The Universe in the Classroom

No. 39 - Summer 1997

© 1997, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112.

Biography of a Star: Our Sun's Birth, Life, and Death

Beth Hufnagel
Michigan State University

They are born. They take shape. They go through a turbulent youth, and then they live out their lives in a predictable pattern. Maybe they have companions they provide for. Someday they rapidly decline and die. Stars, in many ways, are just like people.

Our Sun, constant and permanent though it may seem, is no exception. Once, people regarded the Sun as a different sort of object than the stars. It ruled the day; stars adorned the night. But over the past few centuries astronomers have come to recognize that the Sun is just one middle-aged member of the vast family of stars. From far away, the Sun would look just like any other star -- a point of light. And like any other star, the Sun is mortal.

The realization that the Sun is a star has done wonders for astronomy. By studying the Sun, the closest star, scientists have learned about all stars. Conversely, by studying the stars, in all their variety, scientists have learned about the past and future of the Sun. This, in turn, has told them about the past and future of life on Earth. After all, the Sun is the ultimate root of our food chain. When the Sun came into being, it provided the light and warmth needed to make Earth a hospitable place. When it dies, our planet will no longer be fit for living things.

Despite the progress of astronomy over the past centuries, our knowledge of stars is by no means complete. Recent advances in astronomy, ranging from instruments capable of observing 100 stars at once to the Keck telescope with its enormous mirror 10 meters (33 feet) across (compared to Hubble Space Telescope's 2.4 meters), have joined forces with powerful new computers to propel our understanding of how stars are born, evolve, and die.

Stayin' Alive
Family History
Just Right
That Time Bomb in the Middle
Activity: Pinhole Protractor

Stayin' Alive

The importance of the Sun to Earth is one of the main reasons scientists want to understand it. In fact, the impetus for solar science early this century came not from astronomers, but from geologists. Using radioactive-isotope dating at the beginning of this century, geologists determined that the oldest rocks on Earth are about 4 billion years old. (More recently, rocks brought back from the Moon, as well as the now-famous Mars meteorite, show that this is a common age for planets in the solar system.) Assuming that Earth and the other planets formed around the same time as the Sun, these rock ages indicate that the Sun came into being 4.5 to 5 billion years ago.

Orion nebula
The womb. The Orion nebula, long a favorite of backyard observers, is the tip of a huge cloud of gas and dust floating in interstellar space. Within the cloud are dense lumps where stars are sired. Photo courtesy of Lick Observatory.

The extreme age came as a surprise to most scientists. Astronomers already knew the basic facts about the Sun. It is simply a huge ball of gas, mostly hydrogen, held together by its own powerful gravity; it gives off light because of some source of energy within it. Astronomers thought that this source of energy was a slow but steady contraction of the Sun under the force of gravity, much as a house slowly settles. But this source of energy would only have kept the Sun alive for 20 million years. Other sources of energy -- say, a huge fire -- would burn out even quicker.

The solution to this age discrepancy is an example of how leaps in scientific understanding frequently involve the insights from disparate fields of study. In the years after World War I, British astronomer Arthur Eddington put together three ideas and boldly proposed a new energy source for the Sun.

First, astronomers knew that the Sun has to be extremely hot and dense in its center if it is to support its own weight. Gas at a high temperature exerts a strong pressure, and this holds up the outer layers of the Sun. Second, physicists had recently compared the weight of four atoms of hydrogen with that of one helium atom. Both the hydrogen quadruplet and the helium are composed of essentially the same number of subatomic particles. Yet the helium weighs less. Third, Albert Einstein's new theory of relativity showed that matter can be converted into energy (E=mc2).

At first glance, these three ideas might seem totally unrelated. But from them, Eddington deduced that the Sun's energy source was a process then unknown on Earth: the nuclear fusion of hydrogen to helium.

The word nuclear has gotten a bad rap. Normally people utter it in the same breath as mass death. But in happier circumstances, nuclear processes are responsible for maintaining all life on Earth. Deep in the hot and dense core of the Sun, hydrogen atoms are squeezed together, or fused, into helium atoms -- roughly akin to crunching a few baseballs together and getting a football. A helium atom has less mass than the hydrogen atoms from which it was created, and this missing mass turns into energy.

Few other schemes can generate as much energy as nuclear fusion. A small amount of hydrogen can produce an immense amount of energy -- which is why nuclear bombs are so destructive, and why the Sun can keeping going for billions of years.

Family History

newborn star
The baby. Swaddled in a disc of dust and gas not much larger than our solar system, this newborn star is a recent addition to the Orion family. The proud parents have the Hubble Space Telescope to thank for the portrait. Photo courtesy of Chris O’Dell of Rice University and NASA.
How did the Sun become hot and dense to begin with? This is the secret of stellar birth. Though we weren't around to witness the birth of our provider, we can read its early life history in the stars. Specifically, we can look out into space and see new stars being born right now.

The closest example is in the Orion constellation, a pattern of bright stars easily visible from the Northern Hemisphere in winter. For thousands of years, the pattern has reminded many viewers of a person with one raised arm, wearing a belt. If you look below the belt, there are four bright, blue stars called the Trapezium. If you look even more closely with binoculars, a fuzzy patch called the Orion nebula becomes visible.

This is a stellar nursery -- an enormous, lumpy cloud of cold gas and dust which is turning into hundreds of new stars. The gas is mostly hydrogen; the dust is something like the dust in a desert storm: basically, microscopic rocks. Within the clouds are hundreds of condensed, cold lumps of gas and dust. A disturbance, such as a blast wave from a nearby stellar explosion, can cause each lump to begin collapsing under its own weight.

We can see many examples of such star-forming regions. It seems that stars, like people, are born in families. For stars, these very large families are called clusters, and we know of 1,500 such clusters. Astronomers presume that the Sun was also born into a family, but, as seems to be typical of clusters, the Sun's probably broke up in the first 100 million years of its life. About two-thirds of stars are actually born with nearby twins or triplets, but the Sun is alone.

Astronomers aren't in complete agreement on where the clouds themselves come from, but it's likely that the gas and dust have more than one source. There is the pristine hydrogen gas synthesized in the creation of the universe [see "The Biggest Bang of Them All," The Universe in the Classroom, first quarter 1997]. There is the gas and dust that our galaxy has pilfered from its satellite galaxies, such as the "Magellanic stream," a streamer of gas ripped out of the nearby Large Magellanic Cloud. And there is the gas and dust from previous generations of stars. When stars die, they blow much of their material back into space, where it can form other stars. Stars in the Galaxy are the ultimate recycling machines: They use gas and dust over and over again.

The sisters. The Pleiades star cluster contains 200 stars, all born 50 or so million years ago. The wisps of dust around the stars might be remnants of the cloud from which the bicenttuplets emerged. Photo courtesy of Mount Wilson Observatory.

When the massive lump of cold dust and gas which became our Sun collapsed, the nuclear forces began to come into play. The weight of all that dust and gas produced great pressure and density at the center, and the friction of the infalling particles released heat. When the temperature in the core reached several million degrees, the hydrogen atoms started to fuse together, forming helium atoms. This released energy, the pressure increased, more atoms fused together, more energy was released, and so on, and so on. A chain reaction started that will go on for billions of years.

The outward pressure created by this nuclear fusion counterbalanced the inward pressure of gravity, and when the two canceled each other out, the natal lump of dust and gas stopped collapsing. Astronomers think this process took about 100 million years. The Sun was born.

Although the embryonic Sun slurped up most of the gas and dust from the lump, some crumbs were left over. As this extra material spun around the center, the centrifugal force prevented it from falling into the center. Instead, it flattened into a whirling disc. Astronomers have seen such discs around many young stars. Within these discs, scientists think that blobs of material clump together into the smallish bodies we call planets, asteroids, and comets.





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