The Universe in the Classroom

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

Just Right

Depending on the size of the original lump of gas and dust, the process of stellar birth can give rise to different sorts of stars. A small lump never develops high enough pressures and temperatures to start nuclear fusion. It is doomed to remain a dark, dismal stellar wanna-be -- a so-called brown dwarf. A larger lump becomes a large star, so hot and bright that it burns itself out in a few tens of millions of years. A lump in the middle, not too small and not too large, becomes a middling star such as the Sun. Which is good: If the Sun had been much smaller, Earth would have been a dark, dead world; much larger, and Earth would have been broiled.

The middle-aged suburbanite. The Sun is a two-car-garage kind of star. Its stability and temperance make it an ideal provider for tender planets. Photo courtesy of Mount Wilson Observatory.

In its early years, the Sun went through a tempestuous youth, whipping up strong winds that cleared the solar system of whatever gas had not been incorporated into a planet. But then the Sun settled down. From studying rocks, fossils, and Antarctic ice, scientists think the Sun has been brightening over time, but only slightly.

And how much longer will it continue to shine? For an idea of the Sun's life expectancy, astronomers look to clusters of stars, such as one named Messier 67, which is about the same age as our Sun. By simulating the life cycles of these stars on a computer, astronomers have ascertained how long stars live. They predict that the Sun will be able to fuse hydrogen into helium in its core at about the same rate for another 5 billion years. (What a relief!) If the Sun were a car, the gas tank would now be half full.

What will happen when the Sun does run out of gas? (Hydrogen gas, that is.) Fortunately, the Sun will still have reserves of hydrogen in the layers that surround the core. The core will heat up this shell of hydrogen. When the shell gets hot enough to fuse hydrogen to helium, the release of energy will carry on there. It is as if the driver of the car poured an extra few gallons into the fuel tank.

But this trick has a price. The source of energy will no longer be the dense, massive core, but rather a shell closer to the surface -- and that will make a big (so to speak) difference in the structure of the Sun. The Sun will puff up until its radius is 30 times greater. It will become a red giant, similar to the star Arcturus, though much smaller than a supergiant such as Betelgeuse (see photo on p. 3). A red giant is red because its exterior cooled from 9,000 to 3,000 degrees Fahrenheit as it expanded; for a star, red means cool. This red-giant stage will last for about 2 billion years.

That Time Bomb in the Middle

The last fling. Its normal life over, Betelgeuse is now a red supergiant — red, because its surface is a comparatively cool red-hot, and supergiant, because it is hundreds of times larger than the Sun and ten times more massive. Its waistline, however, is not a sign of might, but of impending death. Photo courtesy of Andrea Dupree, Ronald Gilliland, NASA, and the European Space Agency.
The striking but now-outdated video Universe, produced by NASA in the 1970s, shows the red-giant Sun engulfing the Earth. Though certainly dramatic, this is now thought to be incorrect. Astronomers have had to scale down their estimates of the size of red giants based on data from the satellite Hipparcos and from the new optical and infrared interferometers -- networks of telescopes which can take images of large, nearby stars. Now we think the Sun will not engulf us when it becomes a red giant.

But that is small comfort. In its retirement from normal core fusion, our previously nurturing star will care little for its planetary children. It will be pumping out a thousand times more energy, making Earth a good approximation to hell. To add insult to injury, the solar wind -- a stream of particles which now gives us fun things such as the aurora borealis -- will become a cyclone that will make radio communication impossible and perhaps evaporate the atmosphere altogether. Looking on the bright side, the red-giant Sun may be warm enough to melt the water-rich but now-frozen moons of Jupiter and Saturn. Humanity, if it is still around, might relocate there.

Meanwhile, what happens to all that helium being produced in the shell? It gently rains onto the dead, but still toasty, core of the Sun, making the core more massive and more compressed. This raises the temperature of the core until suddenly -- and I really do mean suddenly, as in seconds -- the helium in the core fires up and begins to fuse itself into carbon. Using the fuel-tank analogy, this is as if the exhaust itself starts to burn.

The end is drawing near. Now the Sun has to rearrange its internal structure all over again, as its source of energy is once again the central core. The Sun will contract back to a bit larger than its original radius and will give off 10 times as much energy as what we are used to now. This phase only lasts another 500 million years, as there are a lot fewer helium nuclei (it took four hydrogen nuclei to make one helium nucleus, and three heliums to make one carbon) and the energy production is much less efficient.

As the Sun exhausts the helium in the core, it desperately staves off the inevitable by resorting again to those reserves in its outer layers. Again the Sun expands. This time, it grows so large that its outer edge is only weakly gravitationally bound to the core. The Sun barely holds itself together anymore. This eleventh-hour attempt at life-support is pitifully ineffective; the final red-giant stage can be maintained for only 100 million years.

At this point, things will really start falling apart. The Sun's outer layers, freed from the gravitational clutches of the core, will waft away. Over the course of about 10,000 years, these layers will spread out into space as an enormous sphere of gas lit up by the now-naked hot core. These layers constitute a "planetary nebula," so called because in a small telescope the gas cloud looks a bit like the disc of a planet (see photo on p. 3). The hot core is now a "white dwarf," a stellar cinder. As a white dwarf, the ex-Sun will glow white-hot for a near-eternity.
white dwarf
The cremation. After stars like the Sun fuse their last atom, they scatter their ashes into space as a so-called planetary nebula, such as Abell 39. At the center of the nebula is what’s left of the star: a slowly decaying “white dwarf.” Photo courtesy of George Jacoby of the National Optical Astronomy Observatories.

Alas, there will be no dramatic explosions to entertain our distant descendants: The Sun would have had to start with at least eight times more mass to die the spectacular death of a supernova. The Sun, modest in life, is subdued in death. After the planetary nebula fades, there is no nuclear fusion at all (no extra fuel, no fuel tank, not even the trunk is left), just a lump of hot carbon and some happy memories. The Sun will be well and truly dead.

The sphere of gas drifts off and eventually is gathered up in a new cloud, and become part of the next generation of star formation. Perhaps one day, the ashes of the Sun will throw their lot in with another star to be born, live, die, and, perhaps, give sustenance to other warm little planets.

BETH HUFNAGEL is a postdoctoral researcher at Michigan State University in East Lansing. As an auditor, she used to ferret out the secrets of corporate finance -- talents now applied to the evolution of Sun-like stars. Her email address is George Musser contributed to this article.



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