Burning Helium and Heavier Elements
When a star has burned much of its hydrogen into helium its fire begins to go out. Diminished combustion means that there is less pressure from escaping light to hold up the outer layers of the star against gravity. The layers fall inward and the temperature rises. If the star is sufficiently massive, its internal temperature may reach 100 million kelvins. At this point helium starts to burn. The only way helium can burn is for three helium atoms to collide together within 100 micro-micro-microseconds of each other. Three helium nuclei colliding nearly simultaneously make a carbon nucleus and a photon with 1.17 micro-microwatt-seconds of energy. Burning three helium atoms makes enough energy for 2.4 million photons of red light. The new light pressure blows the outer layers of the star far away from the center. When a star is burning its helium, its diameter is much greater than the diameter it had when it was burning hydrogen. This makes the outer layers cool and red. The star swells up and becomes a red giant. The central temperature increases, but the temperature of the outer layers drops.
An example of a red giant is Betelgeuse, the right shoulder star of Orion, the Hunter. Its reddish color is visible without field glasses or a telescope. If our yellow Sun started burning helium now and swelled up to the size of Betelgeuse, its surface would reach the Earth. Another red giant is Antares. This star is larger than the orbit of Mars, which in turn is larger than the Earth’s orbit. When will our Sun start to burn helium and swell up, engulfing and burning the Earth? It hasn’t done so for almost 5 000 million years, and it will continue to behave well for another 5 000 million years.
Nuclei repel each other more and more strongly for each proton they have. Higher and higher temperatures must prevail to make the heavier elements burn. If the original mass of a star is the same as the mass of our Sun or as much as 40 percent more, the star’s central temperature will never reach the ignition temperature of elements heavier than helium. After the star burns all its hydrogen and helium it will glow as a white dwarf until most of the internal heat escapes, and then the star will fade. Our Sun appears ideally suited to supply the Earth with energy over the long term.
A larger star will have a much more dramatic end. After a red giant star burns most of its helium, the central fire sinks down and the escaping light pressure diminishes, just as they did when the star had burned most of its hydrogen. Once again the outer layers of the star begin to fall toward the center, producing even more heat and pressure in the interior. Until the central temperature rises to the ignition temperature of the heavy elements there will be no further combustion. But if the original mass was large enough, all the heavy elements will begin to burn almost at the same time. For a few days the resulting conflagration will make the star brighter than 100 000 million suns combined, brighter than all the stars in a typical galaxy. The star becomes a supernova. Long before all the remaining elements are burned up, the light pressure blows the star to bits. This flings most of the heavy elements into space as a cold dust rich in all the elements. There may be a small, dense core, all that remains of the star.
A supernova too near the Earth would be very bad for us now, but the supernovas of the second morning were good for us. They made the heavy elements needed for life. The final steps of nucleosynthesis made the stars of the second morning explode. Their inner layers blew out into space and cooled. When the heavy nuclei were cool enough, they captured electrons. All 92 elements were present in the clouds of dust and ashes left from the first stars. This dust had to consolidate in the crust of Earth-size planets before the elements could combine into the rich variety of combinations needed for life.
Some supernovas explode in regions where many stars are packed close together. Astrophysicists call these “star-forming regions” because the dust from supernovas may fall into new stars. If the new stars are sufficiently massive, they will in turn become supernovas and produce more dust. During all of this process the dust is bathed in light, whether the dust is incorporated in stellar interiors or drifting in a star-forming region near the remnants of the supernova that made the dust. When some of the dust and ashes finally drifted away from the remnants of supernovas, out of the star-forming regions, and spread through the arms of the Milky Way, the second morning ended and the third evening began.
Many ancient peoples had the idea that the complex could build up from combinations of simple elements. Both they and modern people thought that only one process made the elements. At last reality has forced them to accept that two processes operating under different conditions in two epochs of intense illumination made the elements. Long ago the Bible said exactly that. Isn’t it strange how Moses wins every time?
An example of a red giant is Betelgeuse, the right shoulder star of Orion, the Hunter. Its reddish color is visible without field glasses or a telescope. If our yellow Sun started burning helium now and swelled up to the size of Betelgeuse, its surface would reach the Earth. Another red giant is Antares. This star is larger than the orbit of Mars, which in turn is larger than the Earth’s orbit. When will our Sun start to burn helium and swell up, engulfing and burning the Earth? It hasn’t done so for almost 5 000 million years, and it will continue to behave well for another 5 000 million years.
Nuclei repel each other more and more strongly for each proton they have. Higher and higher temperatures must prevail to make the heavier elements burn. If the original mass of a star is the same as the mass of our Sun or as much as 40 percent more, the star’s central temperature will never reach the ignition temperature of elements heavier than helium. After the star burns all its hydrogen and helium it will glow as a white dwarf until most of the internal heat escapes, and then the star will fade. Our Sun appears ideally suited to supply the Earth with energy over the long term.
A larger star will have a much more dramatic end. After a red giant star burns most of its helium, the central fire sinks down and the escaping light pressure diminishes, just as they did when the star had burned most of its hydrogen. Once again the outer layers of the star begin to fall toward the center, producing even more heat and pressure in the interior. Until the central temperature rises to the ignition temperature of the heavy elements there will be no further combustion. But if the original mass was large enough, all the heavy elements will begin to burn almost at the same time. For a few days the resulting conflagration will make the star brighter than 100 000 million suns combined, brighter than all the stars in a typical galaxy. The star becomes a supernova. Long before all the remaining elements are burned up, the light pressure blows the star to bits. This flings most of the heavy elements into space as a cold dust rich in all the elements. There may be a small, dense core, all that remains of the star.
A supernova too near the Earth would be very bad for us now, but the supernovas of the second morning were good for us. They made the heavy elements needed for life. The final steps of nucleosynthesis made the stars of the second morning explode. Their inner layers blew out into space and cooled. When the heavy nuclei were cool enough, they captured electrons. All 92 elements were present in the clouds of dust and ashes left from the first stars. This dust had to consolidate in the crust of Earth-size planets before the elements could combine into the rich variety of combinations needed for life.
Some supernovas explode in regions where many stars are packed close together. Astrophysicists call these “star-forming regions” because the dust from supernovas may fall into new stars. If the new stars are sufficiently massive, they will in turn become supernovas and produce more dust. During all of this process the dust is bathed in light, whether the dust is incorporated in stellar interiors or drifting in a star-forming region near the remnants of the supernova that made the dust. When some of the dust and ashes finally drifted away from the remnants of supernovas, out of the star-forming regions, and spread through the arms of the Milky Way, the second morning ended and the third evening began.
Many ancient peoples had the idea that the complex could build up from combinations of simple elements. Both they and modern people thought that only one process made the elements. At last reality has forced them to accept that two processes operating under different conditions in two epochs of intense illumination made the elements. Long ago the Bible said exactly that. Isn’t it strange how Moses wins every time?