When English poet Alfred, Lord Tennyson wanted to describe the bright stars of the Pleiades against their misty background, he called them "a swarm of fireflies tangled in a silver braid." His poetic vision, expressed in simple yet beautiful metaphor, was an entirely effective means to give his readers insight into a subject that is so far away, so immense that it almost defies comprehension.
As long as man has lived on earth, he has watched the skies, utterly fascinated, perhaps overwhelmed by the magnificence of the display. The study of the heavens may well be man's oldest science—proof of his relentless quest to understand more and more about the structure of the universe. But he has had to wait for the growth of science and coming of sophisticated tools to win most of the knowledge he now possesses. His understanding of the nature of the universe has been gained largely through the study of stars—their birth, development and eventual death.
Much indeed has been discovered about the life of stars within the past decade, but where and how a star's story begins are still matters of educated speculation. One theory contends that there was originally one primeval atom. Perhaps ten billion years ago that single atom burst, and from the radioactive explosion of the proton-electron-neutron-meson materials came every other star, planet and galaxy in the universe. A more common theory holds that there was no beginning point. The billions of stars in the galaxies were always there, forming and re-forming themselves out of what Tennyson called "silver braid."
Scientists call this substance by the less romantic name of "the field of nebulosity"—interstellar gases floating everywhere in space in cloudlike formations. There appears to be a continual flow of movement in space, and sooner or later some of these gas clouds come together, contract, and become dense enough to form a new star. Newton's Law of Gravitation holds that a massive body will tend to collapse under its own gravitational attraction. Yet the stars, most of them far bigger than the sun (which is more than 300,000 times as massive as the earth) keep their balance.
The secret is in their substance. The gases forming a star are so hot and dense that they exert a counter-pressure to the outside pressure. The sun, for example, is estimated to have a central pressure more than a billion times the air pressure at the surface of the earth. And the sun is but a relatively small star that appears big only because it is, by astronomical standards, so near—just 93,000,000 miles mean distance from earth. The sun's light reaches earth in a little over eight minutes, with light traveling at 186,000 miles a second. The next nearest star after the sun, Proxima Centauri, needs four and one-half years for its light to reach us.
Because of its nearness, the sun is a comparatively easy-to-study and spectacular example of how a star lives. Basically, the sun is a sphere of hot gas a little less than one million miles in diameter, with about three hundred thousand times as much mass (weight) as the earth has. The surface temperature is about 10,000 degrees F., rising steeply in the interior to a peak at the center which is believed to reach millions of degrees.
In the sun, as in other stars, is a thermonuclear crucible which is constantly synthesizing the lighter atoms of the gases into heavier atoms. For example, the sun burns 564 million tons of hydrogen into helium each second, liberating as much energy as the explosion of several billion H-bombs.
A series of calculations have been carried out by nuclear physicists for stars more massive than the sun. They show that as the helium content is built up in the star, the central areas shrink, releasing gravitational energy, while the outer areas expand. The star must make a drastic reorganization to the sudden new production of energy in its interior. Nuclear physicists believe that not only helium is formed in the interior of stars, but all other elements as well.
It takes a temperature of one or two hundred million degrees to transform helium into the main isotopes of carbon, oxygen and neon. And it takes temperatures of from two to five billion degrees to make the nuclei of atoms like iron and nickel. The common carbon atom is twelve times heavier than the hydrogen atom; iron is 56 times heavier; uranium is 238 times.
Whatever the atom activity in the center of the star, eventually the energy will move outward to the surface and be radiated into space. This heat radiation is what is seen as the star's twinkle.
Estimates of the age of stars range from ten to twenty million years for youngsters and from ten to twenty billion years for celestial senior citizens. As they pass "middle age," something may go wrong with the inner thermostatic controls, and the star can no longer shine on steadily and uneventfully, quietly manufacturing heavier and heavier atoms. Aging stars appear to become what astronomers call blue giants, then red giants, and eventually white dwarfs.
The aging sequence begins when a star's hydrogen supply runs low. As the star approaches extinction, its old age shows up on the telescope in changes in color and brightness, and it may appear to pulsate.
A white dwarf is a "dead" star, that is, a star that no longer shines because its nuclear fires have died down and it has cooled off. One authority has called the white dwarf "the burned out skeleton of a star," destined to float anonymously through space for eternity. Thus, the last stages of a star's story are spent as a white dwarf.
It is thought that the death of a star comes from violent explosion, whether from unstable internal conditions or from collision with another star. Time after time the skies have yielded what appears to be a brilliant, new star—a nova. It is not really a new star, but for a short time it looks like something new in the sky as the explosion, with its flare-up of light-producing energy, fills the space around with its dying radiance. What appears to be a new star is more likely an old star blowing its mass back into space in the form of interstellar gas and leaving behind its skeleton—a white dwarf.
A truly spectacular phenomenon is the blow-up of a supernova, an explosion so brilliant that some have appeared to be fifty million times more luminous than the sun.
On July 4, 1054, Chinese and Japanese astronomers studied a dazzling new light in the sky, a light that outshone all others. It was so bright that for a week it could even be seen in the daytime, and its visibility at night lasted until April 17, 1056. Then the "star" faded from the sky, and no more was known of it.
Centuries later astronomers noted a small, faint nebulosity, or gas cloud, in the constellation Taurus, the same position the star had occupied. Because of its shape, it was named the Crab Nebula, and it is visible today. At the center are two small stars, probably white dwarfs. Studies through the spectrograph show the nebula expanding at the rate of about 700 miles a second, with its filaments reaching out into space for billions of miles. Calculating back from its steady rate of expansion, astronomers have been able to establish its date of origin. The nebula was indeed the star which had lighted the Oriental skies in 1054. The explosion which caused the light had actually occurred some 4,000 years before the supernova had first been seen.
Since what is now seen happened forty centuries ago, scientists aren't sure what has happened to the supernova of the Crab Nebula. The materials may have been dispersed through space, and parts already gathered elsewhere to make a new star.
Astronomers have made spectroscopic observations indicating that very young stars are richer in metals than the older stars. "Second generation" stars may have formed from the material of the old, exploded stars. There is speculation that the sun may be a third generation star, as it contains all kinds of heavy atoms.
The stars in earth's galaxy number about 100 billion, spread out in a space so great that light takes a hundred thousand years to travel from one side to the other. There are other galaxies, beyond counting, many millions of light-years distant from earth. Like earth's galaxy, they are composed of stars, with interstellar gases. As earth's galaxy is continually moving and evolving, so, scientists speculate, are the other galaxies. But since it is all a matter of deduction from what scientists know today, it may be that a discovery in the next month will give an entirely new perspective.