death of a previous generation of stars. Astronomers have identified hundreds of "stellar nurseries" comprised of the remnants of exploded stars, called nebulae, in which new stars are forming.

Within these vast nebulae, composed largely of hydrogen, the gravitational attraction between the hydrogen atoms eventually begins pulling them towards each other. As this process proceeds, the atoms begin to coalesce into a swirling, spherical body of gas. Gravity continues to force the atoms into a smaller, denser volume. The formation of this protostar may require millions of years.

By the way, some astronomers believe it is during the formation of the protostar that the formation of planets begins. The spinning material may create a disk surrounding a central core. As the central core condenses into the star, the material in the disk may form initially into bands (similar to those surrounding the planet Saturn) and over time coalesce into planets in orbit around the star. There are also other theories of planetary formation; these will be explored in a future article.

The friction created between the rapidly moving atoms of the increasingly dense protostar eventually results in a core temperature sufficient to begin fusing the hydrogen into helium, with the accompanying release of gamma radiation. This "flash point" varies depending on the mass of the star, but is in excess of 15 million degrees F. With this initial thermonuclear explosion, a star is born.

The gravity deriving from the tremendous mass of the new star continues to push inward, trying to collapse it further, but the outward pressure of the sustained thermonuclear reaction in the core pushes back, and these forces reach an equilibrium state at which they balance each other. This leads to a stable condition lasting about 10 billion years for a typical star, during which it remains a uniform size and emits a relatively constant amount of heat and light. Our sun is about halfway through this part of its life.

When the supply of hydrogen in the core has all been converted to helium, there is no more fuel for fusion, and the process ceases. The gravity force then lacks its balance and the star resumes its collapse, again shrinking in diameter. This causes the core to become even denser and hotter, eventually reaching an extreme temperature at which the helium atoms fuse into carbon, releasing a tremendous quantity of gamma radiation. This in turn creates even more heat, and causes the hydrogen surrounding the core to reach a temperature at which it ignites into helium.

This burst of outward pressure overbalances gravity and causes the star to swell to an incredible size. As it expands it cools again, and the fusion reactions cease. The star is glowing red hot at this stage, but is no longer producing gamma radiation and white light. This is called a red giant. In its red giant state, our sun will expand to engulf and absorb the inner planets Mercury and Venus. On Earth, the sun will fill half the sky, the oceans will boil, and the rocks will be melted into lava.

As the star cools, gravity again collapses it, resulting in another fusion event and subsequent expansion and cooling. The star may endure several of these cycles of expansion and contraction. These may be thought of as its death throes, as it is consuming the last of its nuclear fuel. During each expansion cycle, its outer gas shell may escape the gravitational field and be thrown off into space. These gases form nebulae around the dying star from which new stars may form.

Finally, all its hydrogen and helium consumed or ejected, gravity operates unopposed and the star collapses completely, into a small sphere about the size of the Earth, composed primarily of carbon. Though extremely dense and hot, this white dwarf eventually cools, leaving only a small, cold black dwarf. This is the ultimate fate of our sun, billions of years hence.

While our sun is very typical in its properties and lifecycle, it is somewhat unusual in that it is solitary. Of stars similar in size to our sun, most (perhaps two-thirds) are locked in a gravitational embrace with at least one companion star. Two coupled stars are called binary systems, and are very common in our galaxy. The science fiction movies showing two or more suns in the sky are actually illustrating science fact for many of our galactic neighbors.

Not all stars were created equal, and not all will face the same end; stars smaller than our sun can live very long, sedate lives. They don't produce as much energy, but can continue fusing hydrogen into helium for even longer than our sun's 10 billion year lifespan. They die in similar fashion, producing dwarf stars.

Very massive stars, much larger than our sun, live much shorter and more violent lives, some measured in only millions, rather then billions, of years. Such stars end in an explosion of literally cosmic proportions known as a supernova, ejecting most of their mass into space in a single cataclysmic event, and creating a huge nebula. The tiny core left behind is unimaginably dense, taking the form of a neutron star or even a black hole.

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